Gas Strut Calculator

Calculate the force per gas strut, mounting-point travel, spring dimensions and hand effort for a hinged lid.

Example values loadedIllustrative mounting geometry only; replace all example dimensions before selecting a spring.

Preliminary 2D model; not a verified equipment specification. Terms and Conditions

Hinge moment balance at the chosen angle
\[ F_{\mathrm{strut}}=\frac{mgx_G}{n\,d_s} \]

Signed lever arms come from the actual bracket coordinates; the spring force is modeled constant unless a selected force is entered.

1

Lid and mounting dimensions

Measure from the hinge center. Angles are measured counterclockwise from horizontal.

Total moving mass including hardware.

Measure along the lid from hinge to free edge.

Along the lid, to the balance point.

Positive toward the lid’s free edge.

Positive upward; below hinge is negative.

Measured from hinge along the lid.

Degrees counterclockwise from horizontal.

Final angle, greater than closed angle.

One or two struts are most common.

Handle force is perpendicular to the lid.

Advanced geometry, spring selection and clearance

Positive to upper side of lid.

Positive above lid centerline.

Angle at which the nominal force balances gravity.

Manufacturer nominal extending force per spring.

Center-to-center maximum mounting length.

Manufacturer travel from compressed to extended.

Illustrative clearance, not a universal requirement.

2

Gas strut force and sizing

Calculated demand is not a selected manufacturer rating.

Required constant force per strut
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N
Calculating example geometry…

Travel and operating checks

    Show calculation stepsGeometry, torque, spring travel and limitations
    1. Enter valid dimensions to see steps.
    3

    Live mounting geometry

    The diagram labels the key parts so users can see where the hinge, brackets, gas strut, and center of gravity are located.

    Gas strut mounting geometryA hinged lid with labeled hinge, fixed mount, moving mount, gas strut, and center of gravity.

    Preview only; does not change the chosen balance angle or the force calculation.

    HingePivot point where the lid rotates.
    Fixed mountBracket attached to the stationary frame.
    Moving mountBracket attached to the rotating lid.
    Gas strutConnects the two mounting brackets.
    Center of gravityPoint where the lid’s weight is modeled to act.

    Blue line: current lid. Dashed gray lines: closed and open positions. The diagram shows geometry, not installation clearances.

    —
    4

    Method, sources and limitations

    Preliminary rigid-lid, planar static calculation.

    Rigid-body hinge equilibrium

    Force demand is calculated from signed moments about the hinge. Spring travel is evaluated across the full opening arc, not only at end positions.

    • Example geometry is illustrative. Illustrative inputs are not an equipment recommendation. Verify the spring rating at its reference position, force progression, end fittings, clearances, attachment strength and independent support where required.
    • 2D rigid lid; identical springs share load equally. No hinge friction, seal friction, wind, shock or spring progression is included in force demand.
    • Force is evaluated at the selected balance angle, not guaranteed to hold the lid at every angle.

    Gas spring sizing and installation guide

    What Size Gas Strut Do I Need?

    The correct gas strut depends on the lid’s mass, center of gravity, mounting points, opening angles, and number of springs. Enter these measurements into the calculator above to determine the ideal balancing force per spring at a chosen angle and the minimum and maximum mounting distances throughout the lid’s travel. Use those results to evaluate a candidate spring’s force, length, stroke, and expected operating effort.

    A lid cannot be sized by weight alone: its weight creates a moment about the hinge, while each spring creates a competing moment through a mounting-dependent lever arm. A spring with enough force may still be unsuitable if it bottoms out, reaches full extension too early, or makes the lid hard to close.

    Enter
    Lid mass and geometry, bracket coordinates, opening angles, and spring count.
    Primary output
    Ideal static balancing force per strut at the selected balance angle.
    Check before buying
    Spring travel, actual force curve, pivot loads, physical clearance, and installation requirements.

    How to Use the Gas Strut Calculator

    Start with real hinge-center and bracket-pivot measurements. The calculator includes illustrative defaults that must be replaced for your application; results update as the inputs change.

    1. Enter the moving lid’s mass and dimensions

      Include the lid and attached hardware. Measure hinge to free edge, hinge to center of gravity, and hinge to handle. Use the unit selectors for kg/lb and mm/in; switching a unit converts the entered physical value.

