Free GEN1 Concrete Estimator – Cement, Sand, Aggregate & Cost. Updated 2026 Prices.
Enter your slab, footing, or pour dimensions below to calculate the volume of GEN1 concrete required and the quantities of cement, sharp sand, and coarse aggregate needed to mix it on site. Ready-mix (volumetric) concrete costs are also estimated. GEN1 concrete is a general-purpose mix suitable for strip footings, trench fill, mass fill, and blinding.
GEN1 concrete is a designated concrete mix classification under the BS 8500-2 standard (Concrete – Complementary British Standard to BS EN 206, Part 2: Specification for constituent materials and concrete), which defines a range of standardised concrete mixes for use in UK construction. GEN1 is one of the GEN (General Purpose