Cement Calculator
Find how many bags of cement and bulk bags of ballast you need to mix your own concrete.
Measure your job
Your answer
For 0.99 m³ of concrete, mixed 1 : 6 ballast.
Bulk bags are 800 kg. Bags are rounded up.
This answer is a guide. For footings, walls or anything that holds weight, check your local building rules or ask a builder or engineer.
Which costs less?
Check the supplier's smallest order and delivery charge first.
UK middle prices from October 2026, with VAT. Cement: £8.39 a 25 kg bag (7 prices from 3 sellers). Bulk bags: £75, building sand £67.35 (3 sellers each). Few prices, so treat them as a rough guide.
Quick answers
- Cement in 1 m³ (1 : 6 ballast)
- About 295 kg, or 12 bags of 25 kg. Plus about 1,865 kg of ballast.
- Cement in 1 m³ (1 : 4 ballast)
- About 15 bags of 25 kg, using the ST5 row (our rule).
- Cement for a 3 × 3 m slab (100 mm)
- With 10% extra: 12 bags of cement and 3 bulk bags of ballast.
- Concrete from one bulk bag
- About 0.43 m³ from an 800 kg bulk bag, by the British Standard figures.
How to work it out
- Work out the m³: length × width × depth, all in metres.
- Add 10% extra for spills.
- For 1 : 6 ballast, multiply the m³ by 295 kg of cement.
- Divide by 25 to get bags. Round up.
- Multiply the m³ by 1,865 kg for the ballast. Divide by 800 for bulk bags.
3 × 3 × 0.1 = 0.9 m³ + 10% = 0.99 m³
0.99 × 295 = 292 kg ÷ 25 → 12 bags
0.99 × 1,865 = 1,846 kg ÷ 800 → 3 bulk bags
Cement and ballast for 1 m³
Cement and stone come from the British Standard table (BS 8500-2), for 20 mm stone. The row for each mix, and the sand and gravel split, are our own rule.
| Mix | Cement bags (25 kg) | Bulk bags (800 kg) | Row (our rule) |
|---|---|---|---|
| 1 : 6 ballast | 11.8 | 2.3 | ST3 |
| 1 : 5 ballast | 13.2 | 2.3 | ST4 |
| 1 : 4 ballast | 15 | 2.3 | ST5 |
| 1 : 2 : 4 sand and gravel | 11.8 | 0.8 + 1.6 | ST3 |
| 1 : 2 : 3 sand and gravel | 13.2 | 0.9 + 1.4 | ST4 |
Sand + gravel: two bulk bag counts. No extra added. Round up only your final total.
By our rule, 1 : 6 ballast and 1 : 2 : 4 share row ST3, so they take the same cement.
Why other guides say fewer bags
| Source | Bags (25 kg) |
|---|---|
| British Standard | 11.8 |
| Cemex | 8–10 |
| Concrete Society | 5 |
Cemex says 1 m³ of concrete weighs about 2.4 tonnes. The British Standard's cement and stone weigh 2,160 kg. Add the water, and that comes close.
The Concrete Society's DIY table adds up to only 1,525 kg, water included. By our sums, that makes about 0.64 m³, so you could run about 36% short.
Cemex gives only the cement, so we can't check its figure this way.
Mainland Aggregates, a merchant, backs the British Standard. It says an 850 kg bulk bag makes about 0.5 m³; we get 0.46.
Gilmore Building Supplies says about 6 bags of cement per bulk bag; we get 5.4 for 850 kg. But its guide also says 7–8 bags per m³ for 1 : 2 : 4. That does not match its own bulk bag figure.
So we use the British Standard. It is not free, so we read Pavingexpert's copy of BS 8500-2:2015+A1:2016. We have not checked the newer 2023 edition.
Which mix for which job
| Mix | Use it for | Or sand + gravel |
|---|---|---|
| 1 : 6 ballast | Footings and fence posts | 1 : 2.5 : 3.5 |
| 1 : 5 ballast | Shed or garage base, house floors | 1 : 2 : 3 |
| 1 : 4 ballast | Paths, drives and steps | 1 : 1.5 : 2.5 |
Sources differ, though. The Concrete Society pairs 1 : 2 : 3 with 1 : 5 ballast. Heidelberg's cement data sheet pairs it with 1 : 4.
