Concrete Mix Ratios and Structural Classes — UK Reference Guide

Standard Mix Ratios (Cement : Sand : Aggregate)

A concrete mix ratio always gives the proportions of cement, sand and coarse aggregate by volume in that order: cement : sand : aggregate. So 1 : 2 : 3 is one part cement to two parts sand to three parts aggregate. You may also see the same proportions written back to front, such as 3 : 2 : 1 for aggregate : sand : cement, particularly on sites that list materials in the order you would shovel them into a mixer. There is no single UK convention for which order a source uses, so check what each number refers to before batching: in every mix in the table above, cement is the smallest number and is listed first. The strengths in the table are also commonly written in MPa (megapascals) rather than N/mm²: the two are the same unit, so the 1 : 2 : 3 mix's “~30 N/mm²” is identical to “~30 MPa”.

Designated Mixes (BS 8500)

For ready-mix concrete ordered from a batching plant, you'll specify a designated mix under BS 8500. These are standardised mixes designed for specific applications:

BS 8500 designations: GEN 3 (C16/20) for general domestic unreinforced strip foundations in low-sulfate ground; FND 2 (C25/30) for sulfate-resistant trench-fill (DC-2, most common). RC designations (e.g. RC25/30) apply to reinforced concrete by exposure class. Your structural engineer or building control will specify the required mix based on BS 8500 Table A.6 and AD-A.

Each strength class gives two numbers in newtons per square millimetre (N/mm²): the first is the characteristic cylinder strength, the second is the characteristic cube strength, and UK practice generally quotes and tests the cube figure, which is the “Cube Strength” column above. N/mm² and MPa (megapascal) are the same unit under different names, with no conversion factor between them, so a cube strength quoted as “20 N/mm²” and “20 MPa” are identical. Ready-mix order sheets and supplier price lists sometimes shorten this to the cube number with an “N” suffix, for example “30N” or “30N/mm²” concrete for a C30-class mix. That is informal trade shorthand rather than a formal BS 8500 designation, and, as with the C30, C35 and C40 naming discussed below, exactly which dual-class it maps to at the higher grades is not always consistent between suppliers, so confirm the class in writing before ordering.

Some older and non-UK sources quote concrete strength in pounds per square inch (psi) instead of N/mm² or MPa. The two scale by a fixed factor: 1 MPa (1 N/mm²) is approximately 145 psi. So a GEN 3 mix at 20 N/mm² is roughly 2,900 psi, and an FND 2 / RC25/30 mix at 30 N/mm² is roughly 4,350 psi. Psi is not a unit used in BS 8500 or in UK structural specifications, so treat any psi figure as an approximate reference for comparison only, not as something to substitute directly into a UK spec.

Quantities Per m³

Approximate quantities of materials needed to produce 1 m³ of mixed concrete:

Quantities are approximate due to moisture content and compaction. Always order 5–10% extra.

C30, C35 and C40: Higher Structural Strength Classes

C30, C35 and C40 are structural concrete strength classes, named after their cube compressive strength in N/mm² at 28 days. In the same cylinder/cube notation already used above for FND 2, FND 3, RC25/30 and RC28/35, C30 is the class C25/30, C35 is C28/35, and C40 is C32/40.

• C30 (C25/30): reinforced foundations, ground-bearing and suspended floor slabs, load-bearing walls, and driveways or hardstanding carrying heavier vehicle loads. This is the same strength class behind FND 2 and RC25/30 in the table above.
• C35 (C28/35): heavier reinforced elements such as beams and columns, multi-storey building frames, industrial floor slabs, and external hardstanding under regular heavy-vehicle loading. This is the same strength class as FND 3 and RC28/35 in the table above.
• C40 (C32/40):high-strength structural elements and heavy-duty industrial flooring, where load capacity matters most. This is also the class this site's own grade selector uses for exposed, watertight and marine-exposure work; some other trade sources place water-retaining and marine-exposure duty at C35 instead, so always confirm the exposure class (BS EN 1992-3 for water-retaining structures) against your engineer's specification rather than the grade label alone.

