How to Calculate Concrete With Waste
Why the exact volume and the amount you order are two different numbers
Why exact volume isn't the order quantity
A geometric volume calculation assumes perfectly smooth, level, exactly-sized formwork and zero material loss. Real job sites don't work that way: subgrades are uneven, forms bow slightly, wheelbarrows spill, and a pump line holds some concrete that never reaches the form. A waste allowance closes that gap.
Typical allowance ranges
There is no code or standard waste percentage. Common estimating conventions are about 5% for a well-formed slab on prepared, level ground and 10% for footings, uneven ground or a first pour, with more for irregular hand-dug trenches. The Concrete Calculator starts at 10%; change it to suit your site.
- 5%: formed slab, prepared/level subgrade
- 10%: standard footing or driveway, average site conditions
- 10-15%: hand-dug or highly irregular excavation
How to apply it correctly
Calculate the exact volume first, from your measurements. Then multiply by (1 + waste percentage) to get the recommended order volume. Keep both numbers visible — the exact figure tells you what the shape actually contains, and the recommended figure is what you tell the supplier. Never discard the exact figure; you may need it later for a cost breakdown or a change order.
Then round up to the increment your supplier sells. Increments and minimum loads vary between suppliers, so ask before you order.
Allowance examples
The exact volumes come from the slab guide and the footings guide. Each recommended figure is the exact volume × (1 + allowance), before rounding to your supplier's increment.
| Pour | Exact | +5% | +10% |
|---|---|---|---|
| 12 × 10 ft slab, 4 in | 1.48 yd³ | 1.56 yd³ | 1.63 yd³ |
| 10 × 10 ft slab, 6 in | 1.85 yd³ | 1.94 yd³ | 2.04 yd³ |
| 40 ft footing, 24 × 12 in | 2.96 yd³ | 3.11 yd³ | 3.26 yd³ |
Worked Example
A footing with an exact volume of 2.2 yd³ and a 10% waste allowance.
Recommended order = 2.2 × 1.10 = 2.42 yd³
If your supplier sells in quarter yards, round up to 2.5 yd³ — check its increment first
FAQ
- Does a bigger project need a bigger waste percentage?
- Not necessarily — waste is driven more by site conditions and formwork quality than by the size of the pour. A large, well-formed slab can still use 5%; a small, irregular footing might need 10%.
- Should the waste percentage ever be shown as already included in a single total?
- No — keeping the exact and waste-adjusted figures separate lets you see exactly how much buffer is built in and adjust it for your own site, rather than trusting a hidden number you can't verify or change.
- How much extra concrete should I order?
- There is no fixed rule. Many estimates use about 5% for a well-formed slab and 10% for footings or uneven ground, and the Concrete Calculator starts at 10%. Round up to your supplier's ordering increment afterwards.