A YouTube tutorial tells you to order 4,000 PSI concrete, and a product label lists strength in pounds per square inch. Your UK supplier, meanwhile, only offers grades such as C20, C25, and C30, and that gap leaves many homeowners unsure how the two systems match up. Ordering the wrong strength can mean a driveway that cracks after its first winter or a patio that costs more than it ever needed to.
Understanding concrete PSI ratings makes it much easier to order the right mix for your job. Most domestic projects in the UK need concrete of roughly 2,900 to 4,350 PSI, the same strength as C20 to C30 grades measured in N/mm². The exact figure depends on where the concrete will go, from a garden path to a driveway or a structural beam. Choosing well protects both your budget and the life of the finished slab.
What PSI Means in Concrete
PSI, or pounds per square inch, is a measure of concrete’s compressive strength. The figure shows how much pressure a hardened sample can carry before it crushes, usually tested 28 days after the concrete is made. A higher PSI rating means the concrete can support heavier loads without failing.
The rating is worked out in a few simple steps:
- Samples are cast from the same batch as the pour
- The samples cure in damp conditions in a laboratory
- A press squeezes each sample until it breaks
- The crushing force is divided by the sample’s surface area
PSI is the American way of describing concrete strength, and it only covers squeezing forces. Research notes that the tensile strength of concrete is only about 10% of its compressive strength, and that gap is why slabs and beams that bend under load need steel reinforcement.
How UK Concrete Strength Compares With PSI
UK suppliers measure strength in newtons per square millimetre instead of PSI. Both describe the same property, and converting between them is simple once you know the figures.
Converting PSI to N/mm²
One newton per square millimetre equals roughly 145 PSI. To convert PSI to N/mm², divide the PSI figure by 145, and to go the other way, multiply the N/mm² figure by 145. For example, 3,000 PSI works out at about 20.7 N/mm², and a 30 N/mm² mix is roughly 4,350 PSI.
N/mm² and megapascals (MPa) are the same unit, and many product sheets and engineering drawings use MPa instead. A handy shortcut is that every 1,000 PSI equals roughly 7 N/mm², and that rule of thumb makes quick checks easy on site. Round to the nearest standard grade when comparing figures, and always confirm the final choice against the grade on your specification.
Cube Strength vs Cylinder Strength
The two systems also test different shapes. American PSI ratings come from cylinder samples, while UK grades are usually quoted using cube samples, and cubes give higher results than cylinders made from the same concrete.
European standards show both figures side by side, such as C25/30, where 25 is the cylinder strength, and 30 is the cube strength in N/mm². Everyday UK names like C25 usually refer to the cube figure. An American 3,000 PSI rating is therefore closer to a UK C25 than its straight conversion of about C20 suggests.
UK Concrete Grades Converted to PSI
This table converts the most common UK grades into approximate concrete PSI ratings, based on 28-day cube strength. It also shows where each grade is typically used.
| Grade | Strength (N/mm²) | Approx PSI | Typical Uses |
| C10 | 10 | 1,450 | Blinding, kerb bedding, fence post bases |
| C15 | 15 | 2,175 | Shed bases, garden paths, mass fill |
| C20 | 20 | 2,900 | Patios, light slabs, unreinforced house floors |
| C25 | 25 | 3,625 | Foundations, footings, general groundworks |
| C30 | 30 | 4,350 | Driveways, garage floors, reinforced slabs |
| C35 | 35 | 5,075 | Heavy-duty driveways, structural work, exposed slabs |
| C40 | 40 | 5,800 | Commercial floors, structural frames, heavy loads |
These figures are a guide for comparing strength, not a substitute for a specification. Your engineer or building control officer will always have the final say on structural work.
What Strength Concrete Is Needed for Common Jobs
The right strength depends on the load, the weather, and whether steel goes into the slab. Common UK jobs typically need the following strengths, shown in both systems.
Paths, Shed Bases, and Blinding
Light jobs that carry foot traffic or small structures need only modest strength. C10 to C20 concrete, or about 1,450 to 2,900 PSI, suits garden paths, shed bases, and the thin blinding layer laid under foundations. Paying for a stronger mix here rarely adds any real benefit.
Patios and Garden Slabs
Patios and garden slabs usually perform well with C20 to C25 concrete, or roughly 2,900 to 3,625 PSI. These slabs need to handle:
- Garden furniture and barbecues
- Foot traffic and wheelbarrows
- Regular frost and rain
Choose C25 where the slab will hold a hot tub, a greenhouse, or other heavy items. A well-compacted sub-base matters just as much as the grade itself.
Foundations and Footings
Most domestic foundations use C20 to C30 concrete, or about 2,900 to 4,350 PSI. Where the soil contains sulfates or the design calls for it, your engineer may specify an FND-designated mix instead. Ground conditions, rather than the size of the building, usually decide the final choice.
