Concrete strength guide
Which concrete grade do you need?
The grade tells you how strong the concrete is once it has cured. Higher grades carry more load and withstand harsher conditions. Here’s what each grade is typically used for.
Grades and typical uses
Filter by what you’re building, or select a grade.
| Grade | What it’s for | Typical uses |
|---|---|---|
| C10 GEN1 | Basic, non-structural | Drainage, floor blinding, trench fill |
| C15 GEN2 | General purpose, non-load-bearing | Unreinforced internal floors, shed bases, kerb bedding |
| C20 GEN3 | Light foundations and domestic work | Fence posts, patios, greenhouse bases |
| C25 | Foundations and domestic work | House extensions, patios, driveways |
| C30 | Residential and commercial | Driveways, garage bases, pavements, house foundations |
| C35 | Heavy-duty external | Heavy garage floors, reinforced walls, loading bays |
| C40 | Structural and infrastructure | HGV parking, agricultural yards, load-bearing beams |
| C45 | Specialised industrial | Silage pits, piling, aggressive ground conditions |
| C50 | Very high strength | Heavy industrial use, exposed conditions |
This is general guidance. For structural work, foundations and anything building control will inspect, the grade must come from your engineer, architect or building control. If your drawings name a mix, order that mix.
Reading a specification
C, GEN and ST in plain English
- C grades
- In everyday UK use, the number after the C is usually the cube strength in N/mm² (the same as MPa) after 28 days. Formal specifications give two numbers, cylinder and cube, such as C20/25. If yours does, quote it to us exactly.
- GEN mixes
- General-purpose designated concretes from BS 8500, used for unreinforced work such as blinding, floors and foundations.
- ST mixes
- Standardised prescribed concretes from BS 8500, where the mix proportions are set by the standard. Common on housing and garage foundations, kerbs and bases.
- N/mm² and MPa
- Two names for the same unit of strength. One MPa is roughly equivalent to 100 tonnes pressing on one square metre.
Similar strengths
By 28-day cube strength. Designated and standardised mixes set other properties too, so use the one your specification names.
| N/mm² | C grade | GEN | ST |
|---|---|---|---|
| 10 | C10 | GEN1 | ST2 |
| 15 | C15 | GEN2 | ST3 |
| 20 | C20 | GEN3 | ST4 |
| 25 | C25 | – | ST5 |
Three things that decide the grade
- Load
Heavier loads and structural parts need higher-strength mixes. A garden path needs a lower grade than a garage floor.
- Exposure
Freeze-thaw, chemicals and constant moisture call for a more durable mix.
- Advice
Tell us what the concrete is for and we’ll suggest a mix. For structural work, follow your designer’s specification.