Order 5 to 10 percent more than the exact calculation for most materials, and 10 to 20 percent more for compacting base stone and loose fill.
Why you need extra
Spillage, uneven ground, settling, compaction, and measuring error all use up material. A small surplus is cheap insurance against a second delivery or a half finished job.
Typical allowances
Concrete: 5 to 10 percent. Mulch and compost: about 10 percent. Topsoil: 10 percent. Gravel and base stone: 10 to 20 percent for compaction. Fill dirt: 15 to 25 percent for compaction. Sand: about 10 percent.
An example
A 10 by 10 foot slab at 4 inches needs 1.23 cubic yards. A 10 percent allowance brings the order to about 1.36 cubic yards. Ready mix plants usually let you order in quarter or half yard steps.
When to order less
Bagged materials are easy to return if unopened, so a smaller allowance may be fine. Precise projects with solid forms can also use less.
Frequently asked questions
Should I add waste to bags?
A few spare bags are cheap, and you can often return unopened ones.
Is 10 percent always enough?
Not for compacted base stone, which can lose 15 to 25 percent when rolled.
What if I order too much?
Excess soil, gravel, or mulch is easy to use elsewhere. Concrete is the exception.