Concrete calculator: m³, tonnes and bags
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Work out the volume, weight and number of bags of concrete for a slab, footing or post holes. Enter your measurements below; every figure we start from is shown and you can change it.
Worked example
| You enter | Value |
|---|---|
| What are you pouring? | Slab or pad |
| Length | 3 m |
| Width | 3 m |
| Depth | 100 mm |
| Density | 2.4 t/m³ |
| Waste allowance | 5% |
| Bag size | 20 kg |
Order: 114 bags
| Result | Value |
|---|---|
| Bags | 114 |
| Weight | 2.27 t |
| Volume (with waste) | 0.945 m³ |
Show the working
- Slab: 3 × 3 × 0.1 m = 0.9 m³
- With 5% waste: 0.945 m³; weight 0.945 × 2.4 t/m³ = 2.27 t
- Bags: 2,268 kg ÷ 20 kg = 113.4, rounded up = 114
How it works
- Slab or footing volume (m³) = length × width × depth
- Post holes (m³) = holes × (hole area − post area) × depth
- With waste: m³ × (1 + waste); weight (t) = m³ × density
- Bags = weight (kg) ÷ bag size (kg), rounded up; or volume (L) ÷ the yield printed on the bag, rounded up
Ready-mix: ask your supplier about minimum loads.
We don't give mix strengths. Follow the maker's instructions for your job.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Density | 2.4 t/m3 | Typical value Not yet checked against the source | tradecalculator.co.uk m³-to-tonnes converter (checked 19 October 2026) Supplier's figure takes precedence. |
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Uneven ground and spillage. |
| Bag size | 20 kg | Common UK size Not yet checked against the source | From your product Bags of 20 and 25 kg are common. |
Regulations and standards
- BS 8500 (concrete): this page gives volumes and bags, not strength classes or mix designs
Explained: BS 8500 (concrete).
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.
FAQs
Guides
How much concrete for fence posts?
Hole size from the post maker, post-fix vs mixed concrete, and how many bags to buy for a row of fence posts.
How to measure a concrete slab
Measure length, width and depth, keep mm and m apart, then choose bags or ready-mix. How much extra to allow for waste.
Working safely with cement
Cement can burn skin and eyes and cutting dust harms lungs. What HSE guidance says about gloves, eye protection and dust.