Concrete calculator
Enter the length, width and depth of your slab, base or path. The calculator gives the volume of concrete in cubic metres, its approximate weight, and how many 20 kg bags of ready-to-use concrete mix that is, with an allowance for waste.
Your details
Measure the area and decide how thick it needs to be.
Figures checked
Working out the volume
Concrete is sold by volume, so the sum is length times width times depth, with everything in metres. Depth is usually given in millimetres, so divide it by 1,000 first: 100 mm is 0.1 metres.
Worked example, a shed base 3 metres by 2.4 metres, 100 mm deep: 3 × 2.4 × 0.1 = 0.72 cubic metres. Add 10 per cent for waste: 0.72 × 1.1 = 0.792 cubic metres. Rounded up, that is about 80 bags of 20 kg ready-to-use concrete (at about 0.01 cubic metres a bag), and weighs roughly 1.9 tonnes.
Getting the ground level matters. Heidelberg Materials points out that every extra centimetre of depth over 25 square metres adds a quarter of a cubic metre of concrete (25 × 0.01 = 0.25).
How thick should the concrete be?
| Job | Concrete thickness | Source |
|---|---|---|
| Garden path or patio | 75 to 100 mm | Heidelberg Materials |
| Shed base | At least 75 mm, on a similar depth of sub-base | Heidelberg Materials |
| Driveway or garage floor | At least 100 mm | Heidelberg Materials |
Heidelberg Materials advises digging deep enough for a 100 mm sub-base and a damp-proof membrane, as well as the concrete itself. The sub-base is not concrete, so do not include it in the depth you enter above. If you are also buying sub-base material, work out its volume separately; our gravel calculator can help.
These are general guides for home jobs. The right thickness and base depend on the ground, the load and what the concrete is for, so check the product guidance or ask a tradesperson if you are unsure.
How many bags of concrete do I need?
Tarmac says one 20 kg bag of Blue Circle Multi-Purpose Concrete “will typically produce around 0.01 m3 of fresh concrete”, with 2 litres of water per bag. That is about 100 bags for a cubic metre. Tarmac gives the same yield for its High Strength Concrete. Its Quick Set Concrete covers about 0.45 square metres at 25 mm thick per 20 kg bag. Other brands and bag sizes vary, so check the yield printed on the bag you buy and divide the volume by it.
The number of bags, and the mixing, adds up quickly on bigger jobs: a 3 metre by 3 metre patio 100 mm deep, with 10 per cent for waste, is 0.99 cubic metres, or about 99 bags.
Mixing your own: concrete mix ratios
If you are mixing your own for a small home job, the Concrete Society gives these example mixes for DIY use, measured by volume (for example, by the bucket):
| Use | Cement : sharp sand : coarse aggregate | Or cement : all-in aggregate (ballast) |
|---|---|---|
| Foundations (for example footings, blinding, fence posts) | 1 : 2.5 : 3.5 | 1 : 6 |
| General use (for example garage and shed bases, domestic floors) | 1 : 2 : 3 | 1 : 5 |
| Paving (for example paths, drives and steps) | 1 : 1.5 : 2.5 | 1 : 4 |
Heidelberg Materials says 1 part cement to 5 parts ballast “is usually perfectly adequate for shed bases”. Add water a little at a time until the mix is workable. For structural work, or where building control is involved, follow the specification for the job rather than a general DIY mix.
Fence posts
If you are setting fence posts in rapid-setting post mix, use the bag guide for the product you buy, not the calculator above. Tarmac’s table for Blue Circle Postcrete gives, for example: a 7.5 cm square post in a hole 20 cm wide and 75 cm deep needs 1 bag; a 10 cm square post in a hole 30 cm wide and 60 cm deep needs 2 bags; a 5 cm round post in a hole 15 cm wide and 60 cm deep needs half a bag. Tarmac says it sets in 5 to 10 minutes. Its bags are 20 kg.
Ordering ready-mixed concrete
Ready-mixed concrete is sold by the cubic metre. Load sizes and delivery terms vary by supplier and change over time. When we checked on 29th September 2026, Tarmac’s small-load service delivered “from 0.5m3 to 6m3”; Heidelberg Materials delivered in six and eight cubic metre trucks, free on a full load, and charged for part loads. For a bigger job, compare a ready-mixed quote with the cost of the bags you would need. Give the supplier the volume from the calculator, including the waste allowance, and ask what strength of mix they recommend for your job.
Common questions
How many 20 kg bags of concrete make a cubic metre?
About 100, using Tarmac’s figure of around 0.01 cubic metres per 20 kg bag of Blue Circle Multi-Purpose Concrete. Check the bag you buy, because yields vary.
How much does a cubic metre of concrete weigh?
It varies with the mix. As a rough planning figure, about 2.4 tonnes (SMC Mini-Mix); Heidelberg Materials puts it at around 2.5 tonnes, and says additions such as steel fibres can change it significantly.
How much concrete do I need for a 3 metre by 3 metre slab?
At 100 mm deep, 0.9 cubic metres, or 0.99 cubic metres with 10 per cent added for waste.
How do I convert cubic yards to cubic metres?
One cubic yard is about 0.765 cubic metres (exactly 0.764554857984, because a yard is legally 0.9144 metres).
Sources
Figures checked 29th September 2026. Tarmac, Blue Circle Multi-Purpose Concrete, High Strength Concrete, Quick Set Concrete and Postcrete product information (December 2023); Tarmac, Minimix small load concrete delivery; Heidelberg Materials, Ready mixed concrete volume calculator, How to lay a concrete base, How to lay a shed base and Pricing guide; SMC Mini-Mix, FAQs; The Concrete Society, Batching by volume (DIY), updated 8th July 2025; Weights and Measures Act 1985, Schedule 1.