How Much Concrete Do I Need? m³, Bags of Cement and Ready-Mix (UK)
A slab 3 × 3 m and 100 mm thick needs 0.99 m³ of concrete, with 10% extra, which weighs about 2.4 tonnes. To mix it yourself at 1:2:4 you need 13 bags of 25 kg cement, 0.70 tonnes of sharp sand and 1.3 tonnes of 20 mm aggregate. For ready-mix, order 1.00 m³. The concrete calculator works it out for your own slab, circle or area, and the mix you choose.
Concrete for common slab sizes
With 10% waste. The ready-mix order is rounded up to the next quarter of a cubic metre, and the last column is the number of 25 kg bags of cement for a 100 mm slab mixed at 1:2:4:
| Slab size | 100 mm thick | 150 mm thick | Cement bags at 100 mm |
|---|---|---|---|
| 2 × 2 m | 0.44 m³ (0.50 to order) | 0.66 m³ (0.75 to order) | 6 |
| 3 × 2 m | 0.66 m³ (0.75 to order) | 0.99 m³ (1.00 to order) | 9 |
| 3 × 3 m | 0.99 m³ (1.00 to order) | 1.48 m³ (1.50 to order) | 13 |
| 4 × 3 m | 1.32 m³ (1.50 to order) | 1.98 m³ (2.00 to order) | 17 |
| 5 × 4 m | 2.20 m³ (2.25 to order) | 3.30 m³ (3.50 to order) | 28 |
| 6 × 4 m | 2.64 m³ (2.75 to order) | 3.96 m³ (4.00 to order) | 34 |
| 6 × 6 m | 3.96 m³ (4.00 to order) | 5.94 m³ (6.00 to order) | 51 |
| 10 × 3 m | 3.30 m³ (3.50 to order) | 4.95 m³ (5.00 to order) | 42 |
How thickness changes the amount
For a 3 × 3 m slab:
| Thickness | Concrete | Weight | Cement bags (1:2:4) |
|---|---|---|---|
| 75 mm | 0.74 m³ | 1.8 tonnes | 10 |
| 100 mm | 0.99 m³ | 2.4 tonnes | 13 |
| 125 mm | 1.24 m³ | 3.0 tonnes | 16 |
| 150 mm | 1.48 m³ | 3.6 tonnes | 19 |
| 200 mm | 1.98 m³ | 4.8 tonnes | 26 |
DIY mixes
Cement, sharp sand and 20 mm aggregate by volume, for a 3 × 3 m slab 100 mm thick:
| Mix | Use | Cement | Sharp sand | 20 mm aggregate |
|---|---|---|---|---|
| 1 : 2 : 4 | General purpose: paths, patios, bases | 314 kg (13 bags) | 0.70 t | 1.31 t |
| 1 : 1.5 : 3 | Stronger: slabs and drives | 399 kg (16 bags) | 0.67 t | 1.25 t |
| 1 : 3 : 6 | Weak: blinding and fill only | 220 kg (9 bags) | 0.73 t | 1.37 t |
A bag of cement is 25 kg. The dry materials pack together when they are wetted, so you need about 1.54 times the finished volume. The quantities are approximate, since the moisture in the sand and the aggregate change the volumes. Use the mix your supplier or engineer specifies for anything structural.
Ready-mix or your own mix?
- Up to about half a cubic metre: mixing yourself or using bagged dry mix is practical, although it takes a lot of effort.
- Above about a cubic metre: a 2.6 m³ slab (6 × 4 m) is a lot of mixing, and ready-mix gives a more consistent result. Order 2.75 m³.
- Plan the pour. Have the sub-base, shuttering and any mesh ready, with enough people to place and level the concrete before it starts to set.
- Minimum loads. A supplier may charge extra for a short load. A small mixer truck or a concrete pump can help with a difficult site.
Before you pour
- Sub-base. Lay and compact about 100 mm of MOT Type 1 under a slab so that the concrete is supported evenly and water drains away.
- Reinforcement. A slab for vehicles usually has mesh (A142 is common for a drive) and movement joints that divide a large area into bays.
- Services. Check for buried gas, water and electricity before you dig. LSBUD (LinesearchbeforeUdig) is a free service.
- Front gardens. 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.
- Curing. Keep the slab damp, or cover it, for several days so that it gains its strength, and protect it from frost.
For paving slabs on a sub-base see the paving calculator, and for stone see the gravel calculator. Estimates only: confirm the quantities with your supplier.
Frequently asked questions
How much concrete do I need for a slab?
Multiply the length, width and thickness in metres, and add 10% for waste. A 3 × 3 m slab 100 mm thick is 9 × 0.1 × 1.1 = 0.99 m³, which is 2.4 tonnes.
How many bags of cement do I need per cubic metre of concrete?
About 13 bags of 25 kg for a 1:2:4 mix: 13 bags of cement for 0.99 m³, with 0.70 tonnes of sharp sand and 1.3 tonnes of 20 mm aggregate.
Should I order ready-mix or mix it myself?
Above about a cubic metre, ready-mix is usually easier and more consistent. A truck brings it mixed, but you need the ground, the formwork and helpers ready, and there may be a minimum load or a charge for a short load.
How thick should a concrete slab be?
Paths and patios are usually 100 mm, and driveways 125 to 150 mm, laid on a compacted sub-base. Check what your plans or the Building Regulations require.
How much does a cubic metre of concrete weigh?
About 2.4 tonnes. The 3 × 3 m slab at 100 mm weighs 2.4 tonnes.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.