About this calculator
Ordering concrete for a slab, path, shed base or post hole? Enter the length and width in metres and the depth in millimetres, and you get the volume to order in cubic metres.
The calculator adds 10% extra by default for spillage, uneven ground and over-digging (change it as you like). For a round base choose round and enter the diameter as the width.
Worked examples
Real numbers, worked out by the same calculator. Press “Use these numbers” to try one above.
4 m × 3 m slab, 100 mm deep
- Order (with 10% extra)
- 1.32 m³
- Exact volume
- 1.2 m³
- Litres
- 1,320
- Weight (about 2,400 kg per m³)
- 3.17 tonnes
- Cubic yards
- 1.73
A 4 m × 3 m slab, 100 mm deep, is 1.2 m³. Order about 1.32 m³ to allow 10% for waste and uneven ground.
Show the working
- Volume = length × width × depth = 4 × 3 × 0.1 (depth in metres) = 1.2 m³.
- Add 10% for spillage, uneven ground and over-excavation: 1.2 × 1.1 = 1.32 m³.
- Set concrete weighs about 2,400 kg per m³, so 1.32 m³ is about 3.168 tonnes.
Garden path 10 m × 1 m, 75 mm deep
- Order (with 10% extra)
- 0.825 m³
- Exact volume
- 0.75 m³
- Litres
- 825
- Weight (about 2,400 kg per m³)
- 1.98 tonnes
- Cubic yards
- 1.08
A 10 m × 1 m slab, 75 mm deep, is 0.75 m³. Order about 0.825 m³ to allow 10% for waste and uneven ground.
Show the working
- Volume = length × width × depth = 10 × 1 × 0.075 (depth in metres) = 0.75 m³.
- Add 10% for spillage, uneven ground and over-excavation: 0.75 × 1.1 = 0.825 m³.
- Set concrete weighs about 2,400 kg per m³, so 0.825 m³ is about 1.98 tonnes.
Round base 1 m across, 150 mm deep
- Order (with 5% extra)
- 0.124 m³
- Exact volume
- 0.118 m³
- Litres
- 124
- Weight (about 2,400 kg per m³)
- 0.3 tonnes
- Cubic yards
- 0.16
A round slab 1 m across, 150 mm deep, is 0.118 m³. Order about 0.124 m³ to allow 5% for waste and uneven ground.
Show the working
- Volume = π × (diameter ÷ 2)² × depth = π × (1 ÷ 2)² × 0.15 = 0.11781 m³.
- Add 5% for spillage, uneven ground and over-excavation: 0.11781 × 1.05 = 0.1237 m³.
- Set concrete weighs about 2,400 kg per m³, so 0.1237 m³ is about 0.296881 tonnes.
How it's calculated
Volume = length × width × depth, with the depth converted from millimetres to metres. A round base uses π × (diameter ÷ 2)² × depth. Then the waste allowance is added. Set concrete weighs roughly 2,400 kg per cubic metre.
Typical depths
- Garden path or light patio: 75 to 100 mm
- Shed base: 100 mm
- Garage or driveway: 100 to 150 mm, often with reinforcement
- Post holes and footings: measure the hole carefully
Check building regulations and get advice for anything structural.
Ordering tips
Ready-mix lorries usually have a minimum load and charge by the cubic metre, so order slightly over and check the ground. For small jobs, bagged mix is sold by volume yield on the bag. Compare the litres figure with the yield printed on the bag to count how many you need.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length by width by depth (in metres), then add 10% for waste.
How heavy is concrete?
About 2,400 kg per cubic metre, so a 4 m × 3 m slab 100 mm deep is about 3 tonnes.
How much extra should I allow?
10% is a common allowance. Use more on uneven ground.
Can I do a round base?
Yes: choose round and enter the diameter in the length box.
Is this a mix recipe?
No. It gives the finished volume. Mix proportions depend on the strength you need.
Formulas tested against hand-worked answers. Last reviewed 29 September 2026. These calculators do arithmetic only; they are not financial, tax or legal advice.