It depends on the material, and for two of the four materials on this site nobody authoritative has published a number. Here is what we found for each, and what the calculators do with it.
| Material | Published allowance | Starts at |
|---|---|---|
| Concrete | 4% to 10% (NRMCA) | 10% |
| Gravel and crushed stone | 10% when stockpiled (one highway manual) | 10% |
| Mulch | none found | 0% |
| Topsoil | none found | 0% |
Every starting value can be changed in the waste field, and setting it to 0 shows the bare quantity.
Concrete: 4% to 10%
NRMCA, the association of ready-mix producers, says the quantity ordered should be 4% to 10% more than the estimate from the plan dimensions, to cover waste, over-excavation and other contingencies. The same guidance notes that hardened concrete in place may be about 2% less in volume than when it was fresh, and that a 4 in slab poured 1/8 in too thick uses 3% more.
Concrete Network gives no percentage, but makes the same point: never order the exact amount, and for a slab add a quarter of an inch to the thickness when you work out the volume.
Both NRMCA documents come from one association, so this counts as one source. The concrete calculator starts at the top of the range, 10%.
Gravel: one figure, from road building
The Washington State Department of Transportation design manual says that if material is to be stockpiled, quantities should be increased by 10% to allow for waste. That is the only published allowance we found, and it was written for highway work, not for a driveway. The gravel calculator starts at 10% and says so.
Two other things move a gravel order more than waste does:
- Compaction. In the same manual, crushed surfacing weighs 1.43 to 1.48 tons per cubic yard loose in the truck and 1.85 tons compacted in place. The same space takes roughly a quarter more material once it is compacted. If your layer will be compacted, enter the finished depth and pick the compacted material type.
- Water. The manual’s weights are dry weights, to be corrected for water content. A California table of material weights says to add 20% for wet gravel. Gravel is sold by weight, so a wet load is less stone for the same tons.
Mulch and topsoil: no authoritative number
We looked for a published allowance for mulch and for topsoil and did not find one from an extension service, an agency or a manufacturer. The figures of 10% to 20% that many websites repeat appear on other calculators and supplier blogs, not in a source we could check. So the mulch calculator and the topsoil calculator start at 0%.
Two facts may help you choose your own figure:
- Scotts says its 0.75 cu ft bag of topsoil covers 4 sq ft at 2 in. By geometry that area and depth is 0.67 cu ft, so the maker’s own coverage leaves about 11% spare.
- The California table says loose, dry earth weighs 30% more per cubic yard once compacted. Soil that will be firmed down takes more than its loose volume suggests.
How to use the waste field
Enter the percentage you have decided on. The breakdown shows the quantity before and after it is added, so you can see what the allowance costs you. If you would rather round up by hand, set the field to 0.