Free Portland Cement Mix Quantities, Bags & Cost Estimator – Updated 2026 Prices
CEM I — also known as Portland cement or Ordinary Portland Cement (OPC) — is the most widely used type of cement in UK construction and the primary binder in the majority of site-mixed, ready-mix, and pre-bagged concrete products. The designation CEM I is defined by BS EN 197-1:2011 (Cement — Part 1: Composition, specifications and conformity criteria for common cements), which classifies all Portland cements in Europe under a common system. CEM I contains a minimum of 95% Portland cement clinker — the calcium silicate-rich material produced by heating limestone and clay to approximately 1,450°C in a kiln — with the remainder being calcium sulphate (gypsum) to control setting time.
CEM I is the reference cement type against which all other cement types (CEM II through CEM VI) are benchmarked. It produces concrete that achieves its characteristic strength more quickly than blended cements (CEM II, CEM III, CEM IV, CEM V), making it particularly valuable where fast strength gain is required — such as in cold weather concreting, precast production, post-tensioned slabs where early stressing is needed, or situations requiring rapid formwork removal. In the UK, CEM I is available in five standard strength classes: 32.5 R, 42.5 N, 42.5 R, 52.5 N, and 52.5 R, where the number indicates the minimum 28-day compressive strength in N/mm² and the letter indicates early strength (N = normal early strength, R = rapid early strength).
Our CEM I concrete calculator UK 2026 calculates the quantities of CEM I cement, ballast (all-in aggregate), sharp sand, and coarse gravel required for any site-mixed concrete pour, based on the selected mix ratio and the volume of concrete needed. It also provides ready-mix and pre-bagged alternatives, with estimated costs at current 2026 UK pricing.
Selecting the correct CEM I strength class for your application ensures the concrete achieves the required strength at the right time. The strength class number defines the minimum compressive strength of a standard mortar prism at 28 days in N/mm²:
Site-mixed concrete using CEM I follows the same prescriptive mix ratios as standard OPC concrete under BS 8500-2. The most common domestic mixes using CEM I are:
When using CEM I 42.5 N or 52.5 N (higher strength classes), slightly less cement is needed per m³ to achieve the same concrete grade compared to using CEM I 32.5 R — but for domestic site mixing, the standard mix ratios above are typically used regardless of strength class, providing a comfortable margin of strength. The key practical difference of choosing a higher strength class is faster early strength gain, which is most beneficial in cold weather or where early loading is needed.
CEM I is not the only cement available for UK concrete work. The choice of cement type significantly affects the concrete's properties, environmental impact, and durability. In 2026, the main alternatives to CEM I used in UK construction are:
CEM I standards and suppliers UK 2026: BSI Group – BS EN 197-1 | Hanson Cement UK | Breedon Group UK | Heidelberg Materials UK
CEM I cement is sold in the UK in 25kg paper bags (most common for site mixing), 1-tonne bulk bags, and by loose bulk delivery in road tankers for large ready-mix plants. In 2026, typical prices are:
CEM I prices have been subject to significant inflationary pressure in 2022–2024 due to high energy costs in cement production (calcination requires approximately 3–4 GJ/tonne of thermal energy). In 2026, prices have moderated slightly from the 2023 peak but remain elevated compared to pre-2021 levels. Always confirm live prices with your local builders' merchant or cement supplier before purchasing.
The table below provides full technical specifications for CEM I cement strength classes as defined in BS EN 197-1:2011, which is the current European standard for common cements in the UK in 2026.
| Strength Class | Min 28-day Strength | Max 28-day Strength | Min 2-day Strength | Min 7-day Strength | Typical Use | 2026 Bag Price (25kg) |
|---|---|---|---|---|---|---|
| CEM I 32.5 N | 32.5 N/mm² | 52.5 N/mm² | — | ≥ 16 N/mm² | General purpose; mortars | £7.00–£8.50 |
| CEM I 32.5 R | 32.5 N/mm² | 52.5 N/mm² | ≥ 10 N/mm² | — | Domestic concrete, fast early gain | £7.50–£9.00 |
| CEM I 42.5 N | 42.5 N/mm² | 62.5 N/mm² | — | ≥ 16 N/mm² | Standard UK grade – most widely used | £7.50–£9.50 |
| CEM I 42.5 R | 42.5 N/mm² | 62.5 N/mm² | ≥ 20 N/mm² | — | Cold weather, fast-track, precast | £8.50–£11.00 |
| CEM I 52.5 N | 52.5 N/mm² | — | ≥ 20 N/mm² | — | High strength concrete (C35+); prestressed | £9.00–£12.00 |
| CEM I 52.5 R | 52.5 N/mm² | — | ≥ 30 N/mm² | — | Specialist precast; very high early strength | £10.00–£14.00 |
Strength values are for standard mortar prisms tested under BS EN 196-1. In-situ concrete strength depends on mix proportions, water:cement ratio, compaction, and curing — typically achieving 80–90% of these values for well-mixed and properly cured concrete.
