Intermediate

Concrete Strength Classes Explained: C25/30, C30/37 & More

17/04/2026 14 min read

When you order ready-mix concrete (RMC) from a batching plant or read a structural specification, you come across codes like C25/30 XC1 S3 D22 Cl 0.40. These aren’t decorative — they describe the mechanical strength, exposure level and consistency of the concrete. Getting even one of these parameters wrong means ordering concrete that will crack, freeze or fail to carry the intended load. This guide gives you every code you need to know so you don’t get caught out on delivery day.

The C25/30 designation: what the numbers mean

Since the European standard EN 206-1 (implemented in the UK as BS EN 206 with the UK National Annex), concrete is designated by its compressive strength class. The code takes the form C XX/YY:

C = Concrete (normal or heavy aggregate concrete, as opposed to LC for lightweight concrete)
XX = characteristic strength measured on a cylindrical specimen (Ø 150 mm × h 300 mm) at 28 days, in MPa
YY = characteristic strength measured on a cube specimen (150 × 150 × 150 mm) at 28 days, in MPa

The cube gives a higher value than the cylinder because of the specimen shape and the confining effect of the press platens. Both measurements describe the same concrete — they are simply two measurement conventions used in different countries (France and Germany historically used cylinders, the UK and Scandinavia used cubes).

Tip — When someone says “C25/30 concrete”, they mean concrete that withstands 25 MPa in compression on a cylinder and 30 MPa on a cube. Not 25+30=55 MPa. This is the number-one mistake made by beginner self-builders when filling out a delivery order.

What about the old “B25” notation?

Before 2004, concrete was designated by its cube strength only (standard NF P 18-305): B20, B25, B30. This notation is obsolete but still widely used in old DTU guides, on building sites and by some tradespeople.

Old notation	Current notation (EN 206)
B16	C12/15
B20	C16/20
B25	C20/25
B30	C25/30
B35	C30/37
B40	C35/45

If a neighbour or mason tells you “use B25 for your slab”, ask whether they mean the old B25 (= current C20/25) or if they’re using it as shorthand for C25/30. The difference affects both the price per m³ and the performance.

Common concrete strength classes in house self-build

For a residential house, you only need 4 classes from the 16 standardised grades (C8/10 through C100/115). There is no need to specify C50/60 — that is bridge and industrial concrete, prohibitively expensive and impossible to place with a shovel.

Class	Typical use	Indicative cement dosage	Indicative RMC price
C16/20	Blinding concrete, levelling, mass fill concrete	250 kg/m³	100-115 €/m³
C20/25	Light foundations, strip footings for single-storey house	280-300 kg/m³	110-125 €/m³
C25/30	Standard house build: foundations, slabs, ring beams, formwork walls	300-350 kg/m³	120-140 €/m³
C30/37	Two-storey houses, foundations on poor soil, load-bearing floors, drive-on garage	350-380 kg/m³	135-160 €/m³
C35/45	Exposed concrete (pool, drive-on terrace), 3+ storey building	380-400 kg/m³	150-180 €/m³

Best practice — For 95% of residential houses, C25/30 is the correct default choice. It covers foundations, concrete slabs, horizontal and vertical ring beams and lintels. It is the all-round concrete specified by the majority of residential structural standards.

Exposure classes (XC, XF, XS, XA, XD)

Mechanical strength alone is not enough. Concrete placed in a freezing zone, near the sea or in contact with aggressive water must be formulated differently (higher cement content, admixtures, lower water/cement ratio). This is the role of exposure classes, coded by letters:

Code	Meaning	Typical house example
X0	No risk (dry interior)	Internal concrete partitions
XC1	Dry or permanently wet (no alternation)	Protected interior concrete, slab under floor tile
XC2	Wet, rarely dry	Buried foundations, strip footings
XC3	Moderate humidity (sheltered exterior)	Exterior walls under render, garage slab
XC4	Cyclic wet/dry	Exterior walls exposed to rain, terraces
XF1	Moderate freeze/thaw, no de-icing salts (vertical facade)	Exterior wall in temperate zone
XF3	Severe freeze/thaw, no salts (horizontal)	Terrace slab in mountain zone
XF4	Severe freeze/thaw + de-icing salts (road)	Driveway in cold region
XS / XD	Sea / chloride salts	House within 3 km of the coast
XA1-3	Chemically aggressive water	Concrete in contact with sulphate-bearing soil

Combining strength class and exposure class

The UK National Annex to BS EN 206 sets minimum strength requirements depending on exposure. You cannot specify C20/25 in XF3 for example — the standard will require at minimum C30/37.

Warning — Many self-builders order “C25/30” without specifying an exposure class. The batching plant then delivers C25/30 XC1 by default — suitable for dry interiors. If you pour an external terrace slab with this concrete, it will spall after the first winter. Always specify the exposure class when requesting a quote.

Consistency classes (slump)

Consistency is the workability of fresh concrete. It determines how easy the concrete is to place. It is measured with the Abrams cone (slump cone test): fill a 300 mm cone, remove it, and measure the slump in mm.

Class	Slump (mm)	Consistency	Typical use
S1	10-40	Stiff	Blinding concrete, kerbs, vibrated industrial slabs
S2	50-90	Plastic	Foundations, mass concrete, strip footings
S3	100-150	Very plastic	Standard for house slabs, ring beams
S4	160-210	Flowing	Pumped slabs, thin sections
S5	≥ 220	Very flowing	Self-compacting concrete (SCC), narrow formwork

The right slump for each application

Question

Foundations from the truck chute: S2 or S3
Slab placed with a screed rail: S3
Slab pumped: S4 (to pass through the pump hose)
Ring beams and lintels: S3 (flows well around reinforcement)
Formwork walls: S4 or S5 if heavily reinforced

Tip — The higher the slump, the easier the concrete is to place — but also the more water it contains, which reduces final strength. Do not ask for S5 “so it flows better” if S3 will do. If you need extra workability, ask the plant to add a plasticiser to the truck (the RMC supplier can do this on site for around 5-10 €/m³).

