Duct Size Calculator
Size round or rectangular ducts by velocity or equal-friction. Convert units, see quick stats, and view worked steps.
Calculation Steps
HVAC Design Guide
Duct Size Calculator: Fast, Code-Aware, and Field-Ready
Use this practical guide beneath the calculator to pick the right duct diameter (round) or dimensions (rectangular) for a given airflow, velocity target, and friction rate. We cover quick steps, what drives the number, examples, and the trade-offs you’ll make on real jobs.
Quick Start
- 1 Enter the airflow (e.g., CFM or L/s). This is the starting point for all duct sizing.
- 2 Pick a method: Velocity-based (choose a target fpm) or Friction-based (choose in. w.c. per 100 ft). You can also solve for velocity if diameter/dimensions are fixed.
- 3 Choose duct shape (round or rectangular). For rectangles, use the calculator’s equivalent round to check velocity and pressure drop.
- 4 Add length and fittings. Use equivalent length for elbows/tees to capture real friction losses.
- 5 Review results: diameter/dimensions, velocity, and friction loss. Adjust until you’re inside reasonable ranges for noise, energy, and space.
Tip: For residential supplies, a velocity target of 600–900 fpm is a practical starting band; for branches, many designers aim a bit lower to keep sound down. Always confirm with project criteria.
Watch-out: Mixing units is the #1 error. Keep CFM ↔ L/s and in. ↔ mm consistent, and re-run after toggling shape or units.
Choosing Your Method
Velocity Method
You choose the target velocity, the calculator finds a duct size.
- Intuitive control over sound and draft risk.
- Fast for branches and takeoffs.
- Great for quick retrofits with tight space.
- May ignore total static unless you check friction.
- Easy to over-speed long runs and create noise.
\( V = \dfrac{Q}{A} \Rightarrow A = \dfrac{Q}{V},\; D_\text{round} = \sqrt{\dfrac{4A}{\pi}} \)
Friction (Equal-Friction) Method
You set a friction rate; the calculator sizes ducts that keep pressure losses aligned across the run.
- Balances branches; good network behavior.
- Connects directly to fan static and energy.
- Standard for many design offices.
- Needs realistic equivalent lengths for fittings.
- May produce larger diameters in tight ceilings.
\( \Delta P = \dfrac{fL}{D}\,\dfrac{\rho V^2}{2} \ \Rightarrow\ \text{pick } \frac{\Delta P}{100\ \text{ft}} \text{ and solve} \)
What Moves the Number the Most
Airflow (Q)
Higher CFM demands more area to keep velocity and noise down. Halving velocity doubles area.
Velocity Target (V)
Higher velocities shrink size but increase sound and friction loss. Keep long runs moderate.
Friction Rate
Tighter (lower) friction rates yield larger ducts and lower fan energy; looser rates save space but raise static pressure.
Shape & Aspect Ratio
Rectangular ducts with extreme aspect ratios raise friction and noise. Squarer rectangles behave better.
Fittings & Equivalent Length
Elbows, tees, and transitions can rival straight length in pressure loss—don’t ignore them.
Roughness & Liner
Flex and internally lined ducts increase friction; spiral/galv round is usually smoother.
Variables & Symbols
- Q Airflow (CFM or L/s)
- V Velocity (fpm or m/s)
- A Area (ft² or m²)
- D Round duct diameter (in or mm)
- ΔP Pressure loss (in. w.c. or Pa)
- L Length or equivalent length (ft or m)
- f Friction factor (dimensionless)
- ρ Air density (lbm/ft³ or kg/m³)
Worked Examples
Example 1 — Size a Round Supply by Velocity
- Given: \(Q = 450\ \text{CFM}\), target \(V = 800\ \text{fpm}\) (quiet branch), straight length 35 ft + two long-radius elbows (≈ 30 ft eq).
- Units: Imperial.
1
Compute area: \( A = \dfrac{Q}{V} = \dfrac{450}{800} = 0.5625\ \text{ft}^2 \).
2
Convert to diameter: \( D = \sqrt{\dfrac{4A}{\pi}} = \sqrt{\dfrac{4(0.5625)}{\pi}} \approx 10.1\ \text{in} \).
Round to a standard size: 10 in or 11 in if noise margin is critical.
3
Add equivalent length: \( L_\text{tot} \approx 35 + 30 = 65\ \text{ft} \). Use the calculator’s friction output to verify total ΔP is within available static.
4
Iterate: If ΔP is high, go to 11 in; if space is tight, check 9 in but ensure velocity and sound remain acceptable.
