How Much Ballast for Concrete? Mix Ratios Explained
Quick answer: For general concrete, use a 1:5 cement-to-ballast mix (ballast is pre-mixed sand and gravel), or 1 cement : 2 sand : 4 gravel if buying separately. To make 1 cubic metre of concrete you need roughly 1.8 tonnes of ballast plus about 6–7 bags (25kg) of cement. Because materials compact when mixed, order a little extra. Ballast is sold by weight, so calculate your volume first, then convert.
Mixing your own concrete for a project — a post base, a small slab, a shed footing — means getting the ballast quantity and mix ratio right, and this is where people commonly under-order and run short mid-pour, which risks a weak joint where fresh concrete meets set. This guide explains the mix ratios, how much ballast and cement you need per cubic metre, and how to work out your volume. Use the calculator above to find your concrete volume first.
What is ballast?
Ballast is a ready-mixed blend of sharp sand and gravel (aggregate), sold together in the right proportions for making concrete. Using ballast saves you buying and measuring sand and gravel separately — you just add cement and water. It’s the convenient choice for most DIY concrete.
If you prefer, you can buy sand and gravel separately and mix them yourself, which gives more control but more work. For most projects, all-in ballast plus cement is simpler and perfectly good.
The mix ratios
The right ratio depends on the job, but the common ones are:
- General-purpose concrete: 1 cement : 5 ballast (or 1 : 2 sand : 4 gravel separately). Suits paths, bases, and general use.
- Strong/structural concrete: 1 cement : 4 ballast (or 1 : 2 : 3). For foundations and load-bearing work.
- Foundations: 1 cement : 6 ballast is sometimes used for mass fill where strength needs are lower.
The 1:5 general mix covers most garden projects. Use the stronger 1:4 for anything structural or load-bearing. Measure by volume (a bucket is fine) and mix thoroughly with enough water to reach a workable, not sloppy, consistency.
How much ballast per cubic metre
To make 1 cubic metre of finished concrete at a general 1:5 mix, you need roughly 1.8 tonnes of ballast and about 6–7 bags of 25kg cement (around 165kg). Note that the finished concrete volume is less than the combined loose volume of the materials, because the cement and fine particles fill the gaps between the aggregate as it mixes and compacts — so you always need more raw material than the finished volume suggests.
Some rough figures by finished volume at a 1:5 mix:
| Finished concrete | Ballast | Cement (25kg bags) |
|---|---|---|
| 0.25 m³ | ~0.45 t | ~2 bags |
| 0.5 m³ | ~0.9 t | ~3–4 bags |
| 1 m³ | ~1.8 t | ~6–7 bags |
Add around 10% to be safe against running short mid-pour.
Working out your volume
Concrete volume is the same as any volume: length × width × depth. For a slab 2m × 1m × 100mm: 2 × 1 × 0.1 = 0.2 m³. For post holes, it’s the hole volume minus the post — a hole 0.3m × 0.3m × 0.6m is 0.054 m³, less the post’s small volume. For a strip footing, it’s length × width × depth of the trench.
Work out the volume first, then use the figures above to find the ballast and cement. The calculator gives you the concrete volume from your dimensions; multiply by 1.8 tonnes/m³ for the ballast at a 1:5 mix.
Mixing and ordering tips
For small jobs, mixing by hand or in a hired cement mixer with bagged ballast and cement works well. For anything over about a cubic metre, consider ready-mix concrete delivered, which is often easier and more consistent than mixing a large volume by hand — a large hand-mixed pour is hard, slow work and risks weak joints if it sets between batches.
Always order a little extra — around 10% — because coming up short mid-pour means either a rushed second batch or a cold joint where new concrete meets partly-set concrete, which weakens the result. It’s much better to have a little spare than to stop halfway.
Common mistakes to avoid
The main mistakes are under-ordering and running short mid-pour, using a too-weak mix for structural work, mixing too wet (which weakens the concrete), and forgetting that finished concrete volume is less than the loose material volume. One more: hand-mixing a large volume that really should be ready-mix — beyond a cubic metre, the effort and the risk of cold joints make delivered concrete the better call.
Frequently asked questions
How much ballast do I need for 1m³ of concrete?
Roughly 1.8 tonnes of ballast plus about 6–7 bags of 25kg cement at a general 1:5 mix. The raw materials exceed the finished volume because fine particles fill the gaps as it compacts, so always allow a little extra.
What is the ratio of cement to ballast?
1:5 (one part cement to five parts ballast) for general-purpose concrete, or 1:4 for stronger structural work. If buying sand and gravel separately, that’s roughly 1 cement : 2 sand : 4 gravel for a general mix.
What’s the difference between ballast and gravel?
Ballast is a ready-mixed blend of sharp sand and gravel, sold together in the right proportions for making concrete. Gravel alone is just the coarse aggregate. Using ballast saves buying and measuring sand and gravel separately.
How much concrete does a bag of cement make?
At a 1:5 mix, a 25kg bag of cement with about 125kg of ballast makes roughly 0.13–0.15 m³ of concrete. So 6–7 bags of cement, with the matching ballast, make about 1 m³. The exact yield depends on the mix and compaction.
Can I use building sand instead of ballast for concrete?
No — building (soft) sand alone makes a weak concrete. Concrete needs the coarse aggregate that ballast provides, or a mix of sharp sand and gravel. Building sand is for mortar, not concrete.
How much extra concrete should I order?
About 10% more than your calculated volume, so you don’t run short mid-pour and risk a weak cold joint. Coming up short means a rushed second batch or a join between set and fresh concrete, both of which weaken the result.