Concrete calculator: volume in m³ and bags you need

Concrete volume is length × width × thickness in metres: a 5 × 4 m slab 12 cm thick needs 2.4 m³. At around 0.01 m³ per 20 kg bag of ready-to-use concrete, that is 240 bags.

Volume: length × width × thickness

Concrete is ordered and mixed by the cubic metre. For a slab or a shed base, multiply length, width and thickness, all in metres: a 5 × 4 m base 12 cm thick is 5 × 4 × 0.12 = 2.4 m³, or 2,400 litres. Thickness is where most mistakes happen: 12 cm is 0.12 m, not 12.

Other shapes work the same way. Six pads of 30 × 30 cm, 80 cm deep, add up to 0.30 × 0.30 × 0.80 × 6 = 0.432 m³. A round column uses the area of a circle, π × radius²: four columns 20 cm across and 2.5 m tall come to π × 0.10² × 2.5 × 4 = 0.314 m³. A strip footing 36 m long, 40 cm wide and 20 cm deep takes 36 × 0.40 × 0.20 = 2.88 m³.

How many bags of concrete per cubic metre

A bag of ready-to-use concrete already contains cement, sand and coarse aggregate: you only add water. The number of bags depends on the product, so the calculator works from named data sheets. Blue Circle, Tarmac's brand, states that one 20 kg bag of its Multi-Purpose Concrete or High Strength Concrete (40N) will typically produce around 0.01 m³ of fresh concrete, a block of 0.33 × 0.33 × 0.1 m. One cubic metre therefore takes 100 bags, or two tonnes of material.

Both sheets ask for about 2 litres of clean water per 20 kg bag, which is 200 litres per cubic metre, and a mixing time of 3 to 5 minutes in a mixer. The concrete should be used within 30 minutes of adding water.

Bags of ready-to-use concrete by volume

Worked out from the Blue Circle data sheets: one 20 kg bag of Multi-Purpose or High Strength 40N concrete makes around 0.01 m³ and takes 2 litres of water. One cubic metre therefore needs 100 bags.

Worked example: a 5 × 4 m base

The 2.4 m³ base needs 240 bags of 20 kg and 480 litres of water, close to five tonnes of bags to unload and mix. At that size, compare the price with a ready-mix lorry or a mix-on-site delivery before you buy bags.

A 6 × 3 m slab 10 cm thick (1.8 m³) takes 180 bags. The six pads need 44 bags and the four round columns 32. The strip footing in the example above would take 288 bags of High Strength 40N. The calculator always rounds up to a whole bag.

Multi-Purpose or High Strength 40N?

Blue Circle lists Multi-Purpose Concrete for most small jobs: paths, steps, floors, bedding in drains and manhole covers, shed and greenhouse bases, and foundations for paving slabs. High Strength Concrete (40N) is the stronger mix, typically reaching 40 N/mm² at 28 days, for bases, drives, garage floors, footings and foundations, and for fixing fences, signs and bollards. Footing size and depth for an extension or a garden wall are set by the approved design and building control, not by this calculator.

Mixing your own: ratios, not bag counts

If you mix cement, sand and aggregate yourself, B&Q gives three common ratios by volume: 1:2.5:3.5 (or 1 part cement to 5 parts ballast) for light wall foundations and fence posts, 1:2:3 (1 to 4 with ballast) for shed and garage bases, and 1:1.5:2.5 (1 to 3.5 with ballast) for steps, paving and drives. Measure with buckets of the same size rather than shovels. These are proportions only: no data sheet we found states how much concrete one 25 kg bag of cement makes, so the calculator does not turn them into bag counts.

Weather and curing

Blue Circle advises against using the concrete below 5 °C or above 30 °C without protecting new work from frost or drying out, and asks you to cover fresh concrete with polythene sheeting or wet hessian for at least 7 days. Compact it fully before finishing.

Common mistakes

• Entering thickness in centimetres in a formula in metres: 5 × 4 × 12 would give 240 m³ instead of 2.4 m³.
• Using a column's diameter as its radius: the volume comes out four times too big.
• Adding extra water to make pouring easier: it weakens the concrete.
• Mixing more bags than you can place in 30 minutes.
• Forgetting the weight: a hundred 20 kg bags is two tonnes.

For a thinner floor layer, use the screed calculator; for a wall, the block calculator; and to convert loose materials, the cubic metres to tonnes converter.