Concrete Mix Ratios Explained: C20 vs C25 vs C30 (Which One Do You Need?)

Ask five different people what “concrete” means and you’ll probably get five different answers — some picturing a bag from the builders’ merchant, others thinking of a lorry pulling up with a rotating drum. But ask a concrete supplier, and the first question back will almost always be: “What grade do you need?”

That single question trips up more DIYers and even some tradespeople than it should. Concrete isn’t one product — it’s a family of mixes, each formulated for a different job, and picking the wrong one is one of the most common (and most expensive) mistakes in small-scale UK construction. Order a mix that’s too weak, and you risk cracking, settlement, or outright failure. Order one that’s stronger than you need, and you’re paying extra for performance your project will never use.

This guide is a complete concrete mix ratio guide covering the three grades that come up in almost every domestic and light commercial job in the UK — C20, C25, and C30 — what the numbers actually mean, the ratios behind each one, and exactly which grade suits which job. We’ll also cover what concrete grade UK foundations actually need, since that’s the question that sends more people searching than any other.

What Do the “C” Numbers Actually Mean?

Before comparing the grades, it’s worth understanding what you’re actually comparing. The “C” in C20, C25, and C30 stands for compressive strength class, and the number that follows is the strength — measured in newtons per square millimetre (N/mm²) — that a standard test cube of that concrete is expected to reach after 28 days of curing.

So C20 concrete is designed to reach 20 N/mm², C25 reaches 25 N/mm², and C30 reaches 30 N/mm². The higher the number, the stronger and more load-bearing the finished concrete, but also the more cement-rich (and more expensive) the mix.

Old Notation vs Current BS EN 206 Notation

If you’ve been quoted a grade like “C25/30” and wondered why there are two numbers, here’s the explanation: since the UK adopted the European standard BS EN 206, grades are officially written with two figures — the first representing cylinder strength and the second representing cube strength. The cube figure is the one that matters in UK practice, since that’s the test method historically used here, and it’s the number most suppliers and merchants still refer to in conversation.

In practice:

C20 (old notation) ≈ C16/20 or C20/25 depending on supplier convention
C25 (old notation) ≈ C20/25 or C25/30
C30 (old notation) ≈ C25/30 or C28/35

This is genuinely one of the most common sources of confusion when ordering concrete, because “C25” in casual conversation and “C25/30” on a delivery ticket aren’t necessarily identical specifications. The safest approach is always to state the application (e.g. “house extension foundation,” “driveway,” “shed base”) rather than just a grade number, and let your supplier confirm the correct BS EN 206 designation. Most reputable ready-mix suppliers will do this automatically once they know what the concrete is for.

C20 Concrete: The General-Purpose Grade

C20 is the entry point for most non-structural domestic work. It’s strong enough for everyday use but not intended for anything that needs to carry significant structural load or resist heavy, repeated vehicle traffic.

C20 Mix Ratio

The traditional site-mix ratio for C20 is:

1 part cement : 2 parts sand : 4 parts aggregate (1:2:4)

This ratio is the one most people mean when they casually refer to “the concrete mix ratio,” since it’s the most commonly hand-batched mix on small UK sites and the default many DIY guides quote.

What Is C20 Concrete Used For?

Garden paths and stepping areas
Shed bases and small outbuilding foundations
Fence post foundations
Non-structural garden slabs
Light-duty patios (where vehicle weight is never expected)
Blinding layers beneath other structural pours

C20 is a sensible, cost-effective choice when the concrete simply needs to be solid and durable underfoot, without carrying vehicles or supporting a structural load above it. It is not suitable for driveways, house foundations, or anything exposed to regular freeze-thaw cycling without additional durability measures, and it shouldn’t be substituted into a specification that calls for a higher grade just to save on material cost.