The table below provides ready-reckoner material quantities per cubic metre of finished concrete for the four most common UK site-mix ratios using CEM I 42.5 N, along with the equivalent designed concrete grade, water requirement, and estimated 2026 material costs.
| Mix Designation | Ratio (C:S:G) | Equiv. Grade | CEM I /m³ | Ballast /m³ | Water /m³ | CEM I Bags (25kg) | Est. Mat. Cost /m³ |
|---|---|---|---|---|---|---|---|
| GEN 1 | 1:3:6 | C8/10 – C16/20 | ~220 kg | ~1,920 kg | ~110 L | ~9 bags | ~£162 |
| GEN 3 | 1:2:4 | C20/25 | ~310 kg | ~1,750 kg | ~170 L | ~13 bags | ~£193 |
| RC25 | 1:1.5:3 | C25/30 | ~370 kg | ~1,650 kg | ~185 L | ~15 bags | ~£211 |
| RC30 | 1:1:2 | C30/37 | ~480 kg | ~1,500 kg | ~215 L | ~19 bags | ~£238 |
Costs are based on CEM I 42.5 N at £8.50/25kg bag and all-in ballast at £44/tonne (bulk delivery, South East England averages 2026). Ready-mix C25/30 delivered is typically £108–£136/m³ — significantly cheaper for pours over 1 m³ once labour is accounted for. All quantities include a standard mix yield factor.
The table below compares CEM I with the principal alternative cement and binder types used in UK concrete construction in 2026, covering composition, early strength, embodied carbon, and typical applications.
| Cement Type | Standard | Clinker Content | Early Strength | Embodied Carbon | Typical UK Application | 2026 Relative Cost |
|---|---|---|---|---|---|---|
| CEM I (OPC) | BS EN 197-1 | ≥ 95% | Fastest | ~0.83 kgCO₂e/kg | General purpose; all applications | Baseline (1.00×) |
| CEM II/A-S (Portland-slag 6–20% GGBS) | BS EN 197-1 | 80–94% | Slightly slower | ~0.70 kgCO₂e/kg | General; improved sulphate resistance | 0.95–1.05× |
| CEM II/B-S (Portland-slag 21–35% GGBS) | BS EN 197-1 | 65–79% | Slower | ~0.55 kgCO₂e/kg | Below-ground; sulphate exposure | 0.90–1.00× |
| CEM III/A (Blast furnace 36–65% GGBS) | BS EN 197-1 | 35–64% | Significantly slower | ~0.40 kgCO₂e/kg | Foundations, marine, aggressive ground | 0.90–1.00× |
| CEM III/B (High-slag 66–80% GGBS) | BS EN 197-1 | 20–34% | Very slow | ~0.25 kgCO₂e/kg | Mass concrete; very aggressive environments | 0.85–0.95× |
| CEM II/A-V (Portland-fly ash 6–20% PFA) | BS EN 197-1 | 80–94% | Slightly slower | ~0.72 kgCO₂e/kg | General purpose; improved workability | 0.95–1.00× |
| CEM I + GGBS (blend at plant) | BS EN 197-1 + BS EN 15167 | 30–70% clinker | Slower than pure CEM I | 0.25–0.60 kgCO₂e/kg blend | Structural RC; most common low-carbon UK spec | 0.90–1.00× |
| Rapid-hardening cement (RHPC equiv.) | CEM I 42.5 R / 52.5 R | ≥ 95% | Very fast | ~0.85 kgCO₂e/kg | Cold weather; precast; fast-track | 1.10–1.50× |
| Sulphate-resisting cement (SRPC) | BS EN 197-1 / BS 4027 | ≥ 95% (low C₃A) | Similar to CEM I | ~0.83 kgCO₂e/kg | Ground with high sulphate content | 1.10–1.30× |
Cement standards and technical guidance: BSI Group – BS EN 197-1 | The Concrete Centre UK | MPA Slag – GGBS UK | UK Quality Ash Association – PFA
The table below provides specification guidance for the most common UK construction applications using CEM I-based concrete, including recommended CEM I strength class, concrete grade, and suitability notes.
