Ballast Calculator
Find how much ballast you need, in tonnes, bulk bags or small bags.
Measure your project
Your answer
That is 1.20 m³.
If you buy bags
If you buy bulk bags
For a slab, footing or anything that holds weight, check your local building rules or ask a builder or engineer.
Which costs less?
This price is for picking it up yourself. Add a delivery fee in "Change prices" if they bring it.
With VAT, Oct 2026. Bulk bags: £75, the middle of only 3 sellers. Small bags: £4.10 at Wickes. Treat both as rough guides.
Quick answers
- A 4 × 3 m slab, 100 mm thick
- About 2.24 tonnes of ballast at 1 : 6. That is 3 bulk bags of 800 kg, before any extra.
- Ballast in 1 m³ of concrete
- About 1,865 kg at 1 : 6, from the British Standard table.
- One bulk bag
- 800 kg, about 0.48 m³ as laid. Wickes says it covers 9.7 m² at 50 mm.
- Small bags
- Wickes sells 22.5 kg bags. About 36 of them weigh the same as one bulk bag.
How to work it out
- Measure the length and width in metres. Multiply them for m².
- Measure the thickness of the slab in mm. Divide by 1,000.
- Multiply m² × thickness for the volume of concrete, in m³.
- For 1 : 6 concrete, multiply by 1.865 tonnes of ballast for each m³.
- Divide the tonnes by 0.8 for bulk bags of 800 kg. Round up.
4 × 3 × 0.1 = 1.2 m³ of concrete
1.2 × 1.865 = 2.24 tonnes of ballast
2.24 ÷ 0.8 = 2.8 → 3 bulk bags
Ballast per m³ of concrete
The British Standard table (BS 8500-2) gives the ballast for each m³ of concrete. A richer mix has more cement, so a little less ballast.
| Mix | Ballast | 800 kg bags | 850 kg bags | 22.5 kg bags |
|---|---|---|---|---|
| 1 : 6 | 1,865 kg | 2.33 | 2.19 | 83 |
| 1 : 5 | 1,835 kg | 2.29 | 2.16 | 82 |
| 1 : 4 | 1,800 kg | 2.25 | 2.12 | 80 |
No extra added. Round up only your final total.
Mainland Aggregates says an 850 kg bag makes about 0.5 m³ of concrete. That is about 1,700 kg of ballast per m³, a little less than the table.
For a patio, Cemex lays the slabs on a concrete bed 50–80 mm thick. Pick "To make concrete" and type that thickness.
For the cement to go with it, use our ballast and cement calculator.
Bulk bag of ballast: what it holds, and its coverage
Wickes says its 800 kg bag covers 9.7 m² at 50 mm. So as laid, ballast weighs about 1.65 tonnes per m³. One bag is about 0.48 m³.
| Depth | 800 kg bulk bag | 22.5 kg bag |
|---|---|---|
| 25 mm | 19.4 m² | 0.55 m² |
| 50 mm | 9.7 m² | 0.27 m² |
| 75 mm | 6.5 m² | 0.18 m² |
| 100 mm | 4.8 m² | 0.14 m² |
Using 1.65 tonnes per m³. Wickes's own figure for its small bag is about 0.3 m² at 50 mm.
Wickes calls its bulk bag a jumbo bag. Bags are not all the same size. Wickes and Travis Perkins sell 800 kg; Mainland Aggregates sells 850 kg. Check the weight on yours.
Why sellers' ballast weights differ
Each seller's cover figure gives a slightly different weight for 1 m³ of ballast.
| Seller | What they say | Per m³ |
|---|---|---|
| Wickes, 800 kg bulk bag | 9.7 m² at 50 mm | 1.65 t |
| Travis Perkins, 800 kg bulk bag | About 10 m² at 50 mm | 1.6 t |
| Wickes, 22.5 kg small bag | About 0.3 m² at 50 mm | 1.5 t |
| Mainland Aggregates | Says so directly | About 1.8 t |
Worked out from each seller's bag weight and cover.
Wickes says its bag weight can change by about 5% with how wet the ballast is. We use Wickes's bulk bag figure, 1.65 tonnes, for ballast as laid.
Bulk bags or small bags?
- Bulk bag: about £75 with VAT, from £52.20 to £77.24. That is about £94 a tonne.
- Small bag: Wickes's 22.5 kg bag is £4.10, about £182 a tonne. B&Q's large bag is £4.20, but its weight is not given.
So small bags cost about 1.9 times as much per tonne. Small bag prices come from only 2 sellers.
For a 75 mm shed base, Heidelberg Materials puts the concrete on packed-down hardcore. Our Type 1 calculator works out that layer.
Questions people ask
How much ballast do I need?
For concrete, about 1,865 kg for each m³ at 1 : 6. A 4 × 3 m slab, 100 mm thick, takes about 2.24 tonnes. That is 3 bulk bags of 800 kg.
How many m³ of ballast are in a bulk bag?
An 800 kg bag holds about 0.48 m³ as laid, from Wickes's cover figure. Mixed into concrete at 1 : 6, it makes about 0.43 m³.
How much does a bulk bag of ballast cover?
Wickes says 9.7 m² at 50 mm deep for 800 kg. Travis Perkins says about 10 m² at 50 mm for its 800 kg bag.
How much does a 25 kg bag of ballast cover?
We found no 25 kg bag at Wickes or B&Q. Wickes sells a 22.5 kg bag and says it covers about 0.3 m² at 50 mm deep.
How much ballast per square metre?
As laid, about 83 kg for each m² at 50 mm, or 165 kg at 100 mm. For concrete 100 mm thick, about 187 kg per m².
Which ballast do I need?
For concrete, UK merchants sell all-in ballast: sharp sand and gravel already mixed. Mainland Aggregates sells it as 20 mm all-in ballast. We found no UK guide on when to pick 10 mm.
How much is a bulk bag of ballast?
About £75 with VAT, the middle of only 3 UK sellers. Prices ran from £52.20 to £77.24.
Where our numbers come from
- Ballast weight: Wickes ballast bulk bag (800 kg covers 9.7 m² at 50 mm)
- Cement and stone per m³: BS 8500-2 Table 10 (as copied by Pavingexpert)
- Mainland Aggregates: 850 kg bulk bags
- Ballast cover: Travis Perkins bulk bag (800 kg, about 10 m²)
- Ballast small bag: Wickes 22.5 kg Major Bag £4.10
- Ballast small bag: B&Q Tarmac All-in Ballast £4.20
- Bulk bag prices: the middle price from 3 UK sellers each (Wickes, Tippers, Lawsons; Oct 2026)
- Shed base: Heidelberg Materials guide (75 mm concrete minimum)
- Patio build-up: Cemex guide to garden walls and paving
- Extra for spills: Sakrete slab calculator (10%)
Made by BuildersCount. Last checked 4 Oct 2026.
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