Concrete Calculator
Ordering too little concrete stops a pour halfway; ordering too much wastes money. This concrete calculator gives you the exact volume for slabs, footings, driveways and posts from three measurements: length, width and thickness.
Enter your dimensions in metric or imperial units, add a waste allowance, and optionally a price per cubic metre or yard to budget the pour. Results update instantly.
Enter valid values to see the results.
Formula
All dimensions must be in the same unit. Metric result is in cubic metres (m³); imperial result is converted from cubic feet to cubic yards (1 yd³ = 27 ft³). A 25 kg bag of dry ready-mix yields about 0.012 m³; an 80 lb bag yields about 0.6 ft³.
How to measure
- Measure the length and width of the area at its longest and widest points. For L-shaped slabs, split the area into rectangles and calculate each separately.
- Measure thickness at several spots if the ground is uneven — use the average. Typical slabs: 10 cm (4 in) for walkways, 15–20 cm (6–8 in) for driveways.
- For footings and post holes, measure depth below grade and the hole diameter or side length.
Worked example
Practical tips
- For volumes above 1.5–2 m³ (2 yd³), ready-mix delivery is almost always cheaper and faster than bagged concrete.
- Compact and level the sub-base before pouring — soft spots cause cracking and increase the volume needed.
- Order 5–10% extra; spillage, uneven ground and formwork tolerances always consume more than the drawing says.
- Pour within 90 minutes of mixing and keep the slab damp for at least 3 days while it cures.
Waste allowance
Add 5–10% for slabs on well-prepared, level ground and up to 15% for footings dug into rough soil, where walls are never perfectly straight.
Recommended slab thickness by application
| Application | Metric | Imperial |
|---|---|---|
| Walkway / garden path | 10 cm | 4 in |
| Patio / shed base | 10–15 cm | 4–6 in |
| Driveway (cars) | 15–20 cm | 6–8 in |
| Garage floor | 15–20 cm | 6–8 in |
| Strip footing | 20–40 cm | 8–16 in |
Common mistakes
- 1 Mixing units — entering thickness in centimetres but length in metres without converting (10 cm = 0.1 m).
- 2 Forgetting waste: a perfectly level excavation is rare, so the real pour is almost always bigger than the drawing.
- 3 Using slab math for post holes — a round hole needs the cylinder formula (π × r² × depth).
- 4 Underestimating bag counts: one 25 kg bag yields only ~12 litres, so a cubic metre needs roughly 85 bags.
Frequently asked questions
How many 25 kg bags of concrete make 1 m³?
About 85 bags. A 25 kg bag of dry mix yields roughly 0.012 m³ (12 litres) of cured concrete, so 1 ÷ 0.012 ≈ 84–85 bags.
How many cubic yards do I need for a 10×10 slab?
At 4 inches thick: 10 × 10 × (4/12) = 33.3 ft³, which is 33.3 ÷ 27 ≈ 1.23 yd³. Add 10% waste and order about 1.4 yd³.
Should I order ready-mix or use bags?
Bags make sense up to about 0.5 m³ (0.65 yd³). Between 0.5 and 2 m³ compare prices; above 2 m³ a ready-mix truck is almost always cheaper per cubic metre and saves a day of mixing.
How thick should a concrete driveway be?
15–20 cm (6–8 in) for passenger cars on a compacted gravel base. Go to 20 cm or add reinforcement if heavy vehicles will use it.
Does the calculator include reinforcement?
No. Rebar or mesh does not change the concrete volume meaningfully, so you can ignore it for ordering purposes.