| Application | Recommended Mix | CEM I Class | Min Depth | Reinforcement | Notes |
|---|---|---|---|---|---|
| Blinding / levelling layer | GEN 1 (1:3:6) | CEM I 32.5 R or 42.5 N | 50–75mm | None | Non-structural; lowest cement content |
| Domestic strip foundation | GEN 3 (1:2:4) | CEM I 42.5 N | 200–300mm | None (or as engineered) | BCB inspection before covering |
| Trench fill foundation | GEN 3 (1:2:4) | CEM I 42.5 N | Full trench | None | Specify SRPC if sulphate ground present |
| Ground-bearing slab (domestic) | GEN 3 / RC25 | CEM I 42.5 N | 100mm | A142 or A193 | Include DPM; Part C Building Regs |
| Domestic driveway | RC25 (1:1.5:3) | CEM I 42.5 N | 125–150mm | A142 or A193 | C25/30 ready-mix preferred for durability |
| Garage floor | RC25 (1:1.5:3) | CEM I 42.5 N | 125–150mm | A193 | Power-float for smooth finish |
| Garden wall footing | GEN 3 (1:2:4) | CEM I 42.5 N | 200mm | None | Width = 3× wall thickness minimum |
| Fence / gate post hole | GEN 1 or rapid-set | CEM I 42.5 R | 600mm deep | None | 42.5 R for faster set; slope top to drain |
| Structural column / beam | RC30 (1:1:2) | CEM I 42.5 N or 52.5 N | As designed | SE-designed loose bar | SE mandatory; ready-mix preferred |
| Cold weather concreting (below 5°C) | GEN 3 or RC25 | CEM I 42.5 R or 52.5 R | As required | As required | R class essential for frost protection |
| Precast products (lintels, steps) | RC25 or RC30 | CEM I 42.5 R or 52.5 N | As designed | As designed | High early strength for rapid demoulding |
| Ground with sulphate (Class DS3+) | GEN 3 or RC25 | SRPC or CEM III/A | 200mm+ | As required | Do NOT use standard CEM I in high sulphate ground |
CEM I is Portland cement with ≥ 95% clinker content, classified under BS EN 197-1:2011. It is the same material as traditional Ordinary Portland Cement (OPC) but designated under the harmonised European standard. "OPC" and "CEM I" are used interchangeably in UK practice.
CEM I 42.5 N is the most widely used cement grade in the UK for domestic and commercial concrete production. The 42.5 refers to a minimum 28-day mortar prism strength of 42.5 N/mm²; N denotes normal early strength gain.
CEM I 42.5 N costs approximately £7.50–£9.50 per 25kg bag (ex-VAT) from UK builders' merchants in 2026. Bulk delivery by tonne bag reduces cost to approximately £220–£320/tonne. Prices remain elevated vs pre-2021 due to energy costs.
A GEN 3 (1:2:4) site mix requires approximately 310 kg of CEM I per m³ of finished concrete — about 13 × 25kg bags. An RC25 mix needs ~370 kg (15 bags); RC30 needs ~480 kg (19 bags).
CEM I 42.5 R achieves ≥ 20 N/mm² at 2 days versus no minimum for 42.5 N. Choose R class for cold weather concreting, fast-track work, precast production, or where early loading of the concrete is required.
CEM I has an embodied carbon of approximately 0.83 kgCO₂e/kg — one of the highest of any common construction material. Replacing 50% with GGBS reduces this to approximately 0.45 kgCO₂e/kg — a saving of over 45%.
CEM I 42.5 N has an initial set time of approximately 45–60 minutes under normal conditions (20°C). In hot weather (>25°C) or with rapid-class cement, initial set can be as fast as 20–30 minutes — use a retarder admixture for large pours in warm conditions.
CEM I has a shelf life of approximately 3 months if stored in a dry, sealed environment. Cement exposed to moisture will partially hydrate and form lumps — partially hydrated cement produces significantly weaker concrete. Never use cement with visible lumps in structural applications.
Standard CEM I is not sulphate-resistant. If the ground contains high sulphate concentrations (ground class DS3 or above per BRE Special Digest 1), specify sulphate-resisting Portland cement (SRPC) or CEM III/A instead.
The ideal free water:cement ratio for domestic site mixes is 0.50–0.55 for GEN 3/RC25, and 0.45 for RC30. Too much water drastically reduces strength and durability. A good mix should be workable but not pourable or sloppy.
Store CEM I bags off the ground on pallets, protected from rain and condensation with polythene sheeting. Never store outdoors uncovered. Use older bags first (FIFO). Store in a cool, dry location away from direct sunlight and moisture sources.
CEM I cement is strongly alkaline (pH ~12–13) and can cause chemical burns to skin and eyes. Always wear gloves, eye protection, and a dust mask when handling. Cement dermatitis (chrome allergy) is a known occupational hazard — use low-chromate cement where available.
