Introduction
A strip footing — also called a continuous footing — is a long, narrow concrete base that carries a wall’s load and spreads it across the soil. Get the width, depth, and concrete volume right and the foundation is quiet for decades; get them wrong and you face settlement cracks and expensive underpinning. Footing design is applied arithmetic: size the width to the load and the soil, set the depth below the frost line, and compute the concrete volume from the trench dimensions.
The Concrete Footing Calculator handles the core calculation: enter the footing’s run length, width, and depth plus a waste allowance, and it returns the required concrete volume in cubic meters and cubic yards, plus the bag count. This guide explains everything behind that number — strip footing sizing, trench dimensions, frost depth, rebar placement, and a full worked example with metric and imperial cross-checks.
What a Strip Footing Is and Where It’s Used
A strip footing runs continuously under a load-bearing wall — exterior foundation walls, interior bearing walls, retaining walls. The wall concentrates the building’s weight into a thin line; the footing widens that line so the pressure on the soil stays within what the ground can carry. Think of it as a snowshoe for the wall: same load, larger contact area, lower pressure.
Strip footings serve homeowners and DIYers (garden walls, garage foundations, small extensions), contractors pricing residential foundations, engineers sizing widths from loads and soil capacity, and ready-mix suppliers quoting the pour. They differ from isolated pad footings (square pads under columns) and full slabs. On sloped sites the footing is built in steps — level sections dropping downhill — so every segment bears on level ground.
Key Terms and Variables
Run length
The total linear length of the footing in meters or feet. For a rectangular footprint, add all four walls, measuring each leg to the outside face of the corners so each corner counts once.
Footing width
The horizontal width of the concrete strip — the most structurally significant dimension. Wider footing = lower soil pressure. Residential footings commonly run 40–60 cm (16–24 in), but the correct value comes from the load calculation, not habit.
Footing depth (thickness)
The vertical thickness of the concrete itself — typically 20–30 cm (8–12 in). This is not the trench depth: the trench must reach the footing’s bearing level below the local frost line. Confusing the two is the most expensive data-entry error in footing estimating; the derivation is in the formula-explained guide.
Soil bearing capacity
The pressure the ground can safely carry: soft clay ~75–100 kPa (1,500–2,000 psf), firm clay or sandy gravel ~150 kPa (3,000 psf), dense gravel or rock 200–400+ kPa. The calculator doesn’t ask for this — you use it before opening the calculator, when sizing the width.
Waste %
The allowance added for trench irregularities, over-excavation, and spillage — 10% for straight trenches in firm ground, 12–15% for hand-dug or irregular ones. Full quantity takeoff is in the estimating guide.
| Typical residential strip footing | Metric | Imperial |
|---|---|---|
| Garden / non-load-bearing wall | 30 cm × 20 cm | 12 in × 8 in |
| Single-story house, firm soil | 45 cm × 25 cm | 18 in × 10 in |
| Two-story house, firm soil | 60 cm × 30 cm | 24 in × 12 in |
These are starting points, not designs — the load calculation sets the width, frost depth sets the trench depth. Frost-line rules are in the code-compliance guide.
The Formulas Involved
Footing math runs in two stages: size the footing, then compute the concrete.
Formula 1: Required footing width
Width = Wall load per unit length ÷ Soil bearing capacity. Example: 28 kN/m on 120 kPa soil → 28 ÷ 120 = 0.233 m (23.3 cm). Imperial cross-check: 1,919 lb/ft ÷ 2,506 psf = 0.766 ft = 9.2 in — identical. Round up to standard widths and local minimums (commonly 45 cm / 18 in), so this footing is built 45 cm wide.
Formula 2: Concrete volume
Volume = Run length × Width × Depth × (1 + Waste%). Use the footing dimensions — the concrete — not the trench dimensions, unless pouring directly against unformed trench walls.
Formula 3: Rebar quantity
Steel = 2 bars × Run length × (1 + lap allowance). Residential footings typically carry two continuous 12 mm (#4) bars on chairs, lapped 40 bar diameters (0.48 m / 19 in) at joints, with ~5% lap allowance on straight runs. Sizes and cover rules are in the material guide.