C25 Concrete: The Versatile Mid-Range Grade

C25 sits in the middle of the range and is, for many UK domestic projects, the most commonly ordered grade of all. It’s strong enough for genuinely structural applications while remaining more workable and less expensive than the higher grades used in commercial construction.

C25 Mix Ratio

The typical site-mix ratio for C25 is:

1 part cement : 1.5 parts sand : 3 parts aggregate (1:1.5:3)

Some suppliers and guides quote a slightly leaner 1:2:3 ratio depending on the aggregate type and target workability, but 1:1.5:3 is the most widely referenced C25 ratio in UK guidance.

C25 Concrete Uses

This is where C25 earns its reputation as the most versatile grade in domestic construction. C25 concrete uses commonly include:

Domestic strip and trench-fill foundations on reasonable, non-sulphate-bearing ground
Driveways for standard car use (though many suppliers now recommend air-entrained paving grades — more on this below)
Floor slabs in garages, workshops, and extensions
Footings for extensions and outbuildings
Structural slabs that need to bear real, sustained load without being subject to heavy commercial traffic

C25’s popularity comes down to a genuinely good balance: it’s strong enough to be specified for load-bearing work, but it’s not so cement-rich that it becomes needlessly expensive for domestic-scale projects. If you’re not sure which grade your project needs and it isn’t purely decorative or purely structural, C25 is very often the right starting point — though you should always confirm against your specific ground conditions and, where applicable, your structural engineer’s specification.

A note on driveways specifically: while C25 is technically capable of taking car loads, many UK ready-mix suppliers now recommend upgrading to an air-entrained paving-grade mix (commonly labelled PAV1, roughly equivalent to C30/37) for driveways, because standard C25 can be prone to surface scaling under freeze-thaw and de-icing salt exposure. If your project is a driveway rather than a foundation or internal slab, it’s worth reading our dedicated guide on driveway concrete thickness and grade before ordering.

C30 Concrete: The Heavy-Duty Structural Grade

C30 is the grade to reach for when the concrete needs to withstand serious, sustained load — vehicle traffic beyond ordinary car use, structural elements, or environments where durability against weather and wear really matters.

C30 Mix Ratio

The typical site-mix ratio for C30 is:

1 part cement : 1 part sand : 2 parts aggregate (1:1:2)

This is a noticeably richer mix than C20 or C25 — more cement relative to aggregate produces a denser matrix, greater compressive strength, and better resistance to abrasion and environmental stress, at the cost of reduced workability and a higher price per cubic metre.

What Is C30 Concrete Used For?

Driveways with heavy or frequent vehicle use, including vans and multiple cars
Commercial and agricultural yards
Garage floors expected to take workshop or heavier vehicle loads
External paving subject to constant traffic and weathering
Structural elements, such as certain foundations on poor or shrinkable clay ground, where a structural engineer specifies trench-fill in a stronger mix
Reinforced slabs and structural work more broadly, often specified as part of a wider RC (reinforced concrete) designation

C30 is rarely hand-mixed on site in any meaningful volume — the ratio needs to be followed precisely to achieve the intended strength, and most suppliers and guides recommend ordering C30 as a designed ready-mix rather than attempting to hand-batch it, particularly for anything load-bearing. If your project genuinely needs C30 or above, that’s usually also a sign the volume involved is large enough that ready-mix delivery makes more practical and financial sense than mixing by hand anyway.

C20 vs C25 vs C30: Side-by-Side Comparison

Grade	Strength (28-day cube)	Typical ratio	Best for	Site-mixable?
C20	20 N/mm²	1:2:4	Paths, shed bases, fence posts, non-structural slabs	Yes, easily
C25	25 N/mm²	1:1.5:3	Foundations, driveways (standard), floor slabs, footings	Yes, with care
C30	30 N/mm²	1:1:2	Heavy-duty driveways, structural work, commercial/agricultural yards	Ready-mix strongly recommended

A simple way to think about it: C20 for things you walk on, C25 for things that support a structure or occasional vehicle weight, and C30 for things that take heavier or more frequent vehicle traffic or genuine structural load.