Concrete containing CEM I binder, when used in structural or ground-bearing elements forming part of a building, must comply with the Building Regulations 2010 (as amended) and the relevant British and European Standards. The key regulatory requirements for domestic CEM I concrete work in 2026 are as follows.
Foundations, ground-bearing floor slabs, structural columns, and beams constructed in CEM I concrete must satisfy the requirements of Approved Document A. Building Control must inspect excavations before concrete is placed, and the foundation concrete before it is covered. For domestic strip foundations, GEN 3 / C20/25 concrete with CEM I 42.5 N is the standard accepted specification. Delivery tickets confirming the concrete grade (for ready-mix) or evidence of correct mix proportions (for site-mix) may be requested by the inspector. Retain all records.
Ground-bearing concrete slabs forming part of a habitable building must incorporate a continuous damp-proof membrane (DPM) — typically 1200-gauge polythene — beneath the CEM I concrete slab, lapped with the damp-proof course in the surrounding masonry. In ground identified as containing elevated sulphate concentrations, radon, or other contaminants, the Building Regulations require additional protective measures before any concrete is placed. Standard CEM I should not be used where the ground sulphate class is DS3 or higher — specify SRPC or CEM III/A instead.
Official Regulatory Resources: GOV.UK Approved Documents A & C | NHBC Standards 2026 | The Concrete Centre UK | Planning Portal UK
CEM I is the European standard designation for Ordinary Portland Cement (OPC) — the most widely used cement in the UK and globally. It is defined in BS EN 197-1:2011 as a cement containing a minimum of 95% Portland cement clinker, with the remainder being calcium sulphate (gypsum) to regulate setting time. The clinker is produced by burning a mixture of limestone and clay at approximately 1,450°C in a rotary kiln — an energy-intensive process that releases CO₂ both from fuel combustion and from the chemical decomposition of limestone (calcination). CEM I forms the binder in virtually all site-mixed, ready-mix, and pre-bagged concrete products used in UK domestic construction.
Both CEM I 42.5 N and CEM I 42.5 R achieve a minimum 28-day strength of 42.5 N/mm² in standard mortar prism tests. The difference is in early strength gain: CEM I 42.5 R (Rapid) must achieve a minimum of 20 N/mm² at 2 days, whereas CEM I 42.5 N (Normal) has no minimum at 2 days and achieves a minimum of 16 N/mm² at 7 days. Choose 42.5 R when you need: faster formwork removal; concreting in cold weather (below 10°C) when normal-class cement hydrates too slowly to resist frost; precast production requiring early demoulding; or any situation where early loading of the hardened concrete is necessary. CEM I 42.5 R is approximately 10–15% more expensive than 42.5 N per bag.
For a GEN 3 (1:2:4) mix — the standard domestic concrete equivalent to C20/25 — you need approximately 310 kg of CEM I cement per m³ of finished concrete (about 13 × 25kg bags). For a stronger RC25 (1:1.5:3) mix (C25/30 equivalent), approximately 370 kg/m³ (15 bags). For an RC30 (1:1:2) mix (C30/37 equivalent), approximately 480 kg/m³ (19 bags). For blinding concrete (GEN 1, 1:3:6), approximately 220 kg/m³ (9 bags). These figures use CEM I 42.5 N and include a standard yield factor. Our calculator above computes the exact quantities for any volume and mix ratio.
CEM I 42.5 N (the standard grade) costs approximately £7.50–£9.50 per 25kg bag (ex-VAT) from UK builders' merchants in 2026 — equivalent to approximately £300–£380 per tonne. CEM I 42.5 R costs approximately £8.50–£11.00 per bag; CEM I 52.5 N approximately £9.00–£12.00 per bag. Prices vary between merchants and regions — London and the South East are typically 10–15% higher than Northern England prices. For large projects, bulk delivery in 1-tonne bags or road tanker at £220–£320/tonne (CEM I 42.5 N) offers significant savings over individual bagged cement. Always confirm live pricing with your local merchant as cement prices have been volatile in recent years.
Standard CEM I is not suitable for concrete in contact with sulphate-bearing ground where the sulphate class is DS3 or higher (as defined in BRE Special Digest 1). Sulphates react with calcium aluminate phases in Portland cement clinker to form expansive compounds (ettringite and thaumasite) that can cause concrete to crack and spall over time. For DS3+ ground sulphate conditions, specify either sulphate-resisting Portland cement (SRPC) — a Portland cement with a low C₃A content — or a blended cement with high GGBS content (CEM III/A or CEM III/B), both of which provide substantially better sulphate resistance. Always conduct a ground investigation to determine the sulphate class before specifying foundations or below-ground concrete.
