How Much Concrete Do I Need? A Simple Estimating Guide
Updated 2026-07-25 · Yardly
Ordering concrete is one of those moments where a small math error turns into a big problem: either you pay for a half-loaded truck you can’t return, or you run short mid-pour and the slab cold-joints. The good news is the math is simple once you know the formula.
The cubic-yard formula
Concrete is sold by the cubic yard. One cubic yard is 27 cubic feet. To estimate a rectangular slab:
(Length × Width × Thickness ÷ 12) ÷ 27 × 1.10 = cubic yards
The ”÷ 12” converts a thickness in inches to feet. The ”× 1.10” is a 10% waste margin for spillage and slight over-excavation. Industry guides such as QUIKRETE’s estimator use the same volume approach before converting to bag counts.
A worked example
A 10 ft × 10 ft slab at 4 inches thick:
- 10 × 10 × (4 ÷ 12) = 33.3 cubic feet
- 33.3 ÷ 27 = 1.23 cubic yards
- 1.23 × 1.10 = 1.36 cubic yards — round up to 1.5 yd³
At about 45 bags of 80-lb mix per cubic yard, that’s roughly 61 bags — which is exactly when most people switch to a ready-mix truck.
Ready-mix vs bags
As a rule of thumb, anything over about one cubic yard is cheaper and far less labor as ready-mix. The Portland Cement Association notes that proper proportioning and placement matter more than brand — so get the volume right first, then focus on a good mix and finishing.
Avoiding a short pour
Always round up, not down. A 10% buffer covers the usual surprises. If your dimensions are irregular, break the shape into rectangles, estimate each, and add them up.
Frequently asked questions
How do I calculate concrete for a rectangular slab?
Multiply length × width × thickness (in feet), then divide by 27 for cubic yards. Add ~10% for waste. Our concrete slab calculator does this instantly.
Is it cheaper to mix my own or order ready-mix?
For jobs over about one cubic yard, ready-mix is usually cheaper and far less labor. Bag mixing makes sense for small pads and repairs.
How long does concrete take to cure?
Concrete reaches most of its strength in 28 days, but you can usually walk on it in 24–48 hours and drive on it after about 7 days, depending on mix and weather.
References: QUIKRETE (concrete estimator), Portland Cement Association (placement guidance). Yardly estimates are for planning only.