BuildCalcs The core calculation
For a rectangular pour, multiply length by width by depth, with every dimension expressed in metres. A 4 m × 3 m slab at 100 mm deep uses a depth of 0.10 m: 4 × 3 × 0.10 = 1.20 m³.
That is the geometric volume. It does not automatically account for uneven excavation, spillage or a base that is slightly deeper than measured. BuildCalcs therefore keeps the measured quantity and any allowance conceptually separate.
Concrete volume by area and depth
| Area | 75 mm | 100 mm | 125 mm | 150 mm |
|---|---|---|---|---|
| 5 m² | 0.38 m³ | 0.50 m³ | 0.62 m³ | 0.75 m³ |
| 10 m² | 0.75 m³ | 1.00 m³ | 1.25 m³ | 1.50 m³ |
| 15 m² | 1.12 m³ | 1.50 m³ | 1.88 m³ | 2.25 m³ |
| 20 m² | 1.50 m³ | 2.00 m³ | 2.50 m³ | 3.00 m³ |
| 25 m² | 1.88 m³ | 2.50 m³ | 3.12 m³ | 3.75 m³ |
| 30 m² | 2.25 m³ | 3.00 m³ | 3.75 m³ | 4.50 m³ |
These values are calculated directly from area × depth. They are reference quantities, not supplier pack or delivery sizes.
Worked project examples
Small shed base
A 3.0 m × 2.4 m base at 100 mm: 3.0 × 2.4 × 0.10 = 0.72 m³. A 10% planning allowance would make 0.792 m³, normally shown as 0.79 m³ before considering how a supplier sells the material.
Garden path
A 9 m path, 0.9 m wide and 75 mm deep: 9 × 0.9 × 0.075 = 0.6075 m³, or 0.61 m³ to two decimals.
Garage-sized slab example
A 6 m × 3 m area at 125 mm: 6 × 3 × 0.125 = 2.25 m³.
Why measured depth matters
Depth has a direct linear effect on volume. Increasing a 100 mm slab to 125 mm increases the concrete volume by 25%. Measure at several points if the excavation is irregular rather than assuming the neat design depth is what exists on the ground.
Allowance is not the same as the calculation
An allowance is a planning decision, not part of the volume formula. Keeping it visible lets you compare the exact geometric quantity with the quantity you may choose to order. Avoid adding an allowance twice—for example once in your dimensions and again as a percentage.
Common errors
- Using millimetres as though they were metres: 100 mm is 0.10 m.
- Calculating area but forgetting depth.
- Rounding every dimension before multiplying.
- Treating a theoretical volume as a supplier's guaranteed delivery or yield.
Enter your actual measurements rather than relying on a generic example.
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