    2. Measure the fixed and moving brackets

      Use the hinge pivot as the coordinate origin. Enter the stationary bracket’s horizontal and vertical offsets and the moving bracket’s distance along the lid. Enter nonzero normal offsets under Advanced Options if the mounting pivot or center of gravity is displaced from the lid’s reference line.

    3. Set the motion and spring count

      Enter closed and open angles in degrees counterclockwise from horizontal. A horizontal lid extending to the right is 0°. For example, a lid moving from 20° to 85° has a closed angle of 20°, not 0°. Choose a whole number of identical struts from 1 through 4.

    4. Choose the balance angle

      The optional balance angle defaults to the open angle when left blank. Balancing at the open angle does not guarantee the lid will hold at every other angle. Use the angle slider to preview geometry; this slider does not change the chosen balance angle or the primary calculated requirement.

    5. Check a particular spring

      In Advanced Options, enter the proposed nominal spring force if known. To check geometric fit, enter both its extended center-to-center length and rated stroke. The calculator’s default travel clearance is 5 mm at each end; change it to your verified installation allowance. An entered force changes the candidate-spring operating-effort analysis, not the theoretical balancing force displayed as the primary result.

    How to Measure Gas Strut Mounting Points

    Measure every attachment from its pivot center, not the edge of a hinge plate, ball socket, or bracket. The diagram uses numbered markers rather than tiny text inside the drawing; the matching labels below remain full-size on phones.

    Gas strut mounting and measurement diagram Side view of an elevated hinged lid. Marker one is the hinge, two the fixed mounting pivot, three the moving mounting pivot, and four the center of gravity. A dark line joins fixed and moving pivots to represent the strut. The lid is shown in blue, its horizontal reference in a dashed line, and downward gravity is an orange arrow. 1234
    Illustrative side view only—not installation dimensions or a collision-clearance check. The blue member is the lid, the dark diagonal is the gas spring, and the orange arrow indicates the downward weight force. Match markers 1–4 to the measurement key below.
    1. Hinge pivot
    The lid’s center of rotation. Use it as the origin for all coordinates, including the fixed bracket and center of gravity.
    2. Fixed mounting pivot
    The spring connection on the stationary frame. Record horizontal offset (positive toward the horizontal lid’s free edge) and vertical offset (positive up; negative below the hinge).
    3. Moving mounting pivot
    The spring connection that rotates with the lid. Measure its distance along the lid from the hinge; enter any perpendicular offset separately.
    4. Center of gravity
    The complete moving assembly’s balance point. For a uniform lid, half its length is an initial estimate, but heavy locks, windows, and reinforcement can move the true balance point.

    The spring pushes along the line joining markers 2 and 3. The perpendicular distance from the hinge to that force line—not merely the distance from hinge to marker 3—determines spring leverage. A bracket 300 mm along the lid may have a much shorter effective moment arm.

    If a center of gravity or mounting point lies outside the modeled centerline, use the relevant normal-offset input. For a wide lid with an off-center single spring, verify twisting and hinge loads separately: the two-dimensional tool does not calculate torsion.

    Gas Strut Force Calculation

    The calculator uses planar rigid-body moment equilibrium about the hinge. Gravity applies a closing or opening moment depending on the center of gravity’s horizontal position; an extending gas strut applies a moment determined by its signed perpendicular lever arm.

    Force required for static balance

    \[F_s=\frac{mgx_G}{n\,d_s}\]

    When gravity acts toward closing and the struts act toward opening, force per strut equals the gravity moment divided by the number of identical struts times their perpendicular opening moment arm.

    The equation requires a positive nonzero opening moment arm for this closing-gravity case. The calculator uses signed moments to handle other positions and warns when a positive extending spring cannot produce the needed balancing moment.

    Installed mounting distance and stroke

    \[L_s(\theta)=\sqrt{(x_B-x_A)^2+(y_B-y_A)^2}\]

    The installed spring’s center-to-center length is the straight-line distance between its two mounting pivots at each lid angle.

    \[S_{\mathrm{used}}=L_{\max}-L_{\min}\]

    Geometric travel is the greatest installed length minus the smallest installed length over the complete movement range. An intermediate position can be the shortest or longest; the endpoints alone are not always sufficient.

