Concrete Mix Ratios Explained
Not sure whether you need a 1:3, 1:4, or 1:5 mix? This guide explains which concrete mix ratio to use for every common UK DIY project — and why it matters.
The three standard mix ratios
Concrete mix ratios describe the proportion of cement to aggregate (ballast, or separate sand and gravel) by volume. "1:4" means one part cement to four parts ballast. More cement = stronger concrete, but also more expensive and harder to work with.
1:3
Strong · Approx C30
Structural footings, retaining walls, load-bearing columns, garage floors, and any work that takes heavy loads. The strongest standard DIY mix.
1:4
Standard · Approx C20
Shed bases, garden paths, patios, fence posts, and most domestic foundations. The safe default for nearly all DIY projects.
1:5
Light · Approx C15
Non-structural fill, bedding for paving slabs, haunching around drain pipes, and areas that won't carry significant weight.
1:6
Weak · Approx C10
Blinding concrete (thin base layer under footings), temporary fill, and non-load-bearing applications where strength isn't critical.
Not sure? Use 1:4. It's strong enough for 90% of garden and domestic projects, easy to mix, and the most cost-effective. Only go stronger (1:3) for structural work or weaker (1:5) for fill and bedding.
Which mix for which job
| Project | Mix | Strength |
|---|
| Shed base | 1:4 | C20 — Standard |
| Garden path | 1:4 | C20 — Standard |
| Patio slab base | 1:4 | C20 — Standard |
| Fence posts | 1:4 | C20 — Standard |
| Structural footings | 1:3 | C30 — Strong |
| Retaining wall base | 1:3 | C30 — Strong |
| Garage floor | 1:3 | C30 — Strong |
| Driveway | 1:3 | C30 — Strong |
| Paving slab bedding | 1:5 | C15 — Light |
| Drain pipe haunching | 1:5 | C15 — Light |
| Blinding layer | 1:6 | C10 — Weak |
Ballast vs separate sand and gravel
Ballast (also called "all-in aggregate" or "mixed aggregate") is a pre-mixed combination of sharp sand and gravel, typically in a 2:1 gravel-to-sand ratio. It's the most common aggregate for DIY concrete because you only need to buy one product and mix it with cement.
Separate sand and gravel gives you more control over the mix. A typical 1:2:3 mix (1 cement : 2 sharp sand : 3 gravel) produces slightly stronger concrete than 1:4 ballast because you can control the aggregate grading. However, for most DIY projects the difference is negligible and ballast is far more convenient.
Quick rule: Ballast bags from B&Q, Wickes, or Travis Perkins are fine for everything up to and including shed bases and footings. Only bother with separate sand and gravel if you're doing structural work where a building inspector will check the mix.
How to mix concrete by hand
1
Measure your materials
Use a bucket as your "part" measure. For a 1:4 mix: one bucket of cement to four buckets of ballast. Keep your measuring consistent — always use the same bucket, filled to the same level.
2
Dry mix first
On a mixing board or in a wheelbarrow, combine the cement and ballast while dry. Turn the pile over with a shovel at least three times until the colour is uniform — you shouldn't see any patches of grey cement or brown ballast.
3
Add water gradually
Make a crater in the middle of your dry mix and add water slowly — about half a bucket per mix to start. Too much water weakens the concrete dramatically. The ideal consistency is like thick porridge: holds its shape when squeezed but isn't dry or crumbly.
4
Mix thoroughly
Turn the wet mix over repeatedly until every part is the same colour and consistency. No dry spots, no puddles. If it's too dry, add tiny amounts of water. If too wet, add a small amount of dry mix. You have about 60-90 minutes before it starts to set.
Know how much you need?
Use our free calculator to find exactly how many bags of cement and ballast to buy.
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Frequently Asked Questions
What is the strongest concrete mix ratio?
For DIY, 1:3 (one part cement to three parts ballast) is the strongest standard mix, producing approximately C30 grade concrete. This is suitable for structural footings, retaining walls, and load-bearing work. Stronger mixes exist (1:2 or 1:1.5) but are rarely needed for domestic projects and use a lot of expensive cement.
What is the best mix for a shed base?
A 1:4 mix (one cement to four ballast) is the standard choice for shed bases. This produces C20 grade concrete, which is more than strong enough for any shed. Pour to a depth of 75-100mm on a compacted sub-base of hardcore. See our
shed base guide for the full step-by-step process.
Can I use building sand instead of ballast?
No — building sand (soft sand) is for mortar, not concrete. Concrete needs sharp sand or ballast, which contains angular gravel pieces that interlock and give the concrete its strength. Using building sand produces a weak, crumbly mix that won't hold up to any load.
How much water do I add to concrete?
As little as possible while still getting a workable mix. A rough guide is about half a bucket of water per bucket of cement, but it depends on how wet your ballast is. The ideal concrete is like thick porridge — holds its shape but isn't dry. Too much water is the most common DIY mistake and dramatically reduces strength.
What is C20, C25, C30 concrete?
The "C" number refers to the compressive strength in newtons per square millimetre (N/mm²) after 28 days of curing. C20 withstands 20 N/mm², C30 withstands 30 N/mm². For context, a 1:4 ballast mix is roughly C20, and a 1:3 mix is roughly C30. Most domestic projects only need C20.
How long does concrete take to set?
Concrete begins to harden within 2-4 hours and is hard enough to walk on after 24-48 hours. However, it takes 28 days to reach full strength. Don't put heavy loads on fresh concrete for at least 3-7 days, and keep it damp (mist with water or cover with polythene) during hot weather to prevent cracking.