Concrete calculator

How to calculate concrete volume

Measure the length, width and depth in metres and multiply them. That gives cubic metres (m³), which is how ready-mix is sold. Then add waste: 10% covers uneven ground and spillage, 15% on a rough sub-base.

A worked example. A shed base 4m by 3m at 100mm deep is 4 × 3 × 0.1 = 1.2m³. Add 10% and it is 1.32m³. Ready-mix is ordered in steps of a quarter of a cubic metre, so you would order 1.5m³.

For a round post hole or pad, the area is π × radius², times the depth. For post holes the calculator also takes the post out of the hole, because the post fills part of it.

How many 25kg bags of concrete per m³?

About 83. A 25kg bag of pre-mixed concrete makes about 12 litres, and a cubic metre is 1,000 litres. So the 1.32m³ shed base needs 110 bags.

Bags make sense for small jobs, up to about 1 to 1.5m³. Beyond that, the weight to carry and the hours at the mixer make ready-mix or a volumetric lorry the better choice. A 90 litre mixer turns out about 60 litres a load, so 1m³ is about 17 loads.

How much ballast and cement for 1m³ of concrete?

For a 1:5 mix (one part cement to five parts all-in ballast), about 300kg of cement and 1.8 tonnes of ballast. Stronger mixes use more cement and a little less ballast.

These are working figures. Suppliers and calculators disagree by 10 to 20%, because sand and ballast vary. Each row here is checked so the materials add up to what placed concrete weighs, about 2.3 to 2.5 tonnes per m³.

1:2:4 concrete mix quantities per m³

A 1:2:4 mix is one part cement, two parts sharp sand and four parts 20mm gravel, roughly C20. Per cubic metre that is about 320kg of cement (13 bags), 0.64 tonnes of sharp sand, 1.28 tonnes of gravel and 160 litres of water.

Which concrete mix do I need?

Ask the supplier for the designated mix by name, and they do the rest. The names come from BS 8500, the British Standard for concrete.

GEN3 is for slabs out of the weather, like a garage floor. Anything outside that sees frost, such as a path, patio or drive, wants PAV1, which has air bubbles mixed in so it does not flake when water in it freezes.

How thick should a concrete slab be?

It depends on what goes on it. These are typical depths for domestic work on a well compacted sub-base. Heavier loads, soft ground or anything structural need an engineer's figure.

How much does concrete cost per m³ in 2026?

Ready-mix runs about £100 to £115 per m³ for GEN mixes, £105 to £130 for C25/30 and £125 to £160 for C30/37 and paving mixes, excluding VAT. Inner London runs higher.

Most suppliers have a minimum load of about 3 to 4m³. Below that, expect a part-load charge of £15 to £30 for every m³ short. A volumetric lorry mixes on site, charges only for what you use and usually costs 15 to 25% more per m³. A line pump is about £275 to £550 a day.

Mixing it yourself: cement is about £7.50 to £9.50 a 25kg bag and ballast £48 to £68 a bulk bag. Materials alone come to roughly £150 to £200 per m³, so mixing on site only saves money on small jobs, and costs you the day.

How much does concrete weigh?

About 2.4 tonnes per m³ (2,400kg). The 1.2m³ shed base weighs about 2.88 tonnes. That matters when you break out an old slab: check the skip or grab lorry's weight limit before you book, because concrete is heavy for its size.

Concrete for fence posts

A 300mm wide hole 600mm deep around a 100mm square post needs about 0.036m³ of concrete once the post is taken out. Blue Circle say one 20kg bag of Postcrete sets a 100mm post in a 200mm by 750mm hole, which is about 0.016m³ a bag, so allow 3 bags for the bigger hole. Choose "Post holes" in the calculator for a whole fence. Setting posts for a deck? The decking calculator counts the posts first.

Concrete slab and driveway quotes: what goes into the price

A tradesperson's quote for a slab or drive is more than the concrete. It includes digging out and removing the spoil, the sub-base and compaction, shuttering, a membrane and mesh where needed, the concrete itself, labour to lay and finish it, and waste removal. Use the calculator for the concrete line, then compare quotes on the rest.