    \(F_s\)
    Force required per strutIdeal extending force for static balance at the chosen angle.N
    \(m\)
    Moving massComplete moving lid and attached hardware.kg
    \(g\)
    Gravitational acceleration9.80665 meters per second squared in the calculator model.m/s²
    \(x_G\)
    Horizontal center-of-gravity coordinateSigned horizontal hinge-to-center-of-gravity separation at the evaluated angle.m
    \(n\)
    Spring countNumber of identical, equally loaded springs.Dimensionless
    \(d_s\)
    Signed spring moment armPerpendicular hinge-to-force-line distance, signed by spring moment direction.m
    \(L_s\)
    Installed spring lengthPivot-center distance at the angle being evaluated.m
    \(\theta\)
    Lid angleCounterclockwise from horizontal.Degrees
    \((x_A,y_A)\)
    Fixed mounting coordinatesStationary pivot location relative to hinge.m
    \((x_B,y_B)\)
    Moving mounting coordinatesRotating pivot location at the evaluated angle.m
    \(S_{\mathrm{used}}\)
    Used geometric travelMaximum minus minimum pivot separation.m
    \(L_{\max}\)
    Maximum pivot separationLargest center-to-center length over full travel.m
    \(L_{\min}\)
    Minimum pivot separationSmallest center-to-center length over full travel.m

    The gravity moment changes as the lid rotates, and so does spring leverage. Static balance at one angle does not imply equal handling force or stable support at every position.

    Worked Example: A 15 kg Hinged Lid

    Consider the calculator’s illustrative geometry: a 15 kg lid, 800 mm long, with its center of gravity 400 mm along the lid. Two identical springs are positioned between a fixed pivot at (100 mm, −200 mm) and a moving pivot 300 mm along the lid, with no perpendicular offset. Find the ideal force per spring at 80°.

    Given values

    Moving mass
    15 kg
    Lid length
    800 mm
    Center of gravity
    400 mm along lid
    Fixed pivot
    (100 mm, −200 mm)
    Moving pivot
    300 mm along lid
    Closed to open
    0° to 80°
    Balance angle
    80°
    Spring count
    2
    Find
    Ideal static force per spring at 80°

    Convert the lengths

    \[\begin{aligned}400\ \mathrm{mm}&=0.400\ \mathrm{m}\\300\ \mathrm{mm}&=0.300\ \mathrm{m}\\100\ \mathrm{mm}&=0.100\ \mathrm{m}\\-200\ \mathrm{mm}&=-0.200\ \mathrm{m}\end{aligned}\]

    All moments use newtons and meters.

    Calculate the gravitational moment

    \[\begin{aligned}x_G&=0.400\cos80^\circ\\&\approx0.06946\ \mathrm{m}\\mgx_G&=15(9.80665)(0.400\cos80^\circ)\\&\approx10.2174\ \mathrm{N\,m}\end{aligned}\]

    At 80°, gravity produces approximately 10.2174 N·m of closing moment.

    Determine spring leverage

    The rotating pivot is approximately (0.05209 m, 0.29544 m), while the fixed pivot remains at (0.100 m, −0.200 m). Their pivot separation is about 0.49775 m. The signed perpendicular spring moment arm is approximately 0.080287 m (80.287 mm), substantially less than the 300 mm hinge-to-moving-mount distance.

    Calculate force per spring

    \[F_s\approx\frac{10.2174}{2(0.080287)}\approx63.63\ \mathrm{N}\]

    Result

    63.63 N per strut ≈ 14.30 lbf per strut

    This is ideal static balance at 80° using a constant-force spring model, not an automatically approved commercial spring or a guarantee of support at other angles.

    How to Interpret Your Results

    The primary force is what an ideal constant-force spring must apply per strut to balance gravity at the selected angle. It is not the combined force of all struts, an actual manufacturer’s force at every stroke position, or the force needed to physically lift the whole lid vertically.

    Calculated force versus selected rating

    The calculated requirement is fixed by the entered lid and geometry. The optional selected spring rating is a separate candidate value used to estimate handling effort. Entering it does not change the primary balance-force requirement.

    Check sensitivity

    With mounting geometry, spring count, balance angle, and center-of-gravity location fixed, a 10% increase in lid mass produces a 10% increase in required ideal balancing force. Reducing the spring moment arm increases required force.

    Check the warnings

    A zero or reversed spring moment arm prevents useful opening assistance at that position. Even when the selected balance angle is valid, loss of leverage elsewhere in the travel makes the single-angle result unsuitable as a hold-open specification.

