DIY Calculators OnlineHow the volume is calculated
Multiply length × width × thickness. Thickness is in inches, so divide it by 12. A cubic yard is 27 cubic feet. Each round footing is a cylinder: π × radius² × depth.
yd³ = (L × W × T ÷ 12 + footings) × (1 + extra) ÷ 27
Premix yield: an 80 lb bag makes about 0.60 ft³, a 60 lb bag about 0.45 ft³. Above a cubic yard or so, ready-mix delivery is usually cheaper and much less work than mixing 45+ bags.
Recommended thickness
| Project | Thickness | Notes |
|---|---|---|
| Walkway, patio | 4" | Foot traffic, furniture |
| Shed or hot tub base | 4–6" | Thicken edges under heavy loads |
| Car driveway | 5–6" | Rebar or wire mesh |
| RV or truck pad | 6–8" | Check local code |
The rebar grid
Bars run both ways at the spacing you choose, kept 3" in from the edges. The stick count adds 10% for overlaps where bars join. Set the grid on chairs or bricks so it ends up in the middle of the slab, not lying on the ground.
Questions
How many bags of concrete for a 10×10 slab?
At 4" thick with 10% extra: 1.36 cubic yards, or 62 bags of 80 lb.
Why order extra?
Uneven ground and spillage use up 5–10%. Running short mid-pour leaves a weak joint where fresh concrete meets set concrete.
Do I need a gravel base?
Usually 4" of compacted gravel under the slab. The paver calculator sizes gravel by area and depth.