How Much Concrete Do I Need for a Slab?
Short answer: multiply length by width in metres, then by the thickness in metres. A 4 × 3 m slab at 100 mm is 1.2 m³, or 1.32 m³ with 10% waste: about 3.17 tonnes, or 120 bags of 25 kg mix. The table below covers slabs from 2 × 2 m to 6 × 6 m.
The slab formula
A slab is a box, so its volume is length × width × thickness. In the UK the length and width are usually measured in metres and the thickness in millimetres, so divide the thickness by 1,000 first.
Order = volume × 1.10 (10% waste)
Weight ≈ order × 2.4 tonnes
25 kg bags = order × 1,000 ÷ 11 litres, rounded up
The 10% waste covers an uneven sub-base, spillage and concrete left in the mixer or barrow. On a well levelled, formed base 5% is often enough; on rough ground allow more.
Concrete for common slab sizes (with 10% waste)
| Slab | Area | 100 mm: m³ | 100 mm: tonnes | 150 mm: m³ | 150 mm: tonnes |
|---|---|---|---|---|---|
| 2 × 2 m | 4 m² | 0.44 | 1.06 | 0.66 | 1.58 |
| 3 × 2 m | 6 m² | 0.66 | 1.58 | 0.99 | 2.38 |
| 3 × 3 m | 9 m² | 0.99 | 2.38 | 1.49 | 3.56 |
| 4 × 3 m | 12 m² | 1.32 | 3.17 | 1.98 | 4.75 |
| 4 × 4 m | 16 m² | 1.76 | 4.22 | 2.64 | 6.34 |
| 5 × 3 m | 15 m² | 1.65 | 3.96 | 2.48 | 5.94 |
| 6 × 3 m | 18 m² | 1.98 | 4.75 | 2.97 | 7.13 |
| 6 × 4 m | 24 m² | 2.64 | 6.34 | 3.96 | 9.50 |
| 6 × 6 m | 36 m² | 3.96 | 9.50 | 5.94 | 14.26 |
Going from 100 mm to 150 mm adds 50% more concrete. Weights are for mixed concrete at about 2.4 tonnes per m³.
Concrete per 10 m² by thickness
Useful when your area is an odd shape: work out the area in square metres (the square metre calculator helps), then scale from this table.
| Thickness | m³ (no waste) | m³ with 10% | Tonnes | 25 kg bags | Area per m³ |
|---|---|---|---|---|---|
| 75 mm | 0.75 | 0.83 | 1.98 | 75 | 13.33 m² |
| 100 mm | 1.00 | 1.10 | 2.64 | 100 | 10 m² |
| 125 mm | 1.25 | 1.38 | 3.30 | 125 | 8 m² |
| 150 mm | 1.50 | 1.65 | 3.96 | 150 | 6.67 m² |
| 200 mm | 2.00 | 2.20 | 5.28 | 200 | 5 m² |
"Area per m³" is how much ground one cubic metre covers before waste. At 100 mm a cubic metre covers 10 m².
Worked example: a 6 × 3 m parking slab at 150 mm
6 × 3 × 150 ÷ 1,000 = 2.7 m³. Adding 10% gives 2.97 m³, which weighs about 7.13 tonnes. In 25 kg bags that would be 2,970 ÷ 11 = 270 bags, far too many to mix by hand, so this is a job for ready-mix or a mix-on-site lorry. On a carefully levelled, formed base with 5% waste the order drops to 2.84 m³.
Check the bag yield on the product you buy: some 25 kg mixes make less than 11 litres, which means more bags. Our guide on bags of concrete per cubic metre covers 20 kg bags too.
Ordering ready-mix concrete
- Order in cubic metres. Suppliers price ready-mix per m³ delivered. Give them the figure with waste, not the bare volume, and ask how they round part loads.
- Ask about minimum loads and part-load charges. Small orders can carry an extra charge, so a 1 m³ pour may cost more per cubic metre than a 3 m³ pour.
- Mix-on-site lorries mix what you use and charge for what is poured, which suits jobs where the volume is hard to pin down.
- Plan access and labour. Check how close the lorry can get, whether you need a barrow run or a pump, and have enough people ready to spread and level before it sets.
- Prepare the base first. A slab normally sits on 75 to 150 mm of compacted hardcore or MOT Type 1. Work out the sub-base with the gravel calculator.
For a house extension, garage or anything structural, follow your engineer's specification and check with building control, as Building Regulations may apply.
Try the concrete calculator (m³ and bags)
Enter your own numbers below, or open the full concrete calculator (m³ and bags) with the formula and more examples.
This guide is general information with example numbers, not financial, tax or building advice. Your costs and results will differ. Terms
Frequently asked questions
How much concrete do I need for a 4 x 3 m slab?
At 100 mm thick it is 1.2 m³, or 1.32 m³ with 10% waste: about 3.17 tonnes or 120 bags of 25 kg mix. At 150 mm it is 1.98 m³ with waste.
How many square metres does 1 m³ of concrete cover?
10 m² at 100 mm thick, 6.67 m² at 150 mm and 13.33 m² at 75 mm, before waste.
How much concrete for a 6 x 6 m slab?
3.96 m³ at 100 mm or 5.94 m³ at 150 mm, both including 10% waste.
Should I use bags or ready-mix for a slab?
Bags suit small bases up to about half a cubic metre. From about 1 m³, ready-mix or a mix-on-site lorry is usually cheaper and much less work.