Foundation Calculator
The Foundation Pour Sets the Tone for Everything Above It
A foundation slab or mat is typically the largest single concrete pour on a residential job, and it's also the least forgiving to get wrong — running short mid-pour risks a cold joint at exactly the point that carries the most load. Working out the volume precisely before the trucks arrive is what keeps the pour continuous.
The Formula
Length and width are entered in feet and depth in inches (or meters and centimeters for metric). The calculator converts depth to feet, multiplies through to get cubic feet, divides by 27 to reach cubic yards — the unit ready-mix concrete is ordered in — and applies a waste allowance, 10% by default, to cover minor over-excavation, spillage, and the fact that a short-load surcharge from the plant often costs more than a modest overage.
Where This Calculation Matters
- Slab foundations — a monolithic slab foundation combines the footing and floor slab into a single pour, sized directly by this length-width-depth formula.
- Mat and raft foundations — larger structures on poor soil sometimes use a thick mat foundation across the full building footprint, where even a half-inch of extra depth adds meaningfully to the total volume.
- Ordering minimums — most ready-mix plants set a minimum load size and charge a short-load fee below it, so knowing the true volume in advance helps decide whether to combine a foundation pour with another nearby job.
Worked Example
A 30 ft × 30 ft foundation slab (900 sq ft) at an 8 in depth, with the default 10% waste allowance:
| Step | Calculation | Result |
|---|---|---|
| Volume | 30 × 30 × (8 ÷ 12) | 600 cu ft |
| In cubic yards | 600 ÷ 27 | 22.22 cu yd |
| With 10% waste | 22.22 × 1.10 | 24.44 cu yd |
How to Use This Calculator
- Select Imperial (ft / in) or Metric (m / cm) units.
- Enter the foundation's Length and Width.
- Enter the pour Depth.
- Optionally adjust the Waste % — defaults to 10%.
- Select Calculate to see the total concrete volume needed.
Related Calculations
Reinforcing the pour before it goes in? The Rebar Calculator sizes the steel grid, and the Footing Calculator handles the strip footings that often surround a foundation slab.