Concrete Footing Calculator
Compute concrete volume or bags required for strip, rectangular, or circular footings with unit-aware inputs and step-by-step math.
Practical Guide
Concrete Footing Calculator: Fast, Code-aware Takeoffs
Use this calculator to size and estimate concrete for strip and isolated footings. Enter geometry, choose units, add waste and reinforcement assumptions, and export numbers that align with field reality.
Quick Start
- 1 Select footing type (strip/continuous, isolated/pad, stepped). Choose units (ft–in or metric).
- 2 Enter geometry for each run or pad: length \(L\), width \(B\), thickness/depth \(t\), and quantity \(n\). For stepped/tapered footings, add each layer.
- 3 Add waste/overbreak (typically 5–12%) to cover trench widening, spillage, and pump priming.
- 4 (Optional) Include rebar layout assumptions if your calculator reports steel length/weight.
- 5 Review the result row for volumes in ft³/m³ and concrete yards/bags, then round up to truck load sizes.
Tip: If you’re replacing soil with a wider trench than drawn, add 2–4 in (50–100 mm) per side into the calculator or increase waste to 10–12%.
Watch-out: Keep units consistent. Convert thickness to feet/meters before computing yards/meters of concrete. The calculator handles this if inputs are set correctly.
Variables & Symbols
- \(L\) Run length (ft or m)
- \(B\) Footing width (ft or m)
- \(t\) Footing thickness/depth (ft or m)
- \(n\) Number of identical runs or pads
- \(V\) Volume
- \(w\) Waste fraction (e.g., 0.10 = 10%)
Choosing Your Method
Method A — Direct Geometry (Most Common)
Enter length, width, and thickness for each footing run or pad. Add quantities where repeats exist.
- Fast and matches how footings are drawn.
- Easy to group by repeat sizes for subdivisions.
- Works with stepped/tiered sections by summing layers.
- Needs careful unit control on thickness.
- Doesn’t size footings for capacity (only estimates volume).
Method B — Schedule Driven (Takeoff from Plans)
Use a footing schedule (e.g., F1: 24×12 in, F2: 36×18 in) with counts per type. The calculator totals the lot.
- Reduces data entry when plan notes already list sizes.
- Enables quick “what-ifs” (change a type once, update all).
- Easy to miss isolated pads hidden on structural sheets.
- Still requires depth confirmation at frost or expansive soils.
Need load-based sizing (not just volume)? That requires allowable bearing pressure, column/wall loads, and code checks. This page focuses on quantity for estimating and ordering.
What Moves the Number the Most
Small width changes (e.g., 18→24 in) have an outsized impact on total yards across long runs.
Depth set by frost line or engineering adds directly to volume. Double thickness → double volume.
Long perimeter walls and many identical pads amplify small geometry tweaks.
Trench slough, irregular excavation, and pump hose priming are real. Typical: 5–12%.
Stepping with grade or tapering to a pier reduces (or increases) volume versus a full-depth prism.
Poor bearing or expansive clays can require wider/thicker footings—or a different system entirely.
Worked Examples
Example 1 — Strip Footing (US/Imperial)
- Type: Continuous (strip) footing under a 40 ft wall
- Width: \(B = 24\text{ in} = 2.0\text{ ft}\)
- Thickness: \(t = 12\text{ in} = 1.0\text{ ft}\)
- Length: \(L = 40\text{ ft}\)
- Waste: \(w = 10\%\)
Example 2 — Isolated Pad Footings (Metric)
- Pads: \(n=4\) identical 1.2 m × 1.2 m × 0.45 m
- Per-pad volume: \(V_1 = 1.2 \times 1.2 \times 0.45 = \mathbf{0.648\ \text{m}^3}\)
- Total: \(V = n V_1 = 4 \times 0.648 = \mathbf{2.592\ \text{m}^3}\)
- Waste: \(w = 7\%\) → \(V_{tot} = 2.592\times1.07 = \mathbf{2.77\ \text{m}^3}\)
Optional Shape — Tapered/Stepped (Frustum)
For a linear taper from bottom area \(A_1\) to top area \(A_2\) with vertical height \(h\):
\[ V_{\text{frustum}} = \frac{h}{3}\,\big(A_1 + A_2 + \sqrt{A_1 A_2}\big) \]
Use one frustum per step and sum with any prismatic sections.
Common Footing Types & Variations
These are typical residential/light-commercial footing patterns. Your structural drawings are the governing document.
| Footing Type | Where Used | Volume Basis | Notes that Affect Takeoff |
|---|---|---|---|
| Strip / Continuous | Perimeter & interior load-bearing walls | Prism \(L\cdot B\cdot t\) | Depth often set by frost; allow for trench widening. |
| Isolated / Pad | Columns, porch posts, deck piers | Prism per pad × quantity | Check anchor bolt/column base projection and cover. |
| Combined / Strap | Two columns sharing one footing | Sum of pad prisms or frustum + strap beam | Strap beam adds volume; watch beam depth. |
| Stepped with Grade | Sloping sites | Sum of steps (prisms) or frustums | Include end returns and keyways if detailed. |
| Thickened Slab Edge | Monolithic slab-on-grade edges | Slab volume + edge thickening | Don’t double-count slab area when adding thickened edge. |
- Confirm local frost depth and minimum cover over rebar.
- Include keyways/dowels only if your calculator models them.
- Isolate pads from plumbing trenches to avoid undermining.
- Round up truck orders—short loads are costly.
Buying, Logistics & Practicalities
Selection Criteria
- Concrete strength: Typical 3,000–4,000 psi (20–28 MPa) unless drawings specify otherwise.
- Slump & workability: Tight trenches may need a higher slump or chute/pump access.
- Rebar: Common #4–#5 (12–16 mm) with minimum cover; tie lap splices per plan.
- Aggregate size: Ensure pump line and rebar spacing can accommodate stone size.
Field Logistics
- Stage trucks to reduce cold joints; confirm site approach and turning radius.
- Compact subgrade and place a thin granular pad if specified; wet muddy bottoms kill bearing.
- Use form oil and check forms for square and elevation before pour.
Sanity Checks & Limits
- This calculator estimates quantity—not structural capacity.
- If soil bearing is low or unknown, engage a geotech/engineer.
- When in doubt, separate pours for pads vs. continuous runs to manage cold joints.
Codes and practices vary by region. Always follow your structural drawings and local building department requirements.