Concrete & masonryFree calculator
Work out how much concrete you need for a slab, patio, shed base or path. Get the volume in cubic yards or cubic metres, the number of pre-mix bags, and compare the cost of bags against ready-mix.
Your shopping list
How to measure
- Measure the length and width of the slab.
- Decide the thickness: 4 in (10 cm) for patios, paths and shed bases, 6 in (15 cm) for driveways.
- Add 5–10% extra for spillage and uneven ground.
The formula
Volume = length × width × thickness Imperial: cubic feet ÷ 27 = cubic yards Bags: an 80 lb bag makes about 0.6 cu ft
Worked example
A 10 × 10 ft slab, 4 in thick, is 33.3 cu ft or 1.23 cu yd. With 5% extra that is 1.30 cu yd, or about 58 bags of 80 lb pre-mix.
Tips before you buy
- Put down a compacted gravel sub-base first so the slab does not crack.
- Order ready-mix slightly over. Running short mid-pour leaves a weak joint.
- Keep new concrete damp for a few days while it cures, especially in hot weather.
Frequently asked questions
Bags or ready-mix?
Bags suit small jobs under about 1 cubic yard (0.75 m³). Above that, ready-mix is usually cheaper and much less work.
How thick should a patio slab be?
4 in (10 cm) is standard for foot traffic. Use 6 in (15 cm) where cars will park.
How many bags of concrete make a cubic yard?
About 45 bags of 80 lb, or 60 bags of 60 lb. In metric, about 90 bags of 25 kg make one cubic metre.
Results are estimates for planning only. Always check quantities with your supplier before you buy.