The UK Concrete Mix Ratio Cheat Sheet
Which mix for which job, how many bags you'll need, and how to pour in cold weather — one page to keep in the shed.
Mix ratios by job (by volume)
Mixing your own from cement, sand and aggregate? These are the standard UK nominal mixes. Ratios are cement : sharp sand : coarse aggregate.
| Job | Mix ratio | Roughly |
|---|---|---|
| Paths, slabs, general purpose | 1 : 2 : 4 | ~C20 — the everyday DIY mix |
| Foundations, heavy / structural slabs | 1 : 1.5 : 3 | ~C25 / C30 — stronger, denser |
How many bags per cubic metre
| Bag size | Yields about | Bags per 1m³ |
|---|---|---|
| 20kg ready-mix | ~0.009 m³ | ~110 bags |
| 25kg ready-mix | ~0.011 m³ | ~90 bags |
These bag yields follow standard UK ready-mix figures (see sources below) and match HomeCalc's concrete calculator, so the cheat sheet and the tool always agree.
Common jobs at a glance
| Job | What you need |
|---|---|
| Fence post hole — 100mm dia × 600mm deep | ~0.005 m³ — roughly 1 bag of Postcrete, or half a 20kg ready-mix bag |
| Larger post — 150mm dia × 750mm deep | About 2–3 bags |
| Garden shed base slab | Minimum 100mm thick |
| Garage / workshop slab | 150mm thick |
Pouring in cold weather
- Protect fresh concrete from freezing for the first 24–48 hours — if it freezes before it sets, ice crystals permanently weaken it.
- Keep it above about 5°C while it sets: cover with insulating blankets or straw, or use a frost-protection admixture.
- Low temperatures slow how fast concrete gains strength, so it takes longer to cure in the cold — leave it longer before loading it.
Sources: UK ready-mix industry conventions (mix ratios & bag yields), UK groundwork & fencing trade (slab depths, post-hole volumes), standard cold-weather concrete guidance. Mix ratios are industry convention, not a regulated standard. Last verified June 2026. General guidance only — verify slab and footing specs with a builder before pouring.