Pavingexpert's copy of the standard allows ballast only up to ST3. For 1 : 5 ballast and 1 : 4 ballast, sharp sand and gravel are better.
- In the calculator, 1 : 2 : 3 sand and gravel matches 1 : 5 ballast (ST4).
- There is no sand and gravel match for 1 : 4 ballast (ST5).
For footings, drives or anything that holds weight, check your local building rules or ask a builder or engineer.
Mix your own, bags or a lorry?
| Concrete | Mix your own | 20 kg bags | Lorry |
|---|---|---|---|
| 0.1 m³ | £92 | £110 | £128 |
| 0.25 m³ | £100 | £275 | £128 |
| 0.5 m³ | £200 | £550 | £128 |
| 1 m³ | £326 | £1,100 | £128 |
| 2 m³ | £576 | £2,200 | £256 |
Middle prices. 20 kg bags at £11, from only 3 sellers. Lorry at £128 per m³ (4 suppliers), from 1 m³, Rock Solid's smallest home order. No delivery added.
Mixing your own costs least from about 0.08 to 0.42 m³. One bulk bag of ballast makes about 0.43 m³. Above that, a lorry costs less at our prices.
Check the lorry terms first
Mister Concrete's minimum is £625 plus VAT; at that price, mixing 1 m³ yourself costs less. Our concrete calculator covers part loads and delivery.
Cement and ballast prices each come from only 3 sellers, so treat them as a rough guide. Our sums leave out mixer hire.
Questions people ask
How many bags of cement do I need for 1 m³ of concrete?
About 12 bags of 25 kg for 1 : 6 ballast, from the British Standard table. Cemex and the Concrete Society give fewer, but only the Concrete Society's fail our weight check.
How much ballast do I need for 1 m³ of concrete?
About 1,865 kg for 1 : 6 ballast. That is 2.3 bulk bags of 800 kg; round up to 3.
How many bags of cement per bulk bag of ballast?
About 5.4 for an 850 kg bulk bag at 1 : 6 ballast, or 5.1 for 800 kg. Gilmore Building Supplies, whose bags weigh 850–1,000 kg, says about 6.
How much cement for 1 tonne of ballast?
About 6.3 bags of 25 kg at 1 : 6 ballast, by the British Standard figures.
Which ballast mix should I use?
The Concrete Society suggests 1 : 6 ballast for footings, and 1 : 5 for a shed or garage base. Heidelberg's cement data sheet gives 1 : 4 ballast for concrete, so sources differ. Check your local building rules or ask a builder or engineer.
Is it cheaper to mix your own concrete?
At our middle prices, mixing your own costs least for about 0.08–0.42 m³. Above that, a ready-mix lorry costs less. Ask each firm about minimum orders and delivery fees before you decide.
How much is a bag of cement?
About £8.39 for 25 kg, with VAT. We took the middle of 7 prices, but from only 3 sellers. Treat it as a rough guide.
Where our numbers come from
- Cement and stone per m³: BS 8500-2 Table 10 (as copied by Pavingexpert)
- Concrete weight: Cemex concrete FAQs (about 2.4 tonnes per m³)
- Concrete per bulk bag: Mainland Aggregates (850 kg of ballast makes about 0.5 m³)
- Mix ratios and cement per bulk bag: Gilmore Building Supplies guide
- DIY mixes: Concrete Society, Batching by volume
- Cement in each mix: Cemex ST mixes (kg of cement per m³)
- Mix ratios and 25 kg bags: Heidelberg General Purpose cement data sheet
- Cement price: the middle of 7 prices from 3 UK sellers (25 kg, Oct 2026)
- Bulk bag prices: the middle price from 3 UK sellers each (Wickes, Tippers, Lawsons; Oct 2026)
- Ballast weight: Wickes ballast bulk bag (800 kg covers 9.7 m² at 50 mm)
- Bag size: Blue Circle Multi-Purpose Concrete data sheet (20 kg makes about 0.01 m³)
- Bag price: the middle of 4 prices from 3 UK stores for 20 kg concrete (Oct 2026)
- Ready-mix price: the middle of 7 prices from 4 UK suppliers (Oct 2026, with VAT)
- Smallest order: Rock Solid Concrete price guide (1 m³ for homes)
- Minimum charge: Mister Concrete price page (£625 + VAT)
- Extra for spills: Sakrete slab calculator (10%)
Made by BuildersCount. Last checked 3 Oct 2026.
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