Concrete
How Much Extra Concrete Should You Order?
Why every concrete pour needs 5-15% extra, where the waste actually goes, how to pick your percentage, and what to do with surplus concrete when the pour ends.
The waste factor is the least intuitive part of concrete estimating: the math says 3.0 yards, so why order 3.5? Because four separate effects each quietly consume concrete on every real pour, and they only ever work in one direction - against you.
Where the extra concrete actually goes
1. Subgrade irregularity (the big one: 3–8%). A base that's "pretty flat" still has low spots, and every one fills with concrete before the slab reaches design thickness anywhere. Half an inch of average low spot on a 4-inch slab is a 12% overrun by itself. This is why careful base preparation directly saves concrete money.
2. Form deflection (1–3%). Wet concrete weighs 150 lb/ft³ and pushes forms outward, bowing 2× lumber between stakes. Your 10.00 ft formed width becomes 10.1 ft at mid-span.
3. Residue in equipment (1–2%). Concrete stays in the chute, the pump line (a 100-ft pump line holds 1/4 yard or more), wheelbarrows, and buckets.
4. Spillage and placement loss (1–2%). Chute repositioning, over-shoots at edges, and screeding surplus off the top all leak material.
Choosing your percentage
| Situation | Waste factor |
|---|---|
| Simple rectangle, compacted screeded base, chute placement | 5% |
| Typical residential slab, decent base | 10% |
| Irregular shape or curves | 10–12% |
| Rough or sloped subgrade | 12–15% |
| Pump placement (add line volume too) | +2–3% over the above |
| Post holes in variable soil | 15% (holes never auger clean) |
The calculator applies your chosen waste factor and rounds to the ordering increment automatically.
Open the Concrete CalculatorThe asymmetry that justifies rounding up
Compare the two failure modes on a 4-yard patio at $170/yard:
| Ordered 0.5 yd too much | Ordered 0.5 yd too little | |
|---|---|---|
| Material cost | $85 wasted | $85 for the balance |
| Fees | Possible small return fee | Second delivery $75–150 + short-load fee $50–150 |
| Schedule | None | 1–2 hr wait while the slab sets |
| Quality | None | Cold joint across the slab - permanent |
| Total | ~$85–120 | ~$250–400 + a defect |
Ordering extra is cheap insurance with a bounded cost. Running short has an unbounded cost that includes the quality of the finished work.
Plan the surplus before the truck arrives
The best crews treat leftover concrete as an asset with a destination decided in advance: two or three 8–12 in form tubes staked plumb for future fence or deck footings, stepping-stone or splash-block molds, a small equipment pad, or curb repairs. Also prepare a washout area - a lined pit or washout bag - because cement slurry is caustic (pH ~12) and must not run to soil, storm drains, or the lawn.
When to call a professional
If your pour needs more than one truck, involves a pump, or the site has significant slope, a concrete contractor's experience coordinating truck spacing and placement rate protects the pour more than any waste factor can. Two trucks arriving too far apart creates exactly the cold joint the extra half yard was meant to prevent.
Frequently Asked Questions
What waste percentage should I add to a concrete order?
5% for simple forms on a compacted, screeded base; 10% for typical residential slabs; 10-15% for irregular shapes, rough subgrades, or pump placement. Never order the bare geometric volume.
What happens if I run out of concrete mid-pour?
The pour stops and a cold joint forms where fresh concrete later meets partially set concrete - a permanent plane of weakness, usually visible. You'll also pay a second delivery fee and possibly a short-load fee for the balance.
What should I do with leftover concrete?
Have molds ready: form tubes for future post footings, stepping-stone molds, or a small pad. Drivers can also take a small surplus back - washout on your site needs a contained area, as cement slurry is caustic and shouldn't reach soil drains.
Do suppliers charge for returned concrete?
Many charge a return/disposal fee per yard. It's still far cheaper than running short. Ask when ordering so the fee doesn't surprise you.
Is the waste factor the same for bagged concrete?
Buy 5-10% extra bags. Unopened bags are returnable at most retailers, which makes over-buying bags essentially free insurance - the opposite of ready-mix economics.