    Opening and closing effort

    The calculator estimates a perpendicular manual force at the entered handle distance for a constant-force spring and reports sampled peak opening and closing efforts. Actual effort can differ due to manufacturer force progression, internal seal friction, hinge friction, and external loads. Increasing the spring rating can reduce opening effort while making the lid harder to close.

    Choosing Gas Strut Force, Length, and Stroke

    A gas spring must provide acceptable force and fit between its mounting points at every position. Compare the candidate product’s nominal force reference, extended center-to-center length, compressed length, rated stroke, and fittings against the calculator outputs.

    Gas spring extended and compressed lengths Two gas springs with the same fixed left pivot. The fully extended spring ends at the farther right pivot and the fully compressed spring ends at the nearer pivot. Numbered markers identify the extended pivot separation, compressed pivot separation, and extra distance that constitutes rated stroke. 123
    Schematic only; the lengths are measured between the connection pivot centers, including the applicable end fittings. Marker 1 identifies extended length; marker 2 identifies compressed length; marker 3 identifies the difference between the two, the rated stroke. These are product dimensions, not necessarily the full travel your mounting geometry uses.
    1. Extended length
    Fully extended center-to-center pivot distance for the proposed spring and its end fittings. It must accommodate the installation’s maximum pivot separation plus your selected extension reserve.
    2. Compressed length
    Fully compressed center-to-center pivot distance. It must be shorter than the installation’s minimum pivot separation by the selected compression reserve.
    3. Rated stroke
    Extended length minus compressed length. This must exceed the mounting geometry’s used travel by the combined end-of-travel allowances.
    Selected nominal force
    The manufacturer’s rated force at its specified reference position and conditions; it need not equal the force that spring produces at your chosen balance angle.

    Example: travel and clearance

    If the mounting pivots range between 250 mm and 400 mm apart, the geometric travel is 150 mm. As an illustrative design choice, allow 10 mm of unused travel at each end. The candidate must then simultaneously satisfy the following conditions:

    \[\begin{aligned}S_{\mathrm{rated}}&\geq170\ \mathrm{mm}\\L_{\mathrm{extended}}&\geq410\ \mathrm{mm}\\L_{\mathrm{compressed}}&\leq240\ \mathrm{mm}\end{aligned}\]

    These are linked fit constraints for the same spring, not three independent choices. The calculator defaults to a 5 mm reserve at each end; the 10 mm figures above are an example input, not the calculator default or a universal rule.

    Do not automatically round the calculated force upward without checking closing effort and mounting loads. A stronger spring can cause unwanted opening or make closing difficult. Confirm a candidate’s actual force progression, tolerances, fittings, mounting orientation, and physical clearance with its manufacturer’s documentation.

    Replacing an existing gas strut

    Record the original part number and marked force, extended center-to-center length, compressed length or rated stroke, and end-fitting geometry. A replacement with the same force but the wrong compressed length can prevent full closure. A dimensionally matching spring with a different force can change lid behavior or bracket loading. If the old spring is worn, its current measured support force may not represent its original rated specification.

    Real-World Mounting and Operating Conditions

    A real gas spring’s force changes along its stroke and between extension and compression. Manufacturer data, temperature, hinge friction, and installation details determine whether the preliminary constant-force result delivers the intended handling behavior.

    Force progression and friction

    As the rod enters the cylinder, displaced gas volume increases internal pressure. Seal friction opposes movement, so actual extending and compressing forces differ. Obtain the selected spring’s force-versus-stroke data, including the reference point used for its nominal rating.

    Temperature and service condition

    Gas pressure and spring force vary with temperature. Cold-weather support and hot-weather closing effort may differ from room-temperature behavior. Check the manufacturer’s stated operating range, force variation, and relevant service-life information rather than applying an arbitrary universal correction.

    Rod orientation and damping

    Conventional gas springs are commonly mounted rod-down according to manufacturer instructions for seal lubrication and intended damping. Verify the required orientation for the particular product and at the relevant lid position; do not assume every gas spring or damper has identical requirements.

    Brackets, twisting, and end stops

    Gas spring force is transmitted directly to the mounting pivots, brackets, and hinge. An off-center single spring may twist a wide lid. Check bracket and fastener capacity, lid stiffness, three-dimensional interference, and mechanical stops independently. The spring’s internal stroke limit should not be presumed to be an acceptable structural end stop.

