Free UK construction calculators Metric and imperial 2026 UK prices
Work out how much all-in ballast and cement you need for a concrete base, post holes or footings, or how much ballast to order on its own. Get the answer in tonnes, bulk bags and 25 kg bags, with the cement and water to go with it.
Ballast Needed
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Check the measurements above. The result updates once they look right.
| Mix | Ballast | Cement Bags | Typical Use |
|---|
Same volume and extra. Your choice is highlighted.
| Option | Quantity | Notes |
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Bag weights vary with moisture, so round up.
This calculator is for UK DIYers and builders who are mixing their own concrete from all-in ballast and cement, or ordering ballast on its own. It works out the volume, adds an allowance for waste, and turns it into tonnes, bags and cement.
The result shows the ballast in tonnes, bulk bags and 25 kg bags, the cement and water, and a drawing of the job with your measurements.
All-in ballast is a ready-blended mix of sharp sand and gravel, usually up to 20 mm. Because the sand and stone are already combined, you only need to add cement and water to make concrete, rather than buying sand and gravel separately. It is sold by builders' merchants in bulk bags of about 800–850 kg, in small 25 kg bags, and as loose loads by the tonne.
Ballast on its own is also used for bedding, levelling and back-filling. For a driveway or path sub-base, crushed MOT Type 1 is usually better, because its angular stones lock together when compacted.
These are the figures the calculator uses for one cubic metre of finished concrete, before any extra for waste.
| Mix (cement : ballast) | Ballast | Cement | 25 kg bags of cement | Water (approx.) |
|---|---|---|---|---|
| 1:4 | 1.70 tonnes | 360 kg | 15 | 180 litres |
| 1:5 | 1.77 tonnes | 300 kg | 12 | 150 litres |
| 1:6 | 1.82 tonnes | 257 kg | 11 | 130 litres |
| 1:8 | 1.89 tonnes | 200 kg | 8 | 100 litres |
So one cubic metre of 1:5 concrete takes a little over two 850 kg bulk bags of ballast and 12 bags of cement. Put the other way, one bulk bag makes roughly half a cubic metre of concrete, which matches what UK suppliers such as Mainland Aggregates quote for a standard bag.
| Mix | Strength | Good for |
|---|---|---|
| 1:4 | Strong | Driveways, garage bases and anything that takes vehicles |
| 1:5 | General purpose | Shed bases, paths, patios, steps and most garden jobs |
| 1:6 | Medium | Foundations, footings, fence and gate posts |
| 1:8 | Lean | Blinding under slabs and bedding for kerbs and edgings |
The ratio is by volume, so use the same bucket or shovel for both cement and ballast. A richer mix (fewer parts of ballast) is stronger but costs more in cement. For footings to walls and extensions, building control may ask for a specified mix or ready-mixed concrete, so check before you start. Our concrete slab calculator covers ready-mixed concrete orders.
Dry materials take up more room than the finished concrete, because the cement and water fill the gaps between the grains. With all-in ballast, the sand already fills many of those gaps, so about 1.25 m³ of loose cement and ballast makes 1 m³ of concrete. Loose ballast weighs about 1.7 tonnes per m³ and loose cement about 1,440 kg per m³.
For post holes, the calculator works out the volume of a round hole and takes off the space taken by a square post, if you enter one. For Ballast Only, it simply multiplies the volume by the density, with no cement.
The volume is 3 × 3 × 0.1 = 0.9 m³, or 0.99 m³ with 10% extra. At 1:5, you need 1.75 tonnes of ballast and 12 bags of cement, plus about 150 litres of water. That is just over two 850 kg bulk bags, so order three, or two bulk bags and a couple of 25 kg bags.
Each hole is 300 mm wide and 600 mm deep, with a 100 mm square post. The ten holes hold 0.36 m³ of concrete, or 0.40 m³ with 10% extra. At 1:6, you need about 730 kg of ballast (one bulk bag, or thirty 25 kg bags) and 5 bags of cement. That is about 73 kg of ballast for each hole.
A trench 10 m long and 600 mm wide is filled with 225 mm of concrete. That is 1.35 m³, or 1.49 m³ with 10% extra. At 1:5, you need 2.63 tonnes of ballast (four bulk bags) and 18 bags of cement. For structural footings, check the depth and mix with building control first.
A 3 m × 2 m area levelled at 100 mm deep is 0.6 m³, or 0.66 m³ with 10% extra. At 1.7 tonnes per m³, that is 1.12 tonnes, so order two bulk bags.
These figures include 10% extra and use 850 kg bulk bags.
| Job | Mix | Ballast | Bulk bags | Cement bags |
|---|---|---|---|---|
| Shed base 2.4 × 1.8 m (8 × 6 ft), 75 mm | 1:5 | 0.63 t | 1 bag (or 26 small) | 5 |
| Shed base 3 × 3 m, 100 mm | 1:5 | 1.75 t | 3 bags | 12 |
| Garden path 10 × 1 m, 75 mm | 1:5 | 1.46 t | 2 bags | 10 |
| Ten fence posts, 300 mm × 600 mm holes | 1:6 | 0.73 t | 1 bag (or 30 small) | 5 |
| Strip footing 10 m × 600 × 225 mm | 1:5 | 2.63 t | 4 bags | 18 |
| Single garage base 6 × 3 m, 150 mm | 1:4 | 5.05 t | 6 bags | 43 |
For a garage base or anything over about 3 m³, ready-mixed concrete is often cheaper and much less work than mixing 40 or more bags of cement by hand.
Bulk bags are filled by weight, and wet ballast weighs more than dry, so the volume in a bag can vary a little. It is always worth rounding up rather than running short halfway through a pour.
Too much water makes concrete easier to place but weaker when it sets, so add it slowly.
About 1.77 tonnes of all-in ballast and 12 bags of cement for a 1:5 mix. That is just over two 850 kg bulk bags.
About six 25 kg bags of cement for each 850 kg bulk bag at a 1:5 mix, five at 1:6 and seven at 1:4.
An 850 kg bulk bag makes roughly half a cubic metre of concrete once cement and water are added.
At 100 mm deep with 10% extra, you need about 1.75 tonnes of ballast and 12 bags of cement at 1:5.
A 1:6 mix of cement to all-in ballast is common for fence and gate posts. Some people use a post-mix product instead, which sets faster.
A tonne of loose ballast is about 0.59 m³. Laid 100 mm deep, it covers roughly 5.9 m². As concrete, a tonne of ballast makes about 0.56 m³ at 1:5.
No. All-in ballast is a mix of sharp sand and gravel. Gravel on its own has no sand, so it needs sand added to make concrete.
Not for concrete. All-in ballast already contains the sand. You only need separate sand if you are using plain gravel or making mortar.
Yes. Switch to Imperial and enter feet and inches. The results still show tonnes and kilograms, because ballast is sold by weight in the UK.
Figures assume loose all-in ballast at 1.7 tonnes per m³, cement at 1,440 kg per m³ and 1.25 m³ of dry materials per m³ of concrete, which matches supplier guidance that an 850 kg bulk bag makes about 0.5 m³. Water uses a water–cement ratio of 0.5. Reviewed November 2026.