How the concrete calculator works
Concrete is ordered and measured by volume in cubic metres (m³). For a slab, base or strip footing, the volume is simply length × width × depth, all in metres. For a round post hole it's the area of the circle multiplied by the depth:
slab or footing: volume = length × width × (depth in mm ÷ 1,000)
post hole: volume = 3.1416 × (diameter ÷ 2)² × depth × number of holes
The calculator then adds an allowance for waste, spillage and uneven ground, and converts the volume into bags of ready-mixed (dry) concrete.
How many bags of concrete per cubic metre?
Bagged ready-mix is a dry blend of cement, sand and aggregate that you mix with water. We assume you need about 2,200kg of dry mix for every cubic metre of finished concrete. On that basis:
| Bag size | Approximate yield per bag | Bags per m³ |
|---|---|---|
| 20kg | 0.009m³ (9 litres) | 110 |
| 25kg | 0.011m³ (11 litres) | 88 |
Different brands quote slightly different yields, roughly 0.009 to 0.011m³ for a 20kg bag, so check the bag and change the figure under "Bag yield assumption" if yours is different. A higher yield per bag means fewer bags.
How deep should a concrete slab be?
These are common rules of thumb for DIY work. For anything structural, such as house extensions or garage foundations, follow your building control officer or engineer.
| Job | Typical concrete depth |
|---|---|
| Garden shed base | 75 to 100mm, on 75 to 100mm of compacted hardcore |
| Patio or path base | 100mm |
| Driveway or garage floor | 100 to 150mm, more for heavy vehicles |
| Fence post holes | About a quarter of the post's length (600mm for a 2.4m post) |
Worked example: a 3m × 3m shed base
- Volume: 3 × 3 × 0.1 = 0.9m³
- Add 10% extra: 0.9 × 1.1 = 0.99m³
- Dry mix: 0.99 × 2,200 = 2,178kg
- 20kg bags: 2,178 ÷ 20 = 108.9, rounded up to 109 bags (or 88 bags of 25kg)
That's more than two tonnes to carry and mix, which is why most people order ready-mixed concrete for anything approaching a cubic metre.
Post holes example
Four post holes 300mm wide and 600mm deep hold 3.1416 × 0.15² × 0.6 × 4 = 0.17m³. With 10% extra that's 0.187m³, or 411kg of dry mix: 21 bags of 20kg or 17 bags of 25kg. If you're using rapid-setting post mix instead, check the bag, as it's usually sold by the number of bags per post.
Bags, mixer or ready-mix lorry?
- Under about 0.25m³ (post holes, small pads): bags mixed in a wheelbarrow or bucket are fine.
- 0.25m³ to 1m³: hire a cement mixer, or buy a builder's bulk bag of ballast and bags of cement to mix your own, which is usually cheaper than bagged ready-mix.
- Over 1m³: get quotes for ready-mixed concrete delivered by lorry or mixed on site. Check there's access for the lorry, or book a concrete pump.
Tips for a good result
- Dig out and compact a hardcore sub-base before pouring a slab, and set up timber formwork to the finished level.
- Add the water gradually. Too much makes concrete weaker and more likely to crack.
- Don't pour if frost is forecast, and keep new concrete damp for the first few days in hot weather.
- Large slabs need expansion joints. A common guide is every 3 to 4m in each direction.
Laying a patio or floor on top? The tile calculator works out tiles and boxes, and the flooring calculator handles laminate and vinyl. For the shed walls, the paint calculator has you covered.
Frequently asked questions
How much concrete do I need for a 3m × 3m slab?
At 100mm deep a 3m × 3m slab needs 0.9m³ of concrete, or about 0.99m³ with 10% extra. That's around 109 bags of 20kg ready-mix, assuming 2,200kg of dry mix per m³.
How many 20kg bags of concrete make 1 cubic metre?
About 110 bags, if each 20kg bag yields around 0.009m³. Brands vary between about 0.009 and 0.011m³ per 20kg bag, so check the yield printed on the bag.
How many bags of concrete do I need for a fence post?
A 300mm-wide hole 600mm deep holds about 0.042m³ of concrete. That's about 94kg of dry mix, so 5 bags of 20kg ready-mix per post, or 6 if you add 10% extra.
How do I convert mm depth to metres?
Divide by 1,000. A 100mm slab is 0.1m deep and a 150mm slab is 0.15m deep. Multiply length × width × depth in metres to get cubic metres.
Is it cheaper to buy bags or ready-mixed concrete?
For small jobs, bags are simplest. For about 1m³ or more, a delivered ready-mix or mix-on-site supplier is usually cheaper than over 100 bags and saves hours of mixing.
Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.