🏗️ Mix Strength Calculator UK
Calculate concrete and mortar mix strength instantly — find your mix design from a required strength class, or check the strength class of an existing mix ratio. BS EN 206 and BS 8500 referenced.
Mix Strength Calculator UK
Three modes — check an existing mix, design a mix for a target strength, or compare two mixes side by side.
Enter your cement : sand : aggregate ratio and water-cement ratio to find the predicted strength class.
Select your required concrete strength class and application — get the recommended mix ratio, W/C ratio, and cement content.
Enter two mix ratios and W/C ratios to compare their predicted strengths side by side.
Mix A
Mix B
💪 Strength Indicator (C10 → C50+)
📋 Mix Details
✅ Suitability Assessment
UK Concrete Strength Classes — C10 to C50
UK concrete strength classes are defined in BS EN 206 and BS 8500. The C designation refers to the characteristic compressive strength in N/mm² — the first number is the cylinder strength, the second is the cube strength. UK practice traditionally uses cube strength, so C25/30 means 25 N/mm² cylinder / 30 N/mm² cube at 28 days.
C25/30 = Characteristic cylinder strength 25 N/mm² / cube strength 30 N/mm². The cube strength is always approximately 20–25% higher than the cylinder strength. UK site testing uses 150mm cubes; European testing uses 150×300mm cylinders.
| Strength Class | Cube Strength | Typical Mix Ratio | W/C Ratio | Cement (kg/m³) | Typical Application |
|---|---|---|---|---|---|
| C10/12 | 12 N/mm² | 1:3:6 | 0.65–0.70 | ~210 | Blinding, non-structural fill |
| C15/20 | 20 N/mm² | 1:2.5:5 | 0.60–0.65 | ~240 | Lean foundations, kerb bedding |
| C20/25 | 25 N/mm² | 1:2:4 | 0.55–0.60 | ~280 | Strip foundations, paths |
| C25/30 | 30 N/mm² | 1:1.5:3 | 0.50–0.55 | ~320 | Foundations, floor slabs, driveways |
| C28/35 | 35 N/mm² | 1:1.5:3 (low W/C) | 0.47–0.52 | ~340 | Reinforced foundations, retaining walls |
| C30/37 | 37 N/mm² | 1:1.25:2.5 | 0.45–0.50 | ~360 | Driveways, exposed slabs, XF1 exposure |
| C35/45 | 45 N/mm² | Designed mix | 0.38–0.45 | ~380 | Structural frames, columns, beams |
| C40/50 | 50 N/mm² | Designed mix | 0.35–0.40 | ~400 | Heavy structural, bridges |
| C50/60+ | 60+ N/mm² | Designed mix + admixtures | 0.28–0.35 | ~450+ | High-rise, pre-stressed, marine |
Concrete above C30/37 should be ordered as a designed mix from a ready-mix plant with certified batch records and cube test results to BS EN 12390. Site-mixed volumetric ratios cannot reliably achieve or verify strength above C30. Always specify designed mixes for structural elements covered by Building Regulations.
Standard Concrete Mix Ratios — UK Site-Mix Guide 2025
Standard concrete mix ratios are expressed as Cement : Sand : Aggregate (by volume or weight). These volumetric mixes are suitable for domestic construction where a designed mix is not required. For any structural work under Building Regulations, confirm specification with your structural engineer or building control surveyor.
1:3:6 Mix — GEN0 / Blinding
1 cement : 3 sand : 6 coarse aggregate. Very lean mix used for blinding layers under foundations and non-structural fill. Equivalent to C10/12. W/C approximately 0.65–0.70.
1:2:4 Mix — GEN3 / General Use
1 cement : 2 sharp sand : 4 coarse aggregate. The classic UK general-purpose mix. Equivalent to C20/25, suitable for domestic strip foundations, garden paths, and non-exposed slabs. W/C approximately 0.55–0.60.