These classes are ordered as designated ready-mix concrete to BS 8500, either by naming the strength class directly (C30, C35, C40, or the full C25/30, C28/35, C32/40 notation) or through a supplier's RC series. RC30, RC35 and RC40 are common trade shorthand for these three classes, but exact supplier naming for the higher grades is not fully standardised across the industry, so always confirm the strength class and designation against your engineer's specification rather than a supplier code alone.

The nominal 1 : 2 : 3 batch in the Standard Mix Ratios table above is traditionally associated with roughly C25/C30-class strength when it is well-batched and well-compacted, which is why it is listed there for general structural use. But “traditionally associated with” is not the same as “guaranteed to achieve”: a shovel-and-bucket volumetric ratio cannot guarantee a structural strength class. A designated mix is produced at a batching plant under quality control: cement, sand and coarse aggregate are weighed rather than shovelled, aggregate moisture is measured and the mix water adjusted for it, and the producer tests cubes from the load to confirm the class has actually been achieved. None of that is possible with a wheelbarrow mix on site. Wet or dry sand, an extra bucket of water for workability, or under-compaction can each cost several N/mm² of finished strength, with no way to check the result without lab-tested cubes. Designated ready-mix, not a volumetric ratio, is how C30 and above is actually specified when the strength class needs to be assured.

Trade sites do publish nominal cement:sand:aggregate ratios for C30, C35 and C40, generally getting richer (more cement, less aggregate) as the class goes up, but published figures disagree with each other from source to source, sometimes attributing the same ratio to different grades entirely: one trade source assigns 1:1.5:3 to C30, another assigns that same ratio to C25 and treats C30 and C35 as prescribed mixes with no nominal ratio given at all. That disagreement makes the point on its own: a nominal ratio is a rough approximation at best, not a specification, and none of them carry the batching control or cube testing that a genuine BS 8500 designated mix does. Our quick-answer guide, What Concrete Mix Ratio Do I Need?, gives a nominal rule-of-thumb ratio for C30 alongside the lower, site-mixable grades; treat that C30 figure, like any volumetric ratio, as an estimate rather than a guaranteed strength class. For C35 and C40 specifically, this page does not repeat a single figure, because the disagreement between sources runs too wide to be useful. For C30 and above on structural or safety-critical work, specify by strength class and order designated ready-mix rather than relying on a ratio found online.

For C30, C35 or C40 work, get the designation (RC, PAV, FND or similar), exposure class and any durability requirements from a structural engineer or building control, then order a designated ready-mix to that specification. Our Concrete Grade Calculator and Concrete Grade Selector can help you cross-check the right BS 8500 designation before you order, but they do not replace an engineer's specification for load-bearing or safety-critical work.

Site-Mixed vs Ready-Mix

For volumes over 1 m³, ready-mixfrom a batching plant is almost always more practical and ensures consistent quality. Minimum order is typically 1 m³ (some suppliers offer 0.5 m³ in a mini-mix truck). Ready-mix costs vary by region, order size and mix specification; current UK guide prices for standard-grade mixes cluster around £95–£145 per m³ delivered outside London and the South East, with London/South East deliveries and small mini-mix loads often running higher, up to roughly £190 per m³ all-in.

For smaller volumes, site mixingwith a cement mixer, bags of cement, and loose aggregates is more cost-effective. One bag of cement (25 kg) makes approximately 0.07–0.08 m³ of concrete in a 1:2:4 mix.

Calculate Your Concrete Needs

• Concrete Mix Calculator — cement, sand, and aggregate for your chosen mix
• Concrete Volume Calculator — volume in m³ for foundations, slabs and paths
• Foundation Calculator — foundation depth, width and concrete volume
• Concrete Grade Calculator — the correct BS 8500 grade, designation and cement content for your application
• Concrete Grade Selector — the right grade and exposure class by application