Worked Example: A 40 m House Footing
A strip footing with 40 m of run, 0.6 m wide, 0.3 m deep, 10% waste:
Step 1 — Geometric volume. 40 × 0.6 × 0.3 = 7.2 m³.
Step 2 — Waste. 7.2 × 1.10 = 7.92 m³.
Step 3 — Cubic yards. 7.92 ÷ 0.7645549 = 10.36 yd³.
Step 4 — Bags. 80-lb bags at 0.017 m³: 7.92 ÷ 0.017 = 465.9 → 466 bags.
Imperial cross-check — 131.23 ft × 1.97 ft × 0.98 ft = 254.3 ft³ = 9.417 yd³; × 1.10 = 10.36 yd³, matching exactly.
Takeaway: at 10.36 yd³ this is ready-mix territory (a truck carries ~8–10 yd³), not 466 bags. The 0.9 m-deep trench moves 40 × 0.6 × 0.9 = 21.6 m³ (28.25 yd³) of soil — over twice the concrete volume, which is why excavation dominates the budget in the cost guide.
Common Variations
- Stepped footings on slopes: level steps dropping ≤ the footing’s thickness per step (typically 0.3 m / 12 in), each ≥ 0.6 m (2 ft) long. Estimate each step as its own rectangle and add.
- Monolithic vs separate pours: garages and sheds often pour footing and slab together with a thickened edge; houses pour the footing first so it can be inspected before the wall goes on.
- Retaining-wall footings: wider than the wall to resist overturning — break the cross-section into rectangles.
Mistakes That Distort Results
- Sizing width by habit, not load. Run width = load ÷ soil capacity; “we always pour 24 inches” ignores both.
- Entering trench depth as concrete depth. The calculator’s depth is the concrete thickness — 0.3 m, not the 0.9 m trench.
- Rebar on the dirt. Bars need chairs every 1.0–1.5 m (3–5 ft) for 75 mm (3 in) cover, or they corrode.
- Zero waste allowance. Trenches are never as neat as drawings; 10% is the minimum.
- Unit mixing. Depth in inches with length in feet, unconverted, skews results 12×.
- Pouring before inspection. Most jurisdictions inspect the open trench and steel first — schedule it, don’t skip it.
Check each of these against the trench before the truck arrives — most are free to fix now and expensive later.
Related Guides
- Concrete Footing Cost Guide — what strip footings cost per linear metre and where the money goes.
- Concrete Footing Formula Explained — the sizing and volume math, derived step by step.
- Concrete Footing Material Guide — concrete mixes, rebar sizes, chairs, and form materials.
- Concrete Footing Pro Tips — pour-day, ordering, and curing advice from the trade.
- Concrete Footing Common Mistakes — the errors that waste concrete and weaken footings.
- Concrete Footing Checklist — everything to verify before the concrete arrives.
Frequently Asked Questions
How wide should a strip footing be?
Width = wall load ÷ soil bearing capacity, rounded up to standard sizes and local minimums (commonly 45 cm / 18 in for houses). A 28 kN/m wall on 120 kPa soil needs 0.233 m (9.2 in) by the math but gets built 45 cm wide.
How deep should a footing trench be?
The footing’s base must sit below the local frost line — commonly 0.6–1.2 m (2–4 ft) — plus the footing’s own thickness. Trench depth and concrete depth are different numbers.
How much concrete do I need for footings?
Volume = run length × width × depth × (1 + waste%). A 40 m × 0.6 m × 0.3 m footing with 10% waste needs 7.92 m³ (10.36 yd³).
Do strip footings need rebar?
Typically two continuous 12 mm (#4) bars on chairs with 75 mm (3 in) cover. Unreinforced footings suit only very light, non-structural work.
Can I pour the footing and slab together?
Yes for light structures (monolithic pour with thickened edge). Houses usually pour the footing separately so it can be inspected first.
Get Your Number
Size the width from load and soil, set the trench below the frost line, compute the concrete from the geometry with a realistic waste allowance. The Concrete Footing Calculator does the arithmetic — your job is the tape measure and the soil assumption.
Disclaimer: Calculator results are planning estimates based on the dimensions you enter. Actual footing requirements vary with soil conditions, local frost depth, trench accuracy, and mix design. Verify volumes with your supplier and consult a licensed professional or your local building authority for structural or code-related decisions.
