Overview
Ballast is a sand-and-gravel blend — cement and water turn it into concrete, so the ratio drives the final strength.
How the calculation works
Ballast weight = area × slab thickness × 1.7 t/m³ before any allowance — a 3m × 3m shed slab at 100mm needs 9 × 0.1 × 1.7 = 1.53 tonnes.
Common site mixes by volume are around 1 part cement to 5-6 parts ballast for paths and shed bases, or 1:4 for stronger sections; ready-mix strength grades replace guesswork on bigger pours.
Bag maths: 25kg bags of ballast mean a 1.5-tonne slab is about 60 bags plus cement — worth comparing against an 850kg bulk bag delivery.
Worked example
A shed base 3m × 2.5m at 100mm thick, using a 1:5 cement-to-ballast mix.
Volume = 7.5 m² × 0.1 = 0.75 m³. Ballast = 0.75 × 1.7 = 1.28 tonnes; with 10% allowance, about 1.4 tonnes (2 bulk bags).
A 1:5 volume ratio with ~1,450kg/m³ ballast density suggests roughly 7 × 25kg bags of cement for this pour.
Ballast at a glance
- Planning density1.70 t/m³
- Typical range1.60–1.80 t/m³
- CategorySand
- CompactionUsually not applied
- SourcePlanning value based on typical UK all-in ballast bulk densities
- Reviewed2026-08-16
Planning values for typical UK supplies; your supplier product data takes precedence.
Ballast coverage guide
| Depth | Area per tonne | Area per 850kg bag | Weight per m² |
|---|---|---|---|
| 25mm | 23.5 m² | 20.0 m² | 43 kg |
| 40mm | 14.7 m² | 12.5 m² | 68 kg |
| 50mm | 11.8 m² | 10.0 m² | 85 kg |
| 75mm | 7.8 m² | 6.7 m² | 128 kg |
| 100mm | 5.9 m² | 5.0 m² | 170 kg |
| 150mm | 3.9 m² | 3.3 m² | 255 kg |
Computed from a 1.70 t/m³ planning density on level ground.
Assumptions and accuracy
1.7 t/m³ is a planning density for typical all-in ballast; grading varies between suppliers.
Mix ratios above are general site guidance, not a specification for structural concrete.
For structural concrete, follow the specified mix design and confirm quantities with your supplier or engineer.
Planning density values
Use supplier product data whenever available. These figures are planning defaults shared by every calculator on the site.
| Material | Category | Planning density | Typical range | Compaction |
|---|---|---|---|---|
| MOT Type 1 | Sub Base | 2.20 t/m³ | 2.00–2.30 t/m³ | Relevant |
| MOT Type 3 | Sub Base | 2.10 t/m³ | 1.90–2.20 t/m³ | Relevant |
| Recycled Type 1 | Recycled | 2.00 t/m³ | 1.80–2.20 t/m³ | Relevant |
| 6F2 / 6F5 crushed aggregate | Recycled | 2.00 t/m³ | 1.80–2.20 t/m³ | Relevant |
| Road planings | Recycled | 2.00 t/m³ | 1.80–2.20 t/m³ | Relevant |
| Sharp sand | Sand | 1.60 t/m³ | 1.50–1.70 t/m³ | Usually no |
| Building sand | Sand | 1.50 t/m³ | 1.40–1.60 t/m³ | Usually no |
| Kiln-dried sand | Sand | 1.50 t/m³ | 1.40–1.60 t/m³ | Usually no |
| 10mm pea shingle | Gravel | 1.60 t/m³ | 1.40–1.70 t/m³ | Usually no |
| 20mm gravel / shingle | Gravel | 1.60 t/m³ | 1.40–1.70 t/m³ | Usually no |
| Decorative gravel | Decorative | 1.50 t/m³ | 1.30–1.70 t/m³ | Usually no |
| Slate chippings | Decorative | 1.60 t/m³ | 1.40–1.80 t/m³ | Usually no |
| Gabion stone | Gravel | 1.50 t/m³ | 1.40–1.60 t/m³ | Usually no |
| Self-binding gravel | Gravel | 1.90 t/m³ | 1.80–2.10 t/m³ | Relevant |
| Crushed concrete | Recycled | 1.90 t/m³ | 1.70–2.10 t/m³ | Relevant |
| Limestone scalpings | Sub Base | 2.00 t/m³ | 1.80–2.20 t/m³ | Relevant |
| Ballast | Sand | 1.70 t/m³ | 1.60–1.80 t/m³ | Usually no |
| Generic aggregate | General | 1.70 t/m³ | 1.50–2.20 t/m³ | Usually no |
How to use this calculator
- Enter the slab or footing area and thickness.
- Keep ballast selected for all-in ballast, or sharp sand plus gravel if buying separately.
- Add your delivery pricing to compare bulk bags with loose loads.
- Choose the cement ratio with your job's strength needs in mind.
When ballast is commonly used
All-in ballast makes garden slabs, shed bases, path edgings, footings for low-rise walls and fence-post concrete — any job where site-mixed concrete beats ready-mix on cost or access.
Frequently asked questions
How much ballast do I need for 1m³ of concrete?
About 1.4-1.5 tonnes of ballast plus around 230-250kg of cement (roughly 10 × 25kg bags) for a 1:5-1:6 site mix. The calculator applies the same densities to your area and depth.
What ballast to cement ratio should I use?
Around 1:5 or 1:6 by volume suits paths and shed bases; 1:4 gives a stronger mix for footings. Structural work needs a specified mix design, not a site rule of thumb.
How many 25kg bags of ballast in a tonne?
Forty. A bulk bag labelled 850kg equals 34 bags, which is why the calculator shows both formats.
How many bags of cement to a ton of ballast?
For a 1:5 volume mix, a tonne (1,000kg) of ballast pairs with roughly 5 × 25kg bags of cement; a 1:6 mix needs about 4.
How much water should I add to a ballast mix?
Use the minimum that makes the mix workable — typically a water-to-cement ratio around 0.4–0.6 by weight. Wetter mixes are easier to place but weaker. Add gradually; you can't take it back out.
References and source notes
- Typical UK all-in ballast bulk density from supplier technical data (planning value).