Concrete calculator
How much concrete a slab needs, and what that is in bags, in cement and ballast, or as a ready-mix order.
What to buy
Every figure is quoted from a named UK manufacturer or merchant and was checked on 11 Sep 2026; the sources are listed below the calculator.
Measure, then add ten per cent
The volume is length times width times depth, with the depth turned into metres first: 100 mm is 0.1 m. What you order is more than that, because ground is never quite level, formwork bows, and some always ends up on the boots. B&Q adds 10% for waste, and the Concrete Society suggests 10 to 15% where there is no formwork, so 10% is a floor rather than a cushion. The allowance box is yours to change.
Bags, mix your own, or ready-mix
Ready-to-use bags have the cement and aggregate already in them, and only need water. A 20 kg bag makes around 0.01 m³ according to Blue Circle, and 0.008 m³ according to Heidelberg Materials, so the page gives both counts. Anything over half a cubic metre is a lot of bags to carry.
Mixing your own uses 25 kg bags of cement and all-in ballast, which is sand and gravel already combined. B&Q's table gives the cement per cubic metre for each mix and about a cubic metre of ballast whatever the mix; a merchant's bulk bag of ballast covers around 10 m² at 50 mm deep, about half a cubic metre, according to Travis Perkins.
Ready-mix arrives mixed, by the cubic metre. Tarmac Minimix delivers from 0.5 m³ and Breedon Minimix from 1 m³, so even a shed base can be a delivery rather than an afternoon with a mixer.
How deep
For a shed base, B&Q says 100 mm of concrete is enough for a lightweight wooden shed, on 75 mm of MOT Type 1. For a patio or path, Heidelberg Materials says 75 to 100 mm, and Marshalls 75 to 100 mm of sub-base. For a driveway, Heidelberg says at least 100 mm, Marshalls 100 to 150 mm of sub-base, and B&Q reinforces a driveway with a steel grid at half its depth. Choosing what the slab is for fills in those depths; change them if your job needs more.
The sub-base
MOT Type 1 is crushed stone that compacts into a firm, free-draining layer under the concrete. It is sold by weight, so the page turns its volume into tonnes: Marshalls' method, and Wickes' stated coverage, both come to about 2.08 tonnes per compacted cubic metre, and merchants sell it in bulk bags of about 800 kg.
Where the figures come from
Bag yields from the Blue Circle Multi-Purpose Concrete and Heidelberg Materials 40N data sheets. Mixes and the waste allowance from B&Q. Ballast coverage from Travis Perkins, sub-base density from Marshalls and Wickes, bulk bag weights from Travis Perkins, Wickes and Breedon, and the weight of concrete from Cemex. All read on 11 September 2026. For the volume of anything that is not a slab, the volume calculator gives it in every unit at once.
Common questions
- How many bags of concrete make a cubic metre?
- Between 100 and 125 of the 20 kg bags of ready-to-use concrete, depending on the brand. Blue Circle says one bag makes around 0.01 m³, and Heidelberg Materials 0.008 m³ for its 40N concrete. That is two to two and a half tonnes of bags for one cubic metre, which is worth knowing before loading a car.
- How thick should a shed base be?
- Around 100 mm of concrete on 75 mm of compacted MOT Type 1. B&Q says a pad for a lightweight wooden shed "would only need to be 100mm deep", and Heidelberg Materials says at least 75 mm. A heavier building, or a garage, wants more.
- What mix should I use?
- B&Q gives 1 part cement to 4 parts all-in ballast for a shed or garage base that is not exposed to the weather, and 1 to 3.5 for paving that is. Heidelberg Materials says 1 to 5 is usually adequate for a shed base. The page uses B&Q's figures, because B&Q is the one source found that gives quantities per cubic metre.
- How much does a cubic metre of concrete weigh?
- About 2.4 tonnes, according to Cemex. So a 100 mm slab weighs about 240 kg for every square metre, which is why a garden path is heavier than it looks.
- Can I order less than a cubic metre of ready-mix?
- Often, yes. Tarmac Minimix delivers loads from 0.5 m³. Breedon Minimix has a minimum order of 1 m³ but says it can often take smaller orders. Heidelberg Materials runs a mini-mix service of 0.5 to 3 m³ in the North East, Yorkshire and Cumbria. Ask for the price of a part load before assuming it is cheaper than bags.