Order 5% to 10% more concrete than the calculated volume. The extra covers uneven ground, forms that bow, spillage and what is left in the mixer or the pump.
How much to add
| Situation | Waste allowance |
|---|---|
| Flat, well-formed slab on a compacted base | 5% |
| Typical slab or sidewalk | 10% |
| Earth-formed footing or trench | 10% to 15% |
| Curved or irregular shapes | 10% to 15% |
| Pumped concrete | Ask the pump operator; some is left in the line |
What the allowance looks like in yards
| Calculated volume | +5% | +10% | +15% |
|---|---|---|---|
| 1 yd³ | 1.05 | 1.10 | 1.15 |
| 2 yd³ | 2.10 | 2.20 | 2.30 |
| 3 yd³ | 3.15 | 3.30 | 3.45 |
| 5 yd³ | 5.25 | 5.50 | 5.75 |
| 8 yd³ | 8.40 | 8.80 | 9.20 |
Where the extra goes
- Uneven subgrade: a base that is half an inch low across a 4 inch slab uses 12.5% more concrete.
- Form movement: forms bow outward under the weight of wet concrete.
- Spillage and handling: wheelbarrows, chutes and tools all carry some away.
- Rounding: ready-mix is sold in quarter yards and bags are whole, so you round up anyway.
Every calculator on this site has a waste slider set to 10%. Start with the concrete calculator.
Frequently asked questions
How much extra concrete should I order?
Add 5% to 10% for most jobs. Use 10% or more on rough ground, in earth-formed trenches or for curved shapes.
What happens if I run short?
The first concrete starts to set before the rest arrives, which leaves a weak line called a cold joint. A second small delivery also costs more than a few extra bags or a slightly larger first order.
What do I do with leftover concrete?
Have a small form ready, such as a stepping stone or a pad for a trash can, so leftover concrete is not wasted.