How to use the concrete calculator
- Choose US (feet, inches, pounds) or UK (metres, centimetres, kilograms) at the top of the calculator.
- Pick the shape. Use slab, footing or wall for anything rectangular — patios, shed bases, driveways, strip footings. Use round column or hole for post holes, piers and tube forms.
- Enter your measurements. For a slab, the thickness is the depth of concrete, not including any gravel or hardcore underneath.
- Leave 5–10% extra for waste, then read off the volume and the number of bags for each common bag size.
The concrete volume formula
Concrete is ordered by volume. Work everything out in one unit first — feet or metres — then multiply.
Slab volume = length × width × thickness Round volume = π × (diameter ÷ 2)² × depth Cubic yards = cubic feet ÷ 27
Bags are worked out from the yield printed on the bag. A typical 80 lb bag of concrete mix makes about 0.60 ft³ of concrete, a 60 lb bag about 0.45 ft³ and a 40 lb bag about 0.30 ft³. In metric terms that is around 2,136 kg of dry mix for every cubic metre, so a 20 kg bag makes about 0.0094 m³ and a 25 kg bag about 0.0117 m³.
Worked example (US)
A 10 ft × 10 ft patio slab, 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.3 ft³. Divide by 27 to get 1.23 cubic yards. With 80 lb bags: 33.3 ÷ 0.60 = 55.6, so buy 56 bags — or about 62 with 10% extra.
Worked example (UK)
A 3 m × 3 m shed base, 100 mm thick: 3 × 3 × 0.1 = 0.9 m³. That is about 1,922 kg of dry mix, or 97 × 20 kg bags (77 × 25 kg bags) before adding extra for waste.
How thick should concrete be?
These are typical thicknesses for domestic work. Your local building code, building regulations or an engineer’s design always takes priority.
| Project | Typical US thickness | Typical UK thickness |
|---|---|---|
| Garden path, walkway | 4 in | 75–100 mm |
| Patio | 4 in | 100 mm |
| Shed base | 4 in | 75–100 mm |
| Driveway (cars) | 4–6 in | 100–150 mm |
| Garage floor | 4–6 in | 100–150 mm |
Slabs normally sit on a compacted base of gravel or hardcore (often 4 in / 100 mm). Use the gravel calculator to work out the base material.
Bags or ready-mix?
Bagged mix is ideal for post holes, small pads and repairs. Once a job goes beyond about 1 cubic yard (roughly 0.75 m³), ready-mixed concrete delivered by truck — or a volumetric mixer in the UK — is usually cheaper and far less work: one cubic yard is about 45 heavy bags to mix by hand. Ready-mix suppliers often have a minimum order, so phone ahead and give them the volume from this calculator.
Tip: measure the depth of your excavation in several places. Ground is rarely level, and a slab that is half an inch (12 mm) deeper than planned uses about 12% more concrete.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
About 45 bags. An 80 lb bag yields roughly 0.60 cubic feet, and a cubic yard is 27 cubic feet (27 ÷ 0.60 = 45). You would need about 60 × 60 lb bags or 90 × 40 lb bags for the same volume.
How many 20 kg or 25 kg bags of concrete are in 1 m³?
Roughly 107 × 20 kg bags or 86 × 25 kg bags per cubic metre, based on about 2,136 kg of dry mix per m³ of finished concrete. Yields differ slightly between brands, so check the bag.
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab needs 1.23 cubic yards (33.3 ft³) — about 56 × 80 lb bags before any extra for waste. A 3 × 3 m slab at 100 mm needs 0.9 m³, or about 97 × 20 kg bags.
How much extra concrete should I order?
Add 5–10%. Ground is rarely perfectly level, forms bow slightly and some concrete is always left in the mixer or barrow. Running short halfway through a pour leaves a weak cold joint, so it is cheaper to have a little left over.
Can I use this for footings, walls and steps?
Yes. Footings and walls are rectangular blocks, so use the slab option and enter the depth as the thickness. For steps, calculate each step as its own block (or use the quantity box when the steps are identical) and add them together.
How much does a cubic yard of concrete weigh?
Normal concrete weighs about 2,400 kg per m³, which is roughly 4,000 lb per cubic yard. That weight is why a full ready-mix truck cannot always drive onto a lawn or driveway.