1:1.5:3 Mix — RC35 / Structural
1 cement : 1.5 sharp sand : 3 coarse aggregate. The standard UK structural domestic mix, equivalent to C25/30. Used for reinforced foundations, floor slabs, and driveways. W/C approximately 0.50–0.55.
Prescribed Mixes — BS 8500-2 Reference
BS 8500-2 defines prescribed mixes (PM) by specification rather than strength testing — suitable for low-risk domestic applications. The most common are:
- GEN0 — Blinding, mass fill (equivalent to ~C8/10)
- GEN1 — Kerb bedding, backing (equivalent to ~C12/15)
- GEN2 — Strip foundations in low-risk ground (equivalent to ~C16/20)
- GEN3 — General purpose, paths, garage floors (equivalent to ~C20/25)
- RC20/25 to RC40/50 — Reinforced concrete, specified by strength class
- PAV1 / PAV2 — Paving concrete with freeze-thaw resistance
For extensions, new builds, and any structural work subject to Building Control approval, order ready-mix concrete with a designed mix specification to BS 8500 / BS EN 206. Ready-mix is typically only marginally more expensive than site-mixing for volumes above 1m³ and gives traceable, tested quality assurance.
Mortar Mix Strength Classes — BS EN 998-2
Mortar strength for masonry work is classified under BS EN 998-2 into strength classes M1 to M20. In the UK, the most commonly used classes for domestic brickwork and blockwork are M2, M4, and M6. The class designation refers to the compressive strength in N/mm² at 28 days.
| Mortar Class | Strength | Mix Ratio (C:L:S) | Mix Ratio (C:S only) | Typical Application |
|---|---|---|---|---|
| M1 | 1 N/mm² | 1:3:12 | — | Heritage / conservation pointing |
| M2 | 2 N/mm² | 1:2:9 | — | Internal non-loadbearing walls |
| M4 | 4 N/mm² | 1:1:6 | 1:4 | Standard external brickwork / blockwork |
| M6 | 6 N/mm² | 1:0.5:4.5 | 1:3 | Below DPC, exposed walls, parapets |
| M12 | 12 N/mm² | — | 1:3 (rich) | Engineering brickwork, manholes |
| M20 | 20 N/mm² | — | Designed mix | Specialist structural applications |
Many builders mistakenly specify strong mortar thinking it is better — this is wrong. Mortar must be weaker than the brick or block it bonds. Hard OPC mortars (M12+) prevent normal movement, causing bricks to crack rather than the mortar joint. For most UK brickwork, M4 (1:1:6) is correct. Use M6 only below DPC or in highly exposed locations.
How to Design a Concrete Mix — UK Step-by-Step Guide
Concrete mix design is the process of selecting proportions of cement, water, fine aggregate (sand), and coarse aggregate to meet specified strength, workability, and durability requirements. The UK method follows The Concrete Centre MX Mix Design Method, aligned with BS 8500 and BS EN 206.
- Step 1 — Specify the requirement: Define the required strength class (e.g. C25/30), exposure class (e.g. XC3), and workability class (e.g. S3) based on the application and environment
- Step 2 — Set maximum W/C ratio: From BS 8500 Table A.4, identify the maximum permitted W/C ratio for the exposure class — this may be more restrictive than strength alone requires
- Step 3 — Determine water content: Based on aggregate size and target slump class — typically 160–200 kg/m³ for 20mm aggregate at S3 workability
- Step 4 — Calculate cement content: Cement = Water ÷ W/C ratio. Check against minimum cement content for the exposure class
- Step 5 — Determine aggregate content: Total aggregate = Fresh concrete density (≈2,400 kg/m³) minus cement, water, and air content
- Step 6 — Split fine/coarse aggregate: For 20mm aggregate, approximately 40% fine aggregate (sand) and 60% coarse aggregate by weight is typical
- Step 7 — Trial mix and test: Produce trial mix, measure workability (slump/flow), cast cubes, and crush at 7 and 28 days to verify strength
For CEM I Portland cement with standard 20mm aggregate, approximate 28-day cube strength can be estimated as: Strength ≈ 97 ÷ (W/C ratio)² N/mm². At W/C 0.50: 97 ÷ 0.25 ≈ 38 N/mm². This is a simplified guide only — actual strength varies with cement source, curing, and temperature.
