Concrete can look simple when it arrives on site, but the number printed on the delivery ticket can make a huge difference to how that slab performs years later. C20, C25, C30 or C40 are not just numbers. They tell you how much compressive strength the concrete is designed to develop, helping determine whether the mix suits a garden path, driveway, foundation or heavily loaded floor.
Choose a grade that is too weak and the concrete may struggle under the demands placed on it. Go far beyond what the project actually needs, and you could spend more money without gaining a useful advantage. The right grade is less about choosing the strongest concrete available and more about matching the mix to the job.
The C-number indicates the concrete’s designed strength, making the difference between C20 and C40 important when matching a mix to the job.
What Concrete Strength Actually Means
Concrete strength almost always refers to compressive strength, the load a mix can carry before it fails. It is measured in newtons per square millimetre, written N/mm², and it is the figure behind every grade name. Concrete is very strong in compression and comparatively weak in tension, which is why reinforcement is added where a structure has to resist bending or pulling forces.
That strength is verified by crushing a sample cube in a testing machine at a set age. The Concrete Society notes that the standard reference is a cube cured and tested at 28 days, which is the point where a mix is expected to have reached its specified grade. Concrete gains around two-thirds of that strength within the first week, then meets its grade figure by 28 days.
Busting the 28-Day Myth:
Concrete does not stop hardening at 28 days. That age is simply the industry’s standard testing benchmark, chosen for consistency. Hydration continues for months and years, and a mix often reaches around 110% of its grade strength by 90 days and keeps climbing slowly after that. Well-kept old concrete is frequently stronger than the day it was signed off.
How Concrete Grades Are Named
Grade names look cryptic until you know the code. The letter C stands for concrete, and the number is its characteristic cube strength in N/mm². C25 concrete reaches 25 N/mm² by its 28-day test, C40 reaches 40 N/mm², and so on up and down the range.
You may also see a dual number such as C25/30. Under European standard BS EN 206, strength is stated as both a cylinder and a cube figure, so C25/30 means 25 N/mm² measured on a cylinder and 30 N/mm² on a cube. UK practice quotes the cube figure, which is why a mix is often called simply C30.
Alongside the plain C-grades sit the designated concretes of BS 8500, named for their purpose:
- GEN for general non-structural work
- FND for foundations, graded by sulfate exposure
- RC for reinforced concrete
- PAV for paving, air-entrained against frost and de-icing salts
The Different Types of Concrete Strengths and Their Uses
Here is the range grade by grade, from the lightest non-structural mix to heavy-duty commercial concrete. Each entry covers its strength, its character, an indicative mix, and the jobs it suits best.
1. C10 Is the Groundwork Filler
C10 concrete sits at the bottom of the range at 10 N/mm², usually specified as GEN1 with an indicative nominal mix of around 1:4:8 (cement to sand to coarse aggregate). Nothing structural rests on it. Its value is the preparation work that happens before the load-bearing concrete arrives, wherever a job needs a firm, clean base or a filler that will never carry weight.
Where C10 earns its place:
- Blinding layers poured over hardcore to level up beneath foundations
- Bedding and haunching for kerbs and edgings
- Pipe surround in drainage runs
- Trench and mass fill
- Non-structural void and cavity filling
2. C15 Handles Light Domestic Work
C15 delivers 15 N/mm² and steps up to gentle domestic duty, designated GEN2 with an indicative mix near 1:3:6. It still sits below structural work, yet it copes with jobs that see occasional rather than sustained load.
Character
Firm and dependable for light work, without the cost of a richer mix. It is the grade for tidy, low-demand jobs around the home.
Where It Fits
Footings for small garden and boundary walls, unreinforced flooring, and pathways, steps and paving beds all suit C15.
3. C20 Is the Domestic All-Rounder
C20 is where household concrete turns genuinely versatile, carrying steady, modest loads without complaint. Its balance of strength and value puts it under a wide spread of everyday domestic work, from bases to light foundations.
| Strength | 20 N/mm² |
| Indicative mix | around 1:2:4 |
| Designation | GEN3 |
| Character | versatile light-domestic all-rounder |
Typical jobs include garage floors, shed and greenhouse bases, conservatory bases, internal floor slabs, footpaths and light patios. Where a project must endure normal use but never carries vehicles or plant, C20 is usually the sensible floor.
4. C25 Is the General-Purpose Workhorse
Few grades earn their keep like C25, Pro-Mix Concrete’s most-ordered mix. Rated at 25 N/mm² with an indicative mix around 1:1.5:3, it bridges light domestic and genuine structural duty, strong enough for footings yet economical enough for a large pour. That balance is why it appears across almost all everyday groundwork.
- Strip and trench-fill footings for house foundations
- Ground-bearing floor slabs
- Driveways and patios under regular traffic
- Bases for garages and outbuildings
- Foundations for single and two-storey extensions
- Mass fill, groundworks and retaining-wall footings
5. C30 Is Built for the Weather
C30 reaches 30 N/mm² and is the first grade made to face the elements head-on. Specified as PAV1, it is supplied as a designed, air-entrained mix rather than a simple nominal ratio, because the fine air content has to be controlled to work.
Why It Lasts Outdoors
Millions of microscopic air bubbles give trapped water room to expand as it freezes, so the surface survives repeated frost. That same air helps it resist the de-icing salts spread on paving through winter.
Best Uses
Driveways, hardstanding, external slabs, exposed patios, paths and workshop floors all belong in C30, where an ordinary mix would pit and flake after a few cold seasons.