What Concrete Grade Does UK Building Regulations Require for Foundations?

This is one of the most searched questions in UK concrete planning, and unfortunately there isn’t a single one-size-fits-all answer — what concrete grade for foundations UK building control accepts genuinely depends on your ground conditions, the type of foundation, and whether the project falls under Building Regulations Approved Document A.

General Guidance

For standard domestic strip or pad foundations on normal, non-sulphate-bearing ground, C20 (old notation), broadly equivalent to C16/20 or C20/25 under BS EN 206, is commonly accepted as a minimum for unreinforced foundations.
For strip and trench-fill foundations more broadly, many UK builders default to C25, since it comfortably clears the minimum bar for most standard ground conditions while giving a margin of safety.
For trench-fill foundations on poor or shrinkable clay ground, or where sulphates are present in the soil, C30 or a specific designated mix (such as an FND-class mix under BS 8500) is often required — this is not a substitution you should make casually, since sulphate-resisting cement and specific mix designs exist precisely to protect the foundation from long-term chemical attack from the ground.
Under Building Regulations Approved Document A, strip foundations must generally be at least 150mm thick, and the minimum foundation depth is typically 450mm, though clay soils and other challenging ground conditions frequently require significantly greater depth — sometimes 900mm to 1,200mm or more.

Why You Shouldn’t Guess This One

Unlike a garden path or a shed base, foundation specification directly affects the structural safety of a building and is subject to Building Control sign-off. Ground conditions vary enormously even within the same street, let alone across the country, and features like sulphate content, water table depth, and soil type (clay, sand, made ground, etc.) can all change what grade and design are actually required. For any project requiring Building Control approval — new builds, extensions, or anything altering the structure of a building — always get written confirmation of the required mix from your structural engineer or Building Control officer, rather than relying on a general guide like this one for the final specification. This article is a starting point for understanding the system, not a substitute for site-specific engineering advice.

Understanding the Mix Ratio Numbers

It’s worth briefly explaining why these ratios work the way they do, since it makes the differences between grades much more intuitive.

A concrete mix ratio describes the proportion of three core ingredients by volume (or sometimes by weight): cement, fine aggregate (sand), and coarse aggregate (gravel or crushed stone), combined with water. As you move from C20 to C25 to C30:

Cement content increases relative to the other ingredients, which is the single biggest driver of strength
Aggregate content decreases proportionally, since the mix becomes richer in the binding paste and less reliant on bulk filler
Workability typically decreases slightly, meaning higher grades can be a little stiffer and less forgiving to place and finish by hand
Cost per cubic metre rises, since cement is the most expensive core ingredient in the mix

This is why simply “adding more cement” to a weaker mix isn’t a shortcut to a stronger one without proper mix design — water content, aggregate grading, and curing conditions all interact with cement content to determine the final strength, and getting the balance wrong can just as easily produce a weaker, more brittle slab as a stronger one.

A Note on Water-Cement Ratio

Alongside the cement-sand-aggregate proportions, the water-cement ratio is one of the most important — and most commonly mishandled — factors in site-mixed concrete. Too much water makes the mix easier to place but significantly weakens the cured concrete, since excess water leaves voids in the matrix as it evaporates. Too little water makes the mix difficult to work with and can leave it under-compacted, which also weakens the final result. As a general guide, a water-cement ratio under roughly 0.55 is commonly targeted for C25-equivalent strength when mixing by hand, though ready-mix suppliers control this far more precisely through designed batching than is realistically achievable on a domestic site with buckets and a mixer.

Site-Mixed vs Ready-Mix: Which Should You Use?

Once you know your grade, the next decision is whether to mix it yourself or order it delivered. As a rough guide used across the industry:

Hand mixing is genuinely practical for volumes up to around 0.5m³ — small paths, single fence posts, tiny patch repairs.
A hired cement mixer extends that practical range somewhat, useful for shed bases and small slabs, typically up to around 1–2m³ depending on time and labour available.
Ready-mix delivery becomes the more cost-effective and far more reliable option for anything above roughly 1–2m³, and is strongly recommended for any C30 pour or anything structural, regardless of volume, because achieving consistent strength by hand-batching a rich mix is genuinely difficult without proper equipment.

If you’re still working out exactly how much concrete your project needs before deciding between the two, our mix design calculator will convert your measurements into an exact volume, then break that down into either ready-mix order quantities or the cement, sand, and aggregate weights you’d need for a site mix — worth running before you commit to either approach.

Common Mistakes When Choosing a Concrete Grade

Assuming “concrete is concrete.” The difference between C20 and C30 isn’t marginal — it’s the difference between a garden path and a load-bearing structural element. Treating them as interchangeable is one of the most expensive assumptions in small-scale construction.
Downgrading to save money on foundations. A cheaper grade might shave a modest amount off the material bill, but if it fails to meet Building Control requirements or genuinely can’t handle the ground conditions, the cost of remedial work — or a failed inspection — dwarfs the saving.
Overspecifying “just to be safe.” Ordering C30 for a garden path isn’t a safety margin, it’s wasted cement. Matching the grade to the actual load and exposure conditions is the goal, not simply picking the strongest option available.
Ignoring exposure and durability requirements. Strength class alone doesn’t guarantee durability — external concrete exposed to freeze-thaw, chlorides (e.g. from de-icing salt or coastal locations), or sulphate-bearing ground often needs additional durability measures (air entrainment, sulphate-resisting cement, increased cover to reinforcement) on top of the base strength class.
Hand-batching a structural mix without proper control. A bucket-ratio “1:1.5:3” mixed loosely on site is not the same as a specified, tested C25 concrete. If Building Control or a structural engineer has specified a grade, that generally means a designed, tested mix — not an approximate hand-batched equivalent.
Not checking ground conditions before choosing a foundation mix. Sulphates, high water tables, and clay shrinkage can all change the required specification, sometimes significantly, regardless of what a general-purpose guide (including this one) suggests as a starting point.

Strength Isn’t the Whole Story: Durability and Exposure Classes

One thing this guide has touched on but deserves its own section: compressive strength class and durability are two different things, and confusing them is a genuinely common source of premature concrete failure in the UK.

A concrete’s strength class (C20, C25, C30, and so on) tells you how much load it can bear once fully cured. It says nothing on its own about how well that concrete will resist the environment it’s placed in — freeze-thaw cycling, sulphates in the ground, chloride exposure from coastal air or de-icing salt, or repeated wetting and drying. That’s where exposure classes come in, and it’s why two slabs poured at the same strength class can age completely differently depending on where and how they were specified.

Common UK Exposure Classes (Simplified)

Exposure class	Typical environment	Minimum grade guidance
XC1	Dry, permanently indoors	C20/25
XC2	Wet, rarely dry (e.g. some foundations)	C25/30
XC3 / XC4	External, moderate to cyclic wet/dry (most UK external concrete)	C28/35 – C30/37
XD1 / XS1	Exposed to chlorides — coastal areas, roads salted in winter	C32/40
XD3 / XS3	Severe chloride exposure (marine structures, heavily salted roads)	C35/45

This is precisely why a driveway — which sits externally, gets rained on, freezes in winter, and is often gritted or salted — is frequently specified in a durability-enhanced paving mix (like PAV1, roughly C30/37) rather than a plain C25, even though C25’s raw compressive strength is technically adequate for the vehicle loads involved. The extra grade isn’t about carrying more weight; it’s about surviving British winters without the surface scaling and flaking away.

If your project sits in a more demanding exposure category — coastal, heavily trafficked with de-icing salt, or in contact with sulphate-bearing ground — it’s worth having this conversation explicitly with your supplier or engineer, rather than assuming that hitting a strength number alone covers you.

Roughly What Each Grade Costs

Grade choice also has a direct, predictable effect on price, since cement is the most expensive component in the mix and higher grades use proportionally more of it. As a general guide for UK ready-mix pricing in 2026:

Grade	Typical price per m³ (delivered, exc. VAT)	Price relative to C20
C20 (general purpose)	£100–£120	Baseline
C25	£105–£130	Roughly 5–10% more
C30 (or PAV1 driveway-grade equivalent)	£115–£145	Roughly 15–20% more

The gap between grades is usually smaller than people expect — often only a few pounds per cubic metre between C20 and C25, for example — which is one more reason it rarely makes sense to under-specify a foundation or driveway purely to save money. The real cost differences on a project tend to come from delivery, pumping, labour, and site access, not from the small premium of stepping up a grade.

Frequently Asked Questions

What’s the difference between C20 and C25 concrete? C25 is the stronger of the two, reaching 25 N/mm² compared with C20’s 20 N/mm² after 28 days. In practice, C20 suits non-structural work like paths and shed bases, while C25 is commonly used for foundations, driveways, and structural floor slabs.

Is C25 concrete strong enough for a driveway? It can handle standard car loads, but many UK suppliers now recommend an air-entrained paving grade (commonly PAV1, broadly equivalent to C30/37) for driveways specifically, since it offers better resistance to frost and de-icing salt damage than standard C25.

What concrete grade do I need for house foundations in the UK? It depends on your ground conditions. C20-equivalent mixes are often accepted for standard strip foundations on good ground, C25 is a common default for domestic strip and trench-fill foundations, and C30 or a specific designated mix is often required on poor or shrinkable clay ground, or where sulphates are present. Always confirm the exact specification with your structural engineer or Building Control officer.

Can I use C30 concrete for a shed base? You can, but it’s generally unnecessary — C20 is more than sufficient for a shed base and considerably cheaper. C30 is better reserved for genuinely heavy-duty or structural applications.

What ratio is C20 concrete? The typical site-mix ratio for C20 is 1 part cement to 2 parts sand to 4 parts aggregate (1:2:4).

What ratio is C30 concrete? The typical site-mix ratio for C30 is 1 part cement to 1 part sand to 2 parts aggregate (1:1:2), a noticeably richer mix than C20 or C25.

Do I need a structural engineer to specify concrete grade? For garden paths, shed bases, or other non-structural work, no — general guidance like this is usually sufficient. For anything that affects the structural integrity of a building, including most foundations, floor slabs supporting a structure above, and retaining walls, a structural engineer’s specification (and Building Control approval, where applicable) is strongly recommended and often legally required.

Quick Reference Summary

C20 (1:2:4): paths, shed bases, fence posts, non-structural slabs — the general-purpose grade
C25 (1:1.5:3): foundations, driveways, floor slabs, footings — the most versatile domestic grade
C30 (1:1:2): heavy-duty driveways, commercial/agricultural yards, structural work — the highest-strength common grade
Foundations: grade depends on ground conditions — always confirm with a structural engineer or Building Control for anything structural
Volume over ~1–2m³, or any structural pour: ready-mix is more reliable than hand-batching

Explore Further

Once you know which grade fits your project, the next step is working out exactly how much you need and what it will cost:

Browse our full Concrete Grades reference — detailed specifications, exposure classes, and BS 8500 designations for every common UK grade from GEN0 through to C40 and above
Use our Mix Design Calculator — enter your project dimensions and get an exact volume, ready-mix order quantity, or site-mix material breakdown (cement, sand, aggregate, and water) in seconds
C20/25 Grade page — full spec sheet, uses, and durability notes
C25/30 Grade page — full spec sheet, uses, and durability notes
C30/37 Grade page — full spec sheet, uses, and durability notes