How much extra concrete to order
Order about 10% more than the math says. That covers uneven ground, forms that bow, spills and the concrete left in the chute and mixer. Trenches and rough subgrade need more; precise forms and post holes can take less.
Why the formula is never enough
The volume formula assumes a perfect box: flat bottom, vertical sides, exact thickness. Real pours are not boxes. The subgrade has low spots that fill with concrete. Forms bow outward under the weight of wet concrete. Some concrete stays in the mixer, on the shovel, and in the wheelbarrow. A little is pushed over the form during screeding. Each of these is small, but together they typically add several percent to what the shape alone holds.
Running short is worse than ordering extra. If the truck leaves before the forms are full, the rest has to come from bags or a second delivery, and the seam between the two pours, a cold joint, is the weakest place in the slab. A few extra bags or a quarter yard left over costs far less.
What 10% covers
- Normal variation in subgrade level, about a quarter to half an inch over a 4 in slab.
- Formwork that moves a little during the pour.
- Material left in the mixer, wheelbarrow, chute and tools.
- Small over-pours at the edges before the screed takes it off.
- Yield variation in bagged mix, which the manufacturer rates as approximate.
When to order more
| Situation | Consider | Why |
|---|---|---|
| Footing trenches dug by hand or machine | 15 to 20% | Walls cave, bottoms are uneven, trench is wider than planned |
| Rough or unlevelled subgrade | 15% | Low spots take extra concrete before the slab reaches full depth |
| Long, narrow pours such as sidewalks and curbs | 10 to 15% | Edge losses are a larger share of a thin pour |
| Pumped concrete | Ask the pump operator | Concrete is left in the pump line and hopper at the end |
| Very small bagged pours | One extra bag | Rounding to a whole bag already adds margin on tiny jobs |
When you can order less
- Post holes with a post in them. The post displaces concrete. A 4 × 4 post in a 10 in hole takes up about 15% of the hole, which roughly cancels the 10% allowance. Setting the waste to 0 for posts gives a closer count.
- Precise, well-braced forms on a compacted, graded base, such as a formed pier or a slab on a prepared gravel bed, may need only 5%.
- Tube forms hold their shape and have a known diameter, so 5% is usually enough.
How the allowance is applied
Waste is added to the volume before anything else is worked out:
Volume to order = exact volume × (1 + waste %)
Then the ready-mix figure is rounded up to the next quarter yard, because that is how plants sell it, and the bag figure is divided by the yield per bag and rounded up to a whole bag. Both roundings add a little more margin on top of the waste allowance. The calculators show the exact volume and the ordering figure separately so you can see each step.
Example
A 12 × 12 ft slab at 4 in is exactly 48 cu ft, or 1.78 cu yd. With 10% waste that is 52.8 cu ft, or 1.96 cu yd, so you order 2 cu yd. The rounding added only 0.04 cu yd here. On a 10 × 10 ft slab the exact figure is 1.23 cu yd, 1.36 with waste, and the order is 1.5 cu yd, so rounding added 0.14 cu yd, about 11% more. The smaller the pour, the more quarter-yard rounding matters, which is another reason bags suit small jobs.
What to do with the extra
Have a use ready: a splash block, a step, a post base, or a small pad for a trash can. Set a form for it before the truck arrives. Leftover ready-mix cannot be returned, and most plants charge to dispose of it.