Yes — CEM I and OPC are the same material. OPC is the traditional British designation used under the superseded BS 12 standard. When the UK adopted the European harmonised cement standard BS EN 197-1 in 2001, OPC was reclassified as CEM I. The composition requirements are effectively identical — minimum 95% Portland cement clinker. In 2026, both terms are still used interchangeably in the UK construction industry, with older textbooks, site instructions, and specification clauses often still referring to "OPC" while newer documents use "CEM I". For any specification or purchasing purpose, CEM I 42.5 N is the direct equivalent of standard OPC.
For a domestic house extension strip foundation, the standard UK specification is a GEN 3 mix: 1 part CEM I : 2 parts sharp sand : 4 parts 20mm gravel by volume, or equivalently 6 parts all-in ballast to 1 part CEM I by volume using pre-blended all-in aggregate. This produces a C20/25-equivalent concrete with approximately 310 kg of CEM I per m³. Building control bodies across the UK routinely accept this specification for standard domestic foundations on firm ground under Approved Document A. Always check with your Building Control Officer before mixing and pouring — they will inspect the excavations before concrete is placed.
CEM I 42.5 N concrete achieves approximately 50% of its 28-day strength at 3 days and 70–75% at 7 days under normal conditions (15–20°C, kept moist). Full design strength is reached at 28 days. CEM I 42.5 R achieves 70% strength at 3–4 days and full 28-day strength earlier — the key advantage of the R class. In cold conditions (5–10°C), strength development slows significantly — CEM I 42.5 N at 5°C may take 10–14 days to reach 70% strength. Never allow fresh CEM I concrete to freeze — this permanently disrupts the hydration process. Cure all concrete by keeping it moist and protected from frost and drying wind for a minimum of 3–7 days.
CEM I cement must be stored carefully to maintain its quality: store bags on wooden pallets off the ground to prevent moisture absorption from below; cover with polythene sheeting to protect from rain, condensation, and humidity; keep in a cool, dry, well-ventilated location away from direct sunlight; use older stock first (first in, first out — FIFO rotation); and use within 3 months of manufacture date printed on the bag. Cement that has absorbed moisture will partially pre-hydrate and form lumps — cement with hard lumps should not be used in structural concrete. If in doubt, test by pressing a small quantity firmly in your palm — good cement should feel cool and powdery with no hard particles.
CEM I Portland cement has one of the highest embodied carbon footprints of any construction material — approximately 0.83 kgCO₂e per kilogram of cement produced. Global cement production accounts for approximately 8% of global CO₂ emissions. For a 1 m³ C25/30 slab using CEM I, the cement alone contributes approximately 307 kgCO₂e — equivalent to driving a petrol car approximately 1,800 miles. In 2026, the most effective strategy for reducing this impact is to replace 30–70% of CEM I with GGBS (Ground Granulated Blast Furnace Slag), which has an embodied carbon of only 0.068 kgCO₂e/kg — over 90% lower than CEM I — while providing equal or better performance in most structural applications.
CEM I 52.5 N is specified when: (1) you need to achieve a high concrete grade (C35/45 or above) while keeping cement content within a practical range; (2) your mix design calls for a reduced cement content for the same target strength — useful in large concrete pours where thermal cracking is a concern; (3) you are producing prestressed or post-tensioned concrete that requires high early strength for early stressing operations; or (4) you are making high-strength precast concrete products where maximising 28-day strength from a given cement content is commercially important. For standard domestic applications using GEN 3 or RC25 mixes, CEM I 42.5 N is entirely adequate and considerably less expensive than 52.5 N.
CEM I contains a minimum of 95% Portland cement clinker. CEM II is a Portland composite cement that contains 6–35% of a secondary constituent — such as GGBS (slag), fly ash, limestone, silica fume, or natural pozzolan — in partial replacement of the clinker. The secondary constituent reduces the overall embodied carbon of the cement and can improve specific properties such as sulphate resistance, chloride resistance, workability, or reduced heat of hydration. CEM II is widely used in UK ready-mix concrete production as a cost-effective and environmentally improved alternative to CEM I. For domestic site mixing where bags are purchased from builders' merchants, CEM I 42.5 N is still the most commonly available product — CEM II alternatives may need to be specially ordered. In ready-mix concrete, CEM II is the default cement type at many UK batching plants in 2026.
Additional Resources 2026: CEM I at Hanson Cement UK | Heidelberg Materials UK | Breedon Group | Concrete guidance at The Concrete Centre | Standards at BSI Group | Buy cement at Travis Perkins | Jewson