Concrete Calculator
Concrete is bought by volume, not by area, so depth is part of the order. This calculator turns your dimensions into cubic metres, cubic yards and an estimated bag count, with a waste allowance applied.
Formula
Volume = length × width × depth
- Keep every measurement in metres to get cubic metres
- A post hole or column is π × radius² × depth
A 3 × 2 m slab at 100 mm: 3 × 2 × 0.1 = 0.6 m³
How to Measure Your Pour
For any rectangular pour — slab, base, path or strip footing:
Volume = Length × Width × Depth
Convert the depth to the same unit as the other two before multiplying. Depth is usually quoted in millimetres while length is in metres, and multiplying them as they stand puts the answer out by a factor of a thousand.
A 4 m × 3 m patio at 100 mm is 4 × 3 × 0.1 = 1.2 m³, or 1.32 m³ once 10% waste is added.
Round Post Holes
Circular pours use the area of a circle instead:
Volume = π × radius² × depth
Four post holes 300 mm across and 600 mm deep come to 4 × (3.14159 × 0.15² × 0.6) = 0.17 m³.
Remember the radius is half the diameter. Using the diameter quadruples the answer.
Bags or Ready-Mix?
A 25 kg bag yields roughly 0.012 m³, so a cubic metre takes about 85 bags. That 4 m × 3 m patio would need around 113 bags — over a tonne to carry and mix by hand.
- Under 0.2 m³: bags are fine
- 0.2 to 0.5 m³: possible with a mixer
- Over 0.5 m³: order ready-mix, usually cheaper as well as easier
Quick Reference
| Slab size | 75 mm deep | 100 mm deep | 150 mm deep |
|---|---|---|---|
| 2 × 2 m | 0.30 m³ | 0.40 m³ | 0.60 m³ |
| 3 × 2 m | 0.45 m³ | 0.60 m³ | 0.90 m³ |
| 4 × 3 m | 0.90 m³ | 1.20 m³ | 1.80 m³ |
| 6 × 4 m | 1.80 m³ | 2.40 m³ | 3.60 m³ |
Frequently Asked Questions
- How much waste should I allow?
- Between 5% and 10%. Use the lower end for a small, tightly formed pour and the higher end for anything dug into open ground, where trenches over-dig and the base is uneven.
- What happens if I order too little?
- Concrete starts curing immediately, so a pause between deliveries creates a cold joint: a permanent line of weakness that cracks. Always round up, because spare concrete is far cheaper than a failed slab.
- What strength concrete do I need?
- C20 to C25 covers general domestic work and shed bases, C25 to C30 suits foundations and slabs, and C30 to C35 is used for driveways carrying vehicle weight. Any engineer specification overrides these rules of thumb.
- How many bags of concrete are in a cubic metre?
- About 85 bags of 25 kg dry mix, since each yields roughly 0.012 cubic metres once mixed. Check your supplier's stated yield, as it varies between products.