How much concrete do I need for a slab?
A 10 × 10 ft slab at 4 inches thick needs about 1.2 cubic yards of concrete — or roughly 1.3–1.4 cubic yards once you add a waste allowance. Here's the formula so you can size any slab with confidence.
The formula
Concrete is ordered by volume in cubic yards:
Cubic yards = length × width × thickness ÷ 27 (all in feet)
Thickness is usually in inches, so divide it by 12 first to convert to feet. There are 27 cubic feet in a cubic yard, which is why we divide by 27 at the end.
A worked example
For a 10 × 10 ft slab, 4 inches thick:
- Convert thickness: 4 ÷ 12 = 0.33 ft
- Volume: 10 × 10 × 0.33 = 33.3 cubic feet
- Cubic yards: 33.3 ÷ 27 = 1.23 cubic yards
- Add 10% waste: about 1.36 cubic yards
Size any slab instantly
Our concrete calculator gives you cubic yards, cubic feet, the number of bags, and an estimated cost — just enter your dimensions.
Open the Concrete CalculatorHow thick should the slab be?
| Project | Typical thickness |
|---|---|
| Walkways and patios | 4 in |
| Driveways (passenger cars) | 4–5 in |
| Driveways (heavy vehicles) | 6 in |
| Shed and garage floors | 4–6 in |
Don't forget the base and reinforcement
The concrete is only part of the order. A lasting slab also needs:
- A compacted gravel sub-base, usually about 4 inches, for a stable, well-draining foundation. Size it with our gravel calculator.
- Reinforcement — rebar or welded wire mesh on load-bearing slabs to control cracking. Plan it with the rebar calculator.
- Excavation for the slab and base depth; our excavation calculator estimates the dirt to remove.
Bags or ready-mix?
For small jobs, bagged mix is convenient — but it adds up fast, taking roughly 45 80-lb bags to make a single cubic yard. Once you're approaching a cubic yard or more, ready-mix delivery is usually cheaper and far less labor than mixing by hand.
What will the concrete cost?
Ready-mix runs about $125–$200 per cubic yard delivered, so the concrete for our 10 × 10 ft example is roughly $170–$270 — plus a possible "short-load" fee if you order less than about a yard. Bagged mix costs more per yard in materials and far more in labor. If you hire a pro to form, pour, and finish the slab, a turnkey job typically runs $5–$10+ per square foot.
Always order a little extra
Add 5–10% to your total for spillage, an uneven subgrade, and slight over-excavation. You can't pause a pour to run to the store, so it's much safer to finish with a little left over than to come up short.