Ordering too little concrete means a cold joint halfway through the pour. Ordering too much costs money and leaves you with a pile to dispose of. The good news is that the maths is simple: length times width times thickness, converted to the unit your supplier sells.
The basic formula
Volume = length x width x thickness. Measure everything in the same unit before you multiply.
In feet and inches (cubic yards)
Concrete in the US is sold by the cubic yard. One cubic yard is 27 cubic feet. Convert the thickness from inches to feet by dividing by 12, multiply, then divide by 27.
Example: a 10 ft x 10 ft patio slab, 4 inches thick.
- Thickness in feet: 4 / 12 = 0.333 ft
- Volume: 10 x 10 x 0.333 = 33.3 cubic feet
- In cubic yards: 33.3 / 27 = 1.23 cubic yards
In metres (cubic metres)
Elsewhere concrete is sold by the cubic metre. Convert millimetres to metres by dividing by 1,000.
Example: a 3 m x 3 m slab, 100 mm thick.
- Volume: 3 x 3 x 0.1 = 0.9 cubic metres
Add a waste allowance
Ground is rarely perfectly level, forms can bow and some concrete is lost when you pour. Adding about 10 percent is a common allowance. For the imperial example, 1.23 x 1.10 = 1.36 cubic yards. For the metric example, 0.9 x 1.10 = 0.99, so you would order 1 cubic metre. Ready-mix suppliers often have a minimum load, so ask about it.
If you are mixing bagged concrete
Check the yield printed on the bag. As a typical guide, an 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45 cubic feet. For the 33.3 cubic feet slab above:
- 80 lb bags: 33.3 / 0.6 = about 56 bags, or about 61 with a 10 percent allowance
- 60 lb bags: 33.3 / 0.45 = about 74 bags
That is a lot of bags to mix by hand, which is why larger slabs usually use ready-mix delivered by truck.
How thick should a slab be?
A typical residential patio or walkway slab is about 4 inches (100 mm). Driveways and areas carrying vehicles are usually thicker, often 5 to 6 inches or more, and sometimes reinforced. Requirements depend on your soil, climate and local building rules, so check with your local authority before you pour.
Do not forget the base
Most slabs sit on a compacted gravel sub-base, commonly about 4 inches. Calculate the gravel the same way, length x width x depth, and order it separately.
Safety
Wet concrete is caustic. Wear gloves, eye protection and waterproof boots, and rinse skin that gets splashed.
Skip the maths
Our free concrete calculator works out the volume, bags and cost in feet or metres. If you are planning several jobs at once, the DIY Project Budget and Materials Planner combines materials, budget and a shopping list in one Excel file.
These are planning estimates. Confirm quantities with your supplier and follow local building rules and safety guidance.
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