Foundation Calculator
Estimate the concrete volume for a strip foundation and any slab or pad footings in the same run.
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Formula and method
Strip volume is the foundation length times its width and height. Each pad or slab adds length times width times thickness. All volumes are summed and the waste allowance is applied to the total.
Strip footing and slab are two volumes
The calculator adds two things: the strip, which is length × width × depth of the section you designed, and the slab, if there is one, which is length × width × thickness × count. The total is the concrete to order, not the footprint of the building.
The section comes from the design, not from the house's floor area. A two-storey blockwork house typically sits on a 400 × 600 mm strip at 1.2 m depth, a timber garden room on 300 × 400 mm. If there is no design, the section has to be calculated for the soil, not guessed from a table.
Trench, blinding and the concrete nobody counts
The trench is usually wider than the footing by the working space, and the concrete fills the trench shape, not a neat rectangle. On clay with a slightly battered trench, 5 % extra is a reasonable allowance; on a clean, boarded trench it is closer to 2 %.
Blinding under the footing is a separate thin layer, typically 50 mm of weak concrete, and it is often forgotten in the order. On a 30 m strip at 400 mm wide it is another 0.6 m³.
Checking before the first truck
Verify the length against the setting-out, not the drawing: a 10 × 8 m building has 36 m of perimeter, and a 400 mm strip is 0.4 m × 0.6 m × 36 m = 8.6 m³ before any internal walls. Internal load-bearing walls add their own strip and are easy to double count or miss entirely.
Order in one delivery per pour and have the vibrator, the level and the spare hands ready. A strip foundation that goes off mid-pour has a cold joint exactly where the load path is continuous.
How to measure the inputs
- Measure the total centre-line length of the strip foundation.
- Measure the trench width and the depth you will actually pour.
- Add pad footings separately with their own length, width and thickness.
Worked example
Foundation length 19.69 ft; Foundation width 11.81 in; Foundation height 1.64 ft; Waste 8 %; Slab length 6.56 ft; Slab width 3.28 ft; Slab thickness 7.87 in; Slab count 1 → Total concrete volume: 1.836 yd³
Reference data
| Section, mm | Concrete, m³/m | Rebar, kg/m |
|---|---|---|
| 300 × 300 | 0.09 | 4 |
| 400 × 300 | 0.12 | 5 |
| 500 × 400 | 0.2 | 8 |
| 600 × 400 | 0.24 | 10 |
Rebar assumes four Ø12 bars plus Ø6 stirrups every 200 mm — about 5 kg per metre.
Practical tips
- Excavate and pour in sections where possible to keep the trench dry.
- Have the trench inspected before pouring.
- Order concrete slightly early — the trench walls soften if left open.
Sources
- СП 63.13330.2018 — Бетонные и железобетонные конструкции
Checked and updated October 22, 2026