Example 2 — Rectangular vs Equivalent Round (Equal Friction)
- Given: \(Q = 1{,}200\ \text{CFM}\), friction rate target \(= 0.08\ \text{in. w.c.}/100\ \text{ft}\).
- Rectangular trial: 10 in × 16 in (area = 160 in² = 1.11 ft²).
- Goal: Check velocity and equivalent round size.
1
Velocity: \( V = \dfrac{Q}{A} = \dfrac{1200}{1.11} \approx 1080\ \text{fpm} \). This is brisk; assess noise risk.
2
Equivalent round (industry rule of thumb): \( D_\mathrm{eq} \approx 1.30 \dfrac{(ab)^{0.625}}{(a+b)^{0.25}} \) with \(a=10\), \(b=16\) (in).
The result is roughly 14 in. Compare friction/velocity to confirm suitability.
3
Iterate rectangle toward a squarer aspect ratio (e.g., 12×14) to shave friction and sound without growing height too much.
Your calculator lets you flip between shapes and see velocity & ΔP instantly—use it to test space vs. performance.
Common Layouts & Variations
Different duct materials and layouts trade space, sound, and energy. Use this table to set expectations before ordering.
| Variation | Typical Use | Impact on Sizing | Notes |
|---|---|---|---|
| Spiral Round (galv) | Main trunks, exposed ceilings | Lower friction → smaller sizes at same ΔP | Neat aesthetics; fewer joints; check seam leakage class. |
| Rectangular (TDC/TDF) | Ceiling plenums, tight joist bays | Aspect ratio matters; extreme ratios increase ΔP/noise | Transitions add equivalent length—model them. |
| Flex (short whip) | Final runouts to diffusers | Higher friction → upsize or shorten runs | Keep straight, stretched, and under 5–6 ft where possible. |
| Internally Lined | Noise control, condensation | Higher friction vs bare metal | Velocity can fall; re-check ΔP and fan static. |
| High-Aspect Rectangle | Very shallow plenums | Often needs larger area for same ΔP | Consider two smaller parallel ducts to reduce noise. |
- Keep branch velocities moderate near grilles to control sound.
- Use gentle transitions (≤ 15°) to reduce turbulence and loss.
- Account for balancing dampers and coil/fan connections in ΔP.
- Verify leakage class and insulation thickness in specs.
Specs, Logistics & Sanity Checks
Selection Criteria
- Performance: Velocity & friction rate inside project limits.
- Space: Fits ceilings/shafts without compromising access.
- Noise: Conservative velocities near occupants.
- Future: Allow damper & filter losses; don’t size to the ragged edge.
Logistics & Installation
- Measure real pathways; note clashes (beams, lights, sprinklers).
- Use equivalent length for elbows/tees; prefer long-radius fittings.
- Seal to the specified leakage class; pressure test representative sections.
- Support spacing and hanger type per spec to avoid deflection.
Code & Standards Note
Local codes, energy standards, and project specifications govern insulation, leakage, fire/smoke, and sound. This article is informational—always verify against contract documents and authority having jurisdiction.
Frequently Asked Questions
What velocity should I target for supply ducts?
There’s no single “right” number. For many comfort systems, trunks often land in the 700–1,100 fpm range and branches lower (e.g., 500–900 fpm) to limit noise. Start with a target, then iterate size until both velocity and friction loss are acceptable for the run length and fittings.
What’s a reasonable friction rate?
Designers commonly test friction rates around 0.05–0.10 in. w.c. per 100 ft for comfort air systems, tightening for quiet/efficient runs and loosening when space is constrained. The “right” value is the one that balances fan static, energy, and ceiling space on your job.
How do I convert a rectangular duct to an equivalent round size?
Use the calculator’s built-in “equivalent round” function, which estimates the round diameter that yields similar friction behavior. A common rule is \( D_\mathrm{eq} \approx 1.30 \frac{(ab)^{0.625}}{(a+b)^{0.25}} \) for a × b (in). Validate velocity and ΔP after conversion.
Why did my calculated size get noisier when I shortened a run?
Shortening length reduces friction loss, which can let velocity creep up if you downsized the duct to “use up” available static. Keep an eye on both velocity and ΔP; don’t chase one at the expense of the other.
Do elbows and tees really matter that much?
Yes. Two or three elbows can add as much pressure loss as dozens of feet of straight duct. Always include equivalent length for fittings—especially branch takeoffs, transitions, and tight elbows near fans or coils.
Should I size returns differently than supplies?
Returns are often sized for lower velocities to control sound at grilles and through filters. Start with a slightly lower target velocity for returns, then check filter and grille pressure drops as part of the total static budget.