6. C35 Steps Into Commercial and Structural Work
Once a job turns commercial or structural, C35 takes over. Its strength comes from a rich, low-water-cement-ratio mix rather than a nominal ratio, which is what lets it stand up to serious wear and sustained load.
| Strength | 35 N/mm² |
| Mix | designed, cement-rich, low water-cement ratio |
| Designation | PAV2 or an RC grade |
| Character | heavy-duty and structural |
Its home is demanding work: commercial and industrial floor slabs, heavy-duty external paving and yards, reinforced and raft foundations, piling mats, loading bays, and agricultural areas that carry machinery daily.
7. C40 Is the Heavy-Duty Grade
C40 is the strongest mix in our standard range at 40 N/mm², engineered as a designed mix for the heaviest and most punishing conditions. It carries sustained structural load and resists aggressive environments that would degrade anything weaker.
What sets C40 apart:
- The highest strength Pro-Mix Concrete holds as standard
- A designed, closely controlled mix rather than a nominal ratio
- Built for heavy, continuous load and harsh exposure
That strength puts it under structural piling, beams and columns, foundations for large and heavily loaded buildings, septic tanks and water-retaining structures, high-load industrial floors, and the busiest commercial and agricultural yards and roads.
Foundation and Paving Mixes
Two families of designated concrete are chosen by the job rather than by a bare C-number. Foundation grades run FND2 through FND4, selected by the sulfate content of the ground so footings survive aggressive soils. The paving grades, PAV1 and PAV2, are the air-entrained mixes already met at C30 and C35, made for external surfaces that face frost and salt. Where the deciding factor is ground condition or exposure rather than load alone, these designated mixes matter as much as the strength figure.
Higher Strength Grades
Beyond C40 lie specialist grades such as C45 and C50, reserved for high-strength and civil engineering work, from high-rise columns to bridges and marine structures. These fall outside most everyday projects. Pro-Mix Concrete standard supply spans C10 to C40, which meets the full run of domestic, commercial and groundwork demands.
Keep in Mind:
Nominal mix ratios are a rough guide for small hand-mixed jobs only. Structural and reinforced work, and any grade from C30 up, should be a designed mix batched to specification, since a shovel-gauged ratio cannot reliably hit the strength.
Concrete Strengths and Uses Compared
| Grade | Strength | Indicative mix | Designation | Best used for |
| C10 | 10 N/mm² | ~1:4:8 | GEN1 | Blinding, kerb bedding, drainage, fill |
| C15 | 15 N/mm² | ~1:3:6 | GEN2 | Light wall footings, pathways, flooring |
| C20 | 20 N/mm² | ~1:2:4 | GEN3 | Shed and garage bases, light foundations |
| C25 | 25 N/mm² | ~1:1.5:3 | ST2 / RC | Footings, slabs, driveways, extensions |
| C30 | 30 N/mm² | Designed mix | PAV1 | Driveways, external slabs, hardstanding |
| C35 | 35 N/mm² | Designed mix | PAV2 / RC | Commercial floors, heavy paving, foundations |
| C40 | 40 N/mm² | Designed mix | RC | Piling, beams, industrial and commercial |
What Determines Concrete Strength
Strength is engineered into the mix, not left to chance. A few factors decide the final figure:
- Water-cement ratio: the biggest lever of all, since less water for a given amount of cement means higher strength
- Cement content: more cement raises strength, which is why richer mixes carry higher grades
- Aggregate quality: the Concrete Society notes aggregate is the largest constituent by volume and directly affects compressive strength, so clean, well-graded stone builds a denser, stronger matrix
- Compaction: vibrating out trapped air lets the mix reach its rated strength
- Curing: keeping the concrete moist as it sets allows strength to develop fully
Get the water-cement ratio wrong on site, and even a well-specified grade underperforms, which is one reason a batched, designed mix is the surer route on any job where strength matters.
What does the C stand for in concrete grades?
The C stands for concrete, and the number is its characteristic compressive strength in newtons per square millimetre at the 28-day test. C30 means 30 N/mm².
Which concrete grade is best for a domestic driveway?
C30 is the usual choice for a driveway. It is air-entrained to resist frost and de-icing salts, and strong enough for regular vehicle traffic.
What grade of concrete is used for house foundations?
Foundations are commonly poured in C25 or an FND-designated concrete, with the exact grade depending on load and ground conditions such as sulfate levels.
Does concrete stop gaining strength at 28 days?
No, 28 days is the standard age for testing, when a mix should meet its specified grade. Concrete keeps hardening for months and years afterwards, often passing 110% of its grade strength within 90 days.
Can I mix these grades by hand from a nominal ratio?
Nominal ratios work as a rough guide for small, non-structural jobs. Structural and reinforced work needs a designed mix batched to specification to hit the strength reliably.
Is a higher concrete grade always better?
No, a higher grade costs more and suits demanding jobs, but over-specifying wastes money. The right grade is the one that matches the load and exposure of your project.
Final Thoughts
The right concrete grade comes down to what the concrete needs to handle once it is in place. Think about the load, ground conditions, exposure and how the concrete will be placed, then check the required specification before ordering. Getting the quantity right matters too, since running short during a pour can create unnecessary delays and joining fresh concrete to an older section may affect the finished result.
For projects across London and the surrounding areas, Pro-Mix Concrete supplies C10 to C40 through ready-mix and mix-on-site delivery, with line and boom pump hire available for sites where placement is more difficult. A free concrete calculator can help estimate the volume needed, while the team can advise on the practical delivery options for the site. Call 0800 772 3808 for a quotation.
- 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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