How to calculate concrete
Concrete is ordered by volume: cubic yards in the US, cubic metres in most other countries. Multiply length × width × thickness, all in the same unit.
cubic feet = length (ft) × width (ft) × thickness (in) / 12
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cubic yards = cubic feet / 27
Watch the thickness: it is usually measured in inches, while length and width are in feet. A 4-inch slab is 4 / 12 = 0.333 ft thick. The calculator asks for inches and converts for you. In metric mode, it asks for metres and centimetres.
Always add a waste allowance, typically 5 to 10%. Forms bow slightly, the ground is never perfectly level and some concrete always stays in the wheelbarrow or mixer. Running short is far worse than having a little left over: ordering more halfway through a pour leaves cold joints and delays.
Worked examples
- 10 × 10 ft patio, 4 inches thick: 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. With 10% waste, that is 1.36 cubic yards (about 1.04 m³), or roughly 62 bags of 80 lb premix, or 82 bags of 60 lb.
- Six square footings, 1 × 1 ft and 36 inches deep: 1 × 1 × 3 × 6 = 18 cubic feet, or 0.67 cubic yards before waste. For round post holes, the cylinder calculator gives a more precise figure.
- Metric slab of 4 × 3 m, 10 cm thick: 1.2 m³, or 1.32 m³ with a 10% allowance.
Bags or ready-mix?
For small jobs, such as setting fence posts or pouring a small pad, premix bags are convenient: just add water. An 80 lb bag yields roughly 0.6 cubic feet of concrete, a 60 lb bag about 0.45 cubic feet. Always check the yield printed on the bag, because it varies by brand.
Above roughly one cubic yard, mixing bags by hand becomes hard work: over 60 bags of 80 lb is more than two tons to lift. At that point ready-mix delivered by truck is usually easier and not necessarily more expensive. Suppliers price per cubic yard, often with a minimum order and short-load fees for small quantities, plus extra for waiting time or a pump.
Which concrete strength?
Concrete is specified by compressive strength, commonly 2,500 to 4,000 psi for residential work. Patios, sidewalks and shed bases usually need less than driveways, garage floors or structural footings, and concrete exposed to freeze-thaw cycles and de-icing salt needs air entrainment. For anything structural, follow local building codes and get advice from an engineer or your building department. This calculator only works out the quantity.
Tips for pouring
- Base: compact the subgrade and add a layer of gravel for drainage.
- Reinforcement: wire mesh or rebar helps control cracking in slabs.
- Forms: build sturdy forms; wet concrete is heavy at about 4,000 lb per cubic yard.
- Curing: keep the concrete moist for the first days and protect it from hot sun, frost and strong wind. Concrete reaches most of its design strength after about 28 days.
For round footings or columns, use the cylinder calculator; for other shapes, the cubic meter calculator. Finishing a floor with tile or laminate? Work out the area with the square meter calculator and the materials with the flooring calculator.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length and width in feet by thickness in inches divided by 12, divide by 27 for cubic yards and add 5 to 10%. A 10 × 10 ft slab at 4 inches is about 1.23 cubic yards, or 1.36 with waste.
How many 80 lb bags make a cubic yard?
About 45 bags, since each yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet. Check the yield on the bag.
How much does a cubic yard of concrete weigh?
About 4,000 lb (roughly 2,400 kg per cubic metre).
When should I order ready-mix?
Above roughly one cubic yard, ready-mix is usually easier than mixing bags yourself.
How thick should a concrete slab be?
Four inches is common for patios and walkways; driveways and garage floors are often 5 to 6 inches. Follow local codes for structural work.
How much waste allowance should I add?
Usually 5 to 10%. Add more for uneven ground or complicated shapes.
Last reviewed: 2026-10-06. Results are estimates for information only.