How this calculator works
Concrete volume is the area multiplied by the thickness, with the thickness in metres (100 mm is 0.1 m). We add your waste allowance, and multiply by 2.4 to get the weight in tonnes. One cubic metre of concrete is the same as 1,000 litres.
To mix it yourself, the dry materials pack together when they are wetted, so you need about 1.54 times the finished volume of dry materials. A 1:2:4 mix is 7 parts: one of cement, two of sharp sand and four of 20 mm aggregate. Cement weighs about 1,440 kg per m³, sharp sand about 1,600 and aggregate about 1,500.
Worked example: a 3 × 3 m patio base, 100 mm thick, with 10% waste: 9 × 0.1 × 1.1 = 0.99 m³, or 2.4 tonnes. To mix it yourself at 1:2:4 you need about 314 kg of cement (13 bags of 25 kg), 0.70 tonnes of sharp sand (0.44 m³) and 1.3 tonnes of aggregate (0.87 m³).
Concrete for common slab sizes (10% waste)
In m³, for 100 mm and 150 mm thick slabs. Order ready-mix in the next quarter m³ up.
| Slab size | 100 mm thick | 150 mm thick | DIY cement bags at 100 mm |
|---|---|---|---|
| 2 × 2 m | 0.44 m³ | 0.66 m³ | 6 |
| 3 × 3 m | 0.99 m³ | 1.48 m³ | 13 |
| 4 × 3 m | 1.32 m³ | 1.98 m³ | 17 |
| 5 × 4 m | 2.20 m³ | 3.30 m³ | 28 |
| 6 × 4 m | 2.64 m³ | 3.96 m³ | 34 |
| 6 × 6 m | 3.96 m³ | 5.94 m³ | 51 |
Tips before you buy
- Above about a cubic metre, ready-mix is usually easier and gives a more consistent result than mixing by hand. Ask about minimum loads and short-load charges, and have the ground, shuttering and helpers ready before the lorry arrives.
- Lay concrete on a compacted sub-base, usually 100 mm of MOT Type 1, so that the slab is supported evenly and water drains away.
- Reinforce slabs that carry vehicles: A142 mesh (6 mm bars at 200 mm centres) is the common choice for a driveway. Use movement joints to divide large areas into bays.
- Check for buried gas, water and electricity services before you dig. The free LSBUD (LinesearchbeforeUdig) service shows what runs near you.
- In England, paving more than 5 m² of a front garden with an impermeable surface needs permeable materials, or drainage to a lawn or border, otherwise it needs planning permission. Check with your council.
- Do not pour in heavy rain or frost. Keep fresh concrete damp, or covered, for about a week so that it cures slowly.
Tools & materials for this project
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Frequently asked questions
How much concrete do I need for a 3m x 3m patio?
A 3 × 3 m slab 100 mm thick is 0.9 m³, or 0.99 m³ with 10% extra, which is about 2.4 tonnes. Order 1 m³ of ready-mix.
How many bags of cement do I need per cubic metre of concrete?
For a 1:2:4 mix, about 13 bags of 25 kg per m³ of finished concrete (roughly 320 kg of cement), with 0.7 tonnes of sharp sand and 1.3 tonnes of aggregate. The exact figure depends on the mix.
How thick should a concrete base be?
100 mm is the usual thickness for paths, patios and shed bases, and 125 to 150 mm for a driveway. Always check your plans or the Building Regulations for structural work.
Is it cheaper to mix concrete myself or order ready-mix?
Mixing is cheaper per cubic metre for small amounts, but it is hard work and the quality varies. Past about a cubic metre, the labour and the risk of a stop-start pour usually make ready-mix the better choice.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.