Driveways and Garage Floors
Driveways face car weight, frost, and de-icing salts, and they need more strength than a patio. C25 to C35 concrete, or about 3,625 to 5,075 PSI, covers most domestic drives and garage floors.
For drives exposed to winter salt, PAV1 is a popular choice. This paving mix is rated C28/35 and contains tiny air bubbles that help the surface resist frost damage. Garage floors without salt exposure usually perform well with C25 or C30.
Reinforced and Structural Work
Beams, columns, suspended slabs, and retaining walls usually need C30 to C40 or higher, about 4,350 to 5,800 PSI and beyond. Structural concrete should always follow an engineer’s specification, and the reinforcement design matters as much as the grade.
Send us the PSI figure from your plans, product specification, or favourite tutorial, and we will match it to the right UK grade before you order. Our team at Pro-Mix Concrete handles these conversions regularly, and we can explain the difference between cube and cylinder figures in plain terms.
Other Factors That Affect the Strength You Need
Strength is only one part of choosing the right concrete. These factors also guide the final grade:
- Load and Traffic: Vehicles, machinery, and heavy storage need stronger mixes than foot traffic. Commercial vans and delivery lorries often push driveways into higher grades.
- Frost and De-Icing Salts: Outdoor concrete in the UK faces repeated freezing and thawing. Exposure classes in BS 8500 set minimum strengths for surfaces that will meet frost or salt.
- Steel Reinforcement: Reinforced concrete needs enough strength and cover to protect the steel from rust. Structural designs usually start at C30 for this reason.
- Ground Conditions: Clay, sulfates, and high groundwater can all affect foundation design. A soil report helps your engineer choose a mix that suits the site.
- Engineer or Building Control Specification: Structural and regulated work must follow the approved design. The specified grade always takes priority over general guidance.
Weighing these factors together leads to a mix that is strong enough without wasting money. A short conversation with your supplier often clears up any doubts before the pour.
Why Higher PSI Is Not Always Better
Stronger concrete sounds safer, but a higher rating does not automatically mean a better result. Stronger mixes cost more, can be stiffer to place, and may shrink and crack more if they are not cured carefully. Richer mixes can also stiffen faster, leaving less time to level and finish the surface. Poor ground preparation or a thin sub-base will still cause problems, whatever the PSI.
Durability also depends on more than strength alone. Frost resistance comes from air-entrained mixes such as PAV1, while steel protection relies on enough concrete cover and careful curing. A C40 slab that dries out too quickly can perform worse at the surface than a well-cured C25.
Every step up in strength usually means more cement in the mix, and cement carries a heavy carbon cost. Figures from the International Energy Agency show that cement production accounted for 6% of total global emissions in 2024. Ordering the grade your job actually needs keeps both the bill and the carbon footprint down.
How Concrete Reaches Its Rated Strength
Concrete only reaches its full rating when it is mixed, placed, and cured properly. These steps help the finished slab perform as designed:
- Keep the water content right, as extra water weakens the mix.
- Compact the concrete well to remove trapped air pockets.
- Keep the surface damp and covered for the first week while it cures.
- Protect fresh concrete from frost and strong sun during the first few days.
Most concrete gains much of its strength in the first week and reaches its rated figure at about 28 days. Rushing the curing stage is one of the most common reasons a slab falls short of its expected strength.
What PSI is ready-mix concrete in the UK?
Most UK suppliers rarely quote PSI because ready-mix concrete is ordered by grade in N/mm². Common domestic grades fall roughly between 2,900 and 4,350 PSI, and a supplier can confirm the exact grade from a PSI figure on request.
Can US PSI recommendations be used for a UK project?
US advice can guide you, but the figures need converting with care. American ratings come from cylinder tests, while UK grades use cubes, and UK work must also meet British Standards and any engineer’s specification.
Bottom Line
Concrete PSI ratings become simple once you can convert them to the N/mm² grades UK suppliers sell. Most domestic jobs need around 2,900 to 4,350 PSI, or C20 to C30, while driveways and structural work call for more. The right choice balances strength with exposure, reinforcement, and budget, and a supplier can confirm the grade before any concrete is ordered.
Any strength rating, in PSI or N/mm², only matters if the concrete that arrives on site actually reaches it. Pro-Mix Concrete produces every grade from C10 to C40 to British Standards and supplies it delivered ready-mixed or mixed fresh on site across London and the surrounding areas. Our team weighs and mixes each batch to the grade on your order, and the strength on paper becomes the strength in your slab.
Talk to our concrete specialists at Pro-Mix Concrete to order the grade your project needs. Where a lower grade does the job just as well, we will tell you, and your costs stay down.
- Dennis Broderick is the founder and owner of Pro-Mix Concrete Company, a trusted name in ready-mix concrete solutions across the UK. With over 20 years of hands-on experience in the construction and concrete industry, Dennis brings unmatched expertise, practical insights, and a commitment to quality on every project - from residential driveways to large-scale commercial developments.
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