Concrete Calculator
Work out how much concrete any rectangular pour needs — in cubic feet, cubic metres and cubic yards — and how many bags of ready-mix that means.
Concrete is ordered by volume, but bought by the bag — and the two do not divide neatly. This calculator gives you both numbers from the same three measurements, with the wastage that real pours always need.
It works in feet and inches for the US, and in metres and millimetres for the UK and Australia. Bag yields are the standard published figures, shown on the page so you can check them.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Measure the length and width of the pour, and its depth (thickness).
- Set wastage — 10% covers spillage and uneven ground.
- Enter your bag size to get a bag count, or leave it blank for a ready-mix volume.
- Add a price per bag if you want a cost estimate.
Worked example
| Pour | 10 ft × 10 ft × 4 in thick |
|---|---|
| Volume | 10 × 10 × (4 ÷ 12) = 33.33 ft³ |
| In cubic yards | 33.33 ÷ 27 = 1.23 yd³ |
| In cubic metres | 33.33 × 0.0283168 = 0.94 m³ |
| With 10% wastage | 33.33 × 1.10 = 36.67 ft³ |
| 80 lb bags (0.60 ft³ each) | ⌈36.67 ÷ 0.60⌉ = 62 bags |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
How many bags of concrete do I need?
It depends on the volume and the bag size. An 80 lb bag yields 0.60 ft³, a 60 lb bag 0.45 ft³, a 40 lb bag 0.30 ft³; a 25 kg bag yields about 0.0125 m³. Enter your pour size and bag size above and the calculator does the division, with wastage included.
How much wastage should I allow for concrete?
Allow 10%. Concrete is lost to spillage, to ground that is not perfectly level, and to over-excavation. For a small pour, 10% is the safe figure; ready-mix suppliers also have a minimum load, so check that before ordering.
When should I order ready-mix instead of bags?
Bags suit small pours — up to about 1 cubic yard. Above that, ready-mix delivered by truck is usually cheaper and far less work. This calculator gives you the cubic yards to quote to the supplier.
How thick should a concrete slab be?
A standard patio or shed slab is 4 in (100 mm) thick. Driveways need 4–6 in (100–150 mm), and heavy-vehicle areas 6 in or more. Always lay concrete on a compacted gravel sub-base.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.