Mix Strength Calculator — Frequently Asked Questions
Common questions about UK concrete and mortar mix strength, classes, and design.
For standard UK domestic strip foundations, C25/30 (mix ratio 1:1.5:3) is the minimum recommended strength class. This corresponds to BS 8500 designated mix RC25/30. In areas with sulfate-bearing ground (class DS-2 or above), specify SRPC (sulfate-resisting Portland cement) or a CEM III/GGBS mix at the same strength class. Always confirm with your building control officer — some local authorities require C30/37 for reinforced strip foundations.
C25/30 and C30/37 differ in both characteristic strength and durability performance:
- C25/30: 30 N/mm² cube strength, W/C ~0.50–0.55, ~320 kg cement/m³
- C30/37: 37 N/mm² cube strength, W/C ~0.45–0.50, ~360 kg cement/m³
- C30/37 is approximately 20–25% stronger and 10–15% more expensive
- C30/37 is required for XF1 freeze-thaw exposure and XC4 carbonation class
- For most domestic UK foundations, C25/30 is sufficient — C30/37 is used for driveways, exposed external slabs, and reinforced retaining walls
For a UK vehicle driveway, specify C30/37 (PAV1 or RC30/37 to BS 8500) with a W/C ratio of 0.45–0.50 and air entrainment at 4–6% to resist freeze-thaw damage. As a site mix, use a 1:1.25:2.5 ratio with minimum 150mm slab thickness, 100mm compacted Type 1 sub-base, and edge restraints. For HGV / heavy vehicle access, upgrade to C35/45 with a designed mix. Add polypropylene fibres at 0.9 kg/m³ to control plastic shrinkage cracking.
For a 1:n:m mix by volume, the cement content per m³ is estimated as follows:
- Total parts by volume = 1 (cement) + n (sand) + m (aggregate)
- Volume of 1 part cement in 1m³ of concrete ≈ 1 ÷ (1 + n + m) × 1.5 (packing factor)
- For 1:2:4 mix: parts = 7. Cement ≈ 1,000 ÷ 7 × cement bulk density (~1,500 kg/m³) ÷ 1,000 ≈ ~270–290 kg/m³
- For 1:1.5:3 mix: parts = 5.5. Cement ≈ ~310–330 kg/m³
Use our Mix Strength Calculator Design mode above for more precise cement content estimates per m³.
The correct mortar mix for pointing depends on the brick type and location:
- Standard external facing brickwork: 1:1:6 (cement:lime:soft sand) — Mortar class M4
- Below DPC / ground level brickwork: 1:0.5:4.5 or 1:3 (cement:sand) — Mortar class M6
- Old soft bricks / heritage: 1:2.5 hydraulic lime:sand — no OPC cement
- Hard engineering bricks (manholes, retaining): 1:3 (C:S) rich mix — Mortar class M12
Never use a stronger mortar than the brick. For standard UK clay bricks, M4 (1:1:6) is the correct specification in almost all cases.
Concrete strength development follows this approximate timeline for CEM I Portland cement at 20°C:
- 1 day: ~20–25% of 28-day strength (initial set complete)
- 3 days: ~40–50% of 28-day strength
- 7 days: ~65–70% of 28-day strength (can strip formwork for most walls)
- 28 days: ~100% characteristic strength (the design benchmark)
- 90 days: ~115–120% — concrete continues to strengthen slowly for years
Cold weather (below 5°C) significantly slows strength gain — protect fresh concrete from frost for at least 48–72 hours after casting.