This page lists every formula, constant and rounding rule the concrete calculator uses, so you can check any result by hand.
Volume formulas
All measurements are converted to feet first. Results are in cubic feet, and dividing by 27 gives cubic yards.
| Shape | Formula |
|---|---|
| Slab | length × width × thickness |
| Slab from square footage | area × thickness |
| L-shaped slab | (overall length × overall width − cut-out length × cut-out width) × thickness |
| Round slab | π × (diameter ÷ 2)² × thickness |
| Footing | length × width × depth |
| Wall | length × height × thickness |
| Post hole or round column | π × (diameter ÷ 2)² × depth |
| Steps | width × rise × run × n × (n + 1) ÷ 2, plus width × landing depth × rise × n |
| Curb and gutter | length × (curb depth × (curb height + flag thickness) + gutter width × flag thickness) |
Unit conversions
| Unit | Feet |
|---|---|
| 1 inch | 0.0833 |
| 1 yard | 3 |
| 1 meter | 3.2808 |
| 1 centimeter | 0.0328 |
One cubic foot is 0.0283 cubic meters. One square yard is 9 square feet, and one square meter is 10.764 square feet.
Bag yields
Yields are taken from the QUIKRETE Concrete Mix data sheet: about 0.30 cubic feet for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb. The manufacturer describes these as approximate.
Rounding rules
- Bag counts round up to the next whole bag.
- The ready-mix order rounds up to the next quarter of a cubic yard.
- Trucks are counted at 10 cubic yards each. Truck sizes vary by supplier.
- The waste allowance is added before any rounding.
Weight
Weight uses 150 lb per cubic foot, a standard figure for normal-weight concrete. That is 4,050 lb per cubic yard. Lightweight and heavyweight mixes differ.
Reinforcement estimates
- Rebar is #4 bar in a grid running both ways, starting at one edge. The length does not include lap splices, so add about 10%.
- Bars are counted in 20 ft lengths. #4 bar weighs 0.668 lb per foot.
- Wire mesh is counted in 5 ft × 10 ft sheets with 10% added for overlap.
- Rebar and mesh are estimated for slabs only. Footings and walls need a design from your local code or an engineer.
Prices
The prices that appear when the page loads are placeholders. Bag and ready-mix prices change by store, region and season, so enter your own to get a real cost.
Limits
The calculator estimates quantity. It does not check whether a slab is thick enough, whether a footing is large enough or whether reinforcement is adequate. Follow your local building code for anything structural.