Concrete Calculator
Enter your dimensions and get the volume in cubic metres, cubic yards and cubic feet, the weight, and how many bags of concrete to buy. Every figure includes a wastage allowance, because running short mid-pour is the one mistake you cannot undo.
How to calculate concrete
Concrete is sold by volume, and volume is length times width times depth. The arithmetic is easy. The part people get wrong is mixing their units.
Volume = Length × Width × Thickness
Convert everything to one unit before you multiply. A slab 4 m long, 3 m wide and 150 mm thick is 4 × 3 × 0.15 = 1.8 m³ — note the 150 mm became 0.15 m. Working in feet: a 10 ft × 10 ft slab at 4 inches is 10 × 10 × 0.333 = 33.3 ft³, and since there are 27 cubic feet in a cubic yard, that is 33.3 ÷ 27 = 1.23 cubic yards.
Then add 5 to 10 per cent. Sub-bases are never perfectly level, formwork bulges, and some concrete stays in the barrow. Ten per cent extra costs a few pounds. Stopping a pour to wait for a second delivery costs you a cold joint through the middle of the slab.
Quick reference: common slab sizes
| Slab size | Thickness | Volume | 80 lb bags | 25 kg bags |
|---|---|---|---|---|
| 8 × 8 ft (2.4 × 2.4 m) | 4 in (100 mm) | 0.79 yd³ / 0.6 m³ | 36 | 52 |
| 10 × 10 ft (3 × 3 m) | 4 in (100 mm) | 1.23 yd³ / 0.94 m³ | 56 | 81 |
| 12 × 12 ft (3.6 × 3.6 m) | 4 in (100 mm) | 1.78 yd³ / 1.36 m³ | 80 | 117 |
| 20 × 20 ft (6 × 6 m) | 5 in (125 mm) | 6.17 yd³ / 4.72 m³ | 278 | 407 |
| 24 × 24 ft (7.3 × 7.3 m) | 6 in (150 mm) | 10.7 yd³ / 8.2 m³ | 481 | 704 |
Anything past about 30 bags is a hard day’s mixing. That is usually the point to start pricing ready-mix instead.
Pick the right calculator
A trench is not a slab and a staircase is neither. Each tool below is shaped around the job:
- Concrete Slab CalculatorSlabs, pads, floors & shed bases
- Concrete Footing CalculatorStrip & pad footings, foundations
- Concrete Wall CalculatorPoured walls & stem walls
- Concrete Column CalculatorColumns, sonotubes & piers
- Concrete Stairs CalculatorSteps & staircases
- Curb & Gutter CalculatorCurbs, gutters & edging
- Concrete Bag CalculatorHow many bags do you need?
- Concrete Cost CalculatorReady-mix & bagged prices
- Mix Ratio CalculatorCement, sand & aggregate quantities
- Concrete Weight CalculatorWeight per m³, yard & cubic foot
- Ballast CalculatorBallast & cement for concrete (UK)
- Sand & Cement CalculatorMortar for patios & bricklaying
- Concrete Block CalculatorBlocks, bricks & mortar for walls
- Post Hole CalculatorFence posts & postcrete
Frequently asked questions
How much concrete do I need?
Multiply length by width by depth in matching units, then add 5 to 10 per cent for waste. The calculator above does the conversion for you and shows the result in cubic metres, cubic yards and cubic feet along with the bag count.
How many bags of concrete make a cubic yard?
45 bags at 80 lb, 60 at 60 lb, or 90 at 40 lb. For a cubic metre you need roughly 86 bags of 25 kg premix. Past about three-quarters of a cubic yard, ready-mix is cheaper and considerably less work.
How much does concrete cost?
Ready-mix runs £100 to £150 per cubic metre in the UK and $125 to $180 per cubic yard in the US at 2026 prices. Bagged concrete works out at roughly double that per unit volume. The cost calculator compares the two for your project.
How thick should a concrete slab be?
100 mm (4 in) covers paths, patios and shed bases. Driveways with cars want 100 to 125 mm. Go to 150 mm for vans, garage floors or anything carrying point loads. Thickening the edges matters more than most people realise, since that is where slabs crack first.
How much does a cubic metre of concrete weigh?
About 2,400 kg, or 2.4 tonnes. A cubic yard comes to roughly 4,050 lb — just over two US tons. Lightweight mixes drop to around 1,750 kg/m³.
What is the difference between cement and concrete?
Cement is the grey powder that binds everything together. Concrete is the finished material: cement plus sand, plus coarse aggregate, plus water. You buy cement to make concrete, or buy it ready-mixed with everything already in there.
Does this calculator work in metric and imperial?
Yes. Every field takes mm, cm, m, inches, feet or yards, and results come back in all three volume units. The site detects whether you are in the US, UK or Europe and sets the defaults, currency and bag sizes to match — the flags in the header switch it manually.
Should I mix my own or order ready-mix?
Under half a cubic metre, mix your own. Over one cubic metre, order it in. Between the two it comes down to access and how much you value your weekend. Volumetric trucks, which mix on site and bill only for what you use, fill that gap well.