Ballast calculator: tonnes per m² 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
Worked example
| You enter | Value |
|---|---|
| What is the ballast for? | Mixing concrete |
| Area | rectangle 4 m × 2.5 m |
| Depth | 100 mm |
| Mix: parts ballast to 1 part cement | 5 parts |
| Waste allowance | 0% |
| Dry-volume factor | 1.54 × |
| Ballast density | 1.8 t/m³ |
| Cement density | 1,440 kg/m³ |
| Cement bag size | 25 kg |
| Bulk bag size | 0.85 t |
| Small bag size | 25 kg |
Order: 15 × cement bags and 2.31 t of ballast
| Result | Value |
|---|---|
| Ballast bulk bags | 3 |
| Ballast | 2.31 t |
| Ballast per m² | 0.231 t/m² |
| Or small bags of ballast | 93 |
| Cement bags | 15 |
| Cement | 369.6 kg |
| Concrete (wet volume, with waste) | 1 m³ |
| Area | 10 m² |
| Depth | Ballast (t/m², no waste) |
|---|---|
| 50 mm | 0.116 |
| 75 mm | 0.173 |
| 100 mm | 0.231 |
| 150 mm | 0.346 |
Show the working
- Area: 10 m²; depth 100 mm, so 1 m³ before waste
- Wet concrete: 1 × 1 (0% waste) = 1 m³
- Dry materials: 1 × 1.54 = 1.54 m³
- Cement: 1.54 ÷ (1 + 5) × 1,440 kg/m³ = 369.6 kg
- Ballast: 1.54 × 5 ÷ (1 + 5) × 1.8 t/m³ = 2.31 t
- Cement bags: 369.6 kg ÷ 25 kg, rounded up = 15
- Per m²: 2.31 t ÷ 10 m² = 0.231 t/m²
- Bulk bags: 2.31 t ÷ 0.85 t, rounded up = 3
How it works
- Loose layer: ballast (t) = area (m²) × depth (m) × ballast density (t/m³) × (1 + waste)
- Concrete: wet volume = area × depth × (1 + waste); dry volume = wet × dry-volume factor
- Concrete: cement (kg) = dry ÷ (1 + r) × cement density; ballast (t) = dry × r ÷ (1 + r) × ballast density, for a 1 : r mix
- Bags = tonnes ÷ bag size, rounded up
Mix ratios are typical mixes quoted by cement makers, not strength classes. For footings or anything structural, follow the engineer's or cement maker's specification.
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 |
|---|---|---|---|
| Waste allowance: Mixing concrete | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Uneven ground and spillage. |
| Waste allowance: A loose layer | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Spreading and settling; compaction varies. |
| Mix: parts ballast to 1 part cement | 5 parts ballast to 1 cement | Our assumption Not yet checked against the source | Our choice: change it to suit your job A mix quoted by cement makers for general concrete (brief v2 G1). Check the cement maker’s or engineer’s spec; this is not a strength class. |
| Dry-volume factor | 1.54 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job Dry materials shrink when water is added (brief v2 G1). Researcher to check against Hanson, Tarmac or Blue Circle mixing guidance. |
| Ballast density | 1.8 t/m3 Sources range 1.7–1.9 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) |
| Cement density | 1440 kg/m3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Typical planning figure with no source yet. Researcher to source it and reclassify as typical. |
| Cement bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common UK cement bag. |
| Bulk bag size | 0.85 t | Common UK size Not yet checked against the source | builder.tools bulk bag figure (checked 19 October 2026) Varies by merchant; check yours. |
| Small bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common small bag. |
Regulations and standards
- BS 8500 (concrete): mix ratios here are not strength classes
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.