Concrete calculator
Enter the size of your slab, footing or post holes. Get the number of bags, the volume to order and the gravel and forms to go with it.
Shopping list
Enter what your store charges for one of each item. Prices stay in your browser.
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How to use the calculator
- Pick the shape: a slab, a footing or wall, or round holes for posts.
- Enter the measurements. Thickness matters most, so measure it in several places.
- Pick the bag size your store sells. The result shows both bags and the volume to quote when ordering ready-mix.
How the numbers are worked out
Slabs, footings and walls
A 10 × 10 ft slab 4 in thick is 10 × 10 × 0.333 = 33.3 cubic feet. Adding 10% for spillage and uneven ground gives 36.7 cubic feet, which is 1.36 cubic yards.
Round holes
Four holes 12 in wide and 36 in deep hold 9.4 cubic feet in total.
Bags
| Bag | Makes about | Bags for 1 cubic yard | Bags for 1 m³ |
|---|---|---|---|
| 40 lb | 0.30 cubic feet | 90 | |
| 60 lb | 0.45 cubic feet | 60 | |
| 80 lb | 0.60 cubic feet | 45 | |
| 20 kg | 9.3 litres | 108 | |
| 25 kg | 11.6 litres | 86 |
Yields vary a little between brands, so check the bag.
Common questions
How many bags of concrete do I need for a 10×10 slab?
A 10 × 10 ft slab 4 inches thick is 33.3 cubic feet. That is 56 bags of 80 lb mix, or 62 with 10% extra for waste. At that size, ready-mix delivery is usually easier.
How many 80 lb bags of concrete make a cubic yard?
45 bags. One 80 lb bag makes about 0.6 cubic feet, and a cubic yard is 27 cubic feet.
How thick should a concrete slab be?
4 inches (100 mm) suits patios, paths and shed bases. Driveways are usually 5 to 6 inches (125 to 150 mm) with reinforcing. Check local rules for anything structural.
How much extra concrete should I order?
Add 10%. Ground is never perfectly level and forms bow outward, so the true volume is almost always more than the measured one.
When should I order ready-mix instead of bags?
Above about 1 cubic yard (0.75 m³). That is 45 bags of 80 lb mix, which is hard to mix fast enough to finish as one pour.
What the list leaves out
- Reinforcing mesh, rebar and chairs.
- Expansion joint strips and curing compound.
- Tool hire, such as a mixer or a plate compactor.
- Steps, curved slabs and sloped pours.