    Self-rise and self-close behavior depend on the net gravity and spring moments at each position, as well as friction and the latch. A force value that balances one angle does not demonstrate either behavior across the operating range.

    Common Gas Strut Problems

    If the lid does not move as intended, check the full installation geometry before assuming the spring simply needs a higher force rating.

    Lid falls closed

    The effective opening moment may be too small because of insufficient spring force, poor bracket leverage, added lid weight, low temperature, or gas loss. Check the force and signed moment arm at the angles where support fails.

    Lid is difficult to close or opens unexpectedly

    The installed spring may produce excessive opening moment. Compare its force curve and the calculator’s estimated closing effort; inspect latch requirements before changing the spring rating or mounting positions.

    Lid stops before fully closing or opening

    The spring may reach its length limit or the hardware may interfere. Compare the full-range minimum and maximum mounting distances against compressed and extended dimensions with adequate end reserves.

    Brackets bend or loosen

    Mounting forces, poor alignment, inadequate reinforcement, or side loading may exceed the hardware’s capacity. Independently verify pivots, fasteners, hinges, brackets, and supporting structure.

    Support the lid independently before inspecting damaged hardware or replacing a failed gas spring; do not work below a heavy lid held only by a suspect spring.

    Calculation Limitations and Safety Checks

    The calculator is a preliminary, static, two-dimensional rigid-lid model. Its geometry and moment calculations are useful for screening mounting layouts, but they do not certify a real spring, complete assembly, or safety-critical installation.

    Idealized spring and load sharing

    The force analysis treats each modeled spring as having constant force and multiple springs as identical and equally loaded. The model does not establish actual spring force progression or unequal loads caused by structural flexibility or alignment.

    Static, planar motion only

    The analysis does not calculate acceleration, impact, wind, slamming, out-of-plane twisting, damping, or structural capacity. The diagram and length checks cannot establish real-world clearance between the full spring body and nearby parts.

    Handling-force estimates are sampled

    Reported peak opening and closing forces are calculated at sampled angular positions under the idealized force model. A sampled maximum is not an independently verified physical worst-case or a guarantee of measured operating effort.

    Independent support where needed

    A gas spring is not automatically a positive safety lock. Heavy, overhead, or otherwise consequential access panels may require a separately rated mechanical support and verification of the complete attachment structure.

    Gas Strut FAQs

    These answers cover common purchase and replacement decisions not fully resolved by a single calculated force value.

    What size gas strut do I need for a 20 lb lid?

    A 20 lb lid has a weight force of about 89 N under standard gravity, but that does not determine its gas strut force rating or physical dimensions. Enter its mass or weight, center of gravity, bracket locations, opening angles, and spring count to calculate its static moment and geometric travel.

    What does 300 N on a gas strut mean?

    It denotes a nominal spring force of approximately 67.44 lbf under the manufacturer’s specified reference conditions. The actual force at your lid angle may differ because of stroke position, progression, friction, and temperature; check the manufacturer’s product data.

    Can I replace a 200 N gas strut with a 300 N strut?

    Not without evaluating the installation. A stronger spring may increase closing effort, cause unintended opening, and increase hinge and bracket forces. Confirm the original design requirement and both products’ force and dimensional specifications.

    How do I convert the calculated strut force to pounds-force?

    Choose lbf in the calculator’s force result unit selector. One pound-force equals approximately 4.44822 N, so 63.63 N is approximately 14.30 lbf. Changing the output unit does not change the underlying physical force.

    Can I use one gas strut instead of two?

    One spring may provide sufficient net moment but can create twisting loads if mounted off-center on a wide lid. Check torsional stiffness, hinge reactions, bracket capacity, and the real load path; equal total force is not proof that the one- and two-spring arrangements are structurally equivalent.

    How do I measure a replacement gas strut?

    Record the original part number, force marking, fully extended pivot-center length, compressed length or rated stroke, and the specific end fittings. Measure to the actual joint centers, not only the metal cylinder or exposed rod.

    Engineering References

    The equations are derived from planar rigid-body moment equilibrium and pivot-to-pivot geometry. Manufacturer technical information is needed for real spring force ratings, progression, damping, and installation practice.

    The worked example was checked by reversing the static moment balance. It is an illustrative preliminary force calculation rather than verification of a specific manufactured spring or installed assembly.

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