Maximum aggregate size (D)

The code D22 (or Dmax 22) means the largest aggregate particle in the mix is 22 mm. This is the standard value for residential construction. You may also find:

D8: fine concrete, micro-concrete for fixings
D10 or D12.5: concrete for thin sections (lintels, closely spaced ring beams, thin slabs)
D16: compromise between workability and economy
D22: house standard — foundations, slabs, formwork walls
D32: mass concrete, thick raft foundations

The smaller the D, the more expensive the concrete per m³ (more cement and sand required for equivalent workability). Stick with D22 unless there is a specific reason not to.

Decoding an RMC delivery order

Here is a typical order for a single-storey house ground-floor slab:

C25/30 XC1 S3 D22 Cl 0.40 — 12 m³

Breakdown:

C25/30: strength 25 MPa cylinder / 30 MPa cube
XC1: protected interior exposure (slab on insulation + floor covering on top)
S3: plastic consistency for placement with a screed rail
D22: maximum 22 mm aggregates
Cl 0.40: maximum chloride content 0.40% (standard for ordinary reinforced concrete)
12 m³: quantity

Warning — The chloride content Cl 0.40 is the maximum permitted for reinforced concrete. For prestressed concrete, the limit drops to Cl 0.20. If you are pouring a slab with standard reinforcement mesh, 0.40 is sufficient. Always check there is no “Cl 1.0” on your delivery note — that would be plain (unreinforced) concrete, which would corrode your reinforcement within a few years.

Site-mixed concrete vs ready-mix: which class for what?

For small volumes (< 1 m³: post foundations, lintels, small screed), you can mix your own concrete with a mixer. For large volumes (foundations, slabs), order ready-mix.

Criterion	Site mixer	RMC truck
Economical volume	< 1 m³	> 2 m³
Guaranteed strength class	C16/20 to C20/25 max	All classes (C16/20 to C50/60)
Dosage accuracy	± 15%	± 3%
Traceability	None (100% your responsibility)	Delivery note + technical data sheet
Admixtures (plasticisers, waterproofers)	Difficult	Standard
Price per m³	70-90 € (materials only)	110-180 € delivered

Best practice — For your house build, use the site mixer for mortar when laying blockwork, small fixings and lintels under 50 L. For everything else — foundations, slabs, ring beams, screed — order ready-mix by truck. It works out cheaper than buying bagged cement and breaking your back for 3 days.

Mix proportions for a C20/25 site-mixed concrete

If you do mix your own concrete (rarely justified on a serious self-build, but useful for small works), here are the reference proportions for 1 m³ of C20/25 concrete (formerly B25):

Component	Quantity per 1 m³
CEM II 32.5 R cement	350 kg (14 × 25 kg bags)
Sand 0/4	700 kg (approx. 450 L)
Gravel 4/22	1,050 kg (approx. 700 L)
Water	175 L (w/c ratio = 0.50)

Scaled to a 130 L (useful capacity) mixer, that gives per batch:

35 kg of cement (1.5 × 25 kg bags)
70 kg of sand (approx. 50 L — 5 buckets of 10 L)
105 kg of gravel (approx. 70 L — 7 buckets)
17.5 L of water

Warning — The water/cement ratio (w/c) is the single most important factor in the final strength of concrete. If you add water “to make it flow better”, you lose 5 to 10 MPa of strength for every 0.1 increase in w/c. A w/c of 0.60 on your C20/25 reduces it to C16/20 in practice. Measure your water with a graduated bucket — never by eye.

Cheat-sheet: which class for my application?

Certifications to check on your RMC supplier

When ordering concrete, verify that your supplier holds the relevant quality certification. In the UK, look for a supplier accredited under the Quality Scheme for Ready Mixed Concrete (QSRMC) or holding a BSI Kitemark for BS EN 206. This guarantees that:

The specified dosage is consistently achieved
Strength is verified by cube tests in an independent laboratory
The delivery note is a legally binding document in the event of a defect claim

Conseil

The major national producers (Lafarge Holcim, Cemex, Eqiom, Vicat, Unibeton) are all certified. Be wary of small, uncertified local plants: you have no guarantee of the actual mix design, and building insurance may refuse to cover a claim linked to untraced concrete.

Common mistakes

1. Confusing C25/30 with “C25”. Plain C25 does not exist in the standard. If someone writes that, ask for the full class.

2. Forgetting the exposure class. The plant delivers XC1 by default. On an external slab, that guarantees spalling within 5 years.

3. Ordering too high a slump for “pour comfort”. You lose strength. Use a plasticiser instead.

4. Reusing concrete that has been waiting too long. After 1.5 to 2 hours from batching, concrete begins to set. Refuse a truck that arrives late unless a set retarder has been added by the plant.

5. Mixing deliveries from two trucks without control. If two trucks arrive 2 hours apart for the same slab, plan a construction joint (projecting reinforcement mesh, raked surface) at the right location.

6. Ignoring the ambient temperature. Below +5 °C or above +30 °C, request a specific mix (winter concrete with an admixture or summer concrete with a retarder). The plant may refuse to deliver outside this range.