Order concrete in Manchester and you will be asked for a C grade. Order it in Michigan and you will be asked for a psi number. Order it in Munich and you will get a dual figure like C25/30. All three describe the same property: how much pressure the hardened concrete can take before it fails.
What the numbers mean
A C grade is the characteristic compressive strength in newtons per square millimetre after 28 days of curing. C25 means 25 N/mm². The American equivalent is pounds per square inch, and the conversion is straightforward: 1 N/mm² is about 145 psi, so C25 lands near 3,600 psi.
The EN 206 dual notation used across the EU quotes two figures — C25/30 is 25 N/mm² measured on a cylinder and 30 N/mm² measured on a cube. Same concrete, two test methods. Cubes read higher because of how the sample is restrained during the test.
Grade comparison table
| UK grade | UK designated mix | EU (EN 206) | US strength | Mix ratio |
|---|---|---|---|---|
| C10 | GEN0 | C12/15 | ~1,500 psi | 1 : 4 : 8 |
| C15 | GEN1 | C16/20 | ~2,200 psi | 1 : 3 : 6 |
| C20 | GEN3 | C20/25 | ~3,000 psi | 1 : 2 : 4 |
| C25 | ST2 / RC25 | C25/30 | ~3,600 psi | 1 : 1.5 : 3 |
| C30 | ST3 / RC30 | C30/37 | ~4,400 psi | 1 : 1 : 2 |
| C35 | RC35 | C35/45 | ~5,000 psi | design mix |
Which grade for which job
- Blinding and mass fill: C10 / GEN0. It is there to give you a clean level surface, not to carry anything.
- Shed bases, paths, patios: C20 / GEN3. The default domestic mix and the one most site-mixed concrete lands on.
- Footings and foundations: C20 to C25, or whatever building control specifies after they inspect the trench.
- Driveways and garage floors: C25 to C30. Cars are heavier than they were, and a driveway takes point loads a patio never sees.
- Anything structural: C30 upwards, to an engineer’s design. Not a decision to make from a table on a website.
Strength is not the only thing to specify
Two more matter for outdoor work. Air entrainment puts microscopic bubbles in the mix that give freezing water somewhere to expand into — essential anywhere that sees frost with de-icing salt. Slump describes how wet the mix is; a low slump is stiffer and stronger but harder to place, and a driver adding water on site to make your life easier is quietly weakening the concrete you paid for.
That last point is worth repeating. Water content drives strength more than cement content does. A rich mix that has been soaked to make it flow will end up weaker than a leaner mix placed properly.
Mixing your own
The ratios in the table above are by volume, not weight. Our mix ratio calculator converts any of them into cement bags, sand and gravel for the volume you need. If you are working from all-in ballast instead of separate aggregates, the ballast calculator does the same for 1:5, 1:6 and 1:8 mixes.
Common questions
What does C25/30 mean?
25 N/mm² cylinder strength and 30 N/mm² cube strength at 28 days, under the EN 206 standard used across the EU. In UK terms it is C25; in US terms roughly 3,600 psi.
What is the difference between C20 and C25 concrete?
Five newtons per square millimetre of design strength, achieved with more cement and less aggregate. C20 suits paths and shed bases; C25 is the sensible minimum for a driveway.
What psi is C30 concrete?
About 4,400 psi. The conversion is 1 N/mm² ≈ 145 psi, so C30 × 145 ≈ 4,350.
Does concrete keep getting stronger after 28 days?
Yes, but slowly. The 28-day figure is the specification point because roughly 90% of the strength has developed by then. Concrete continues to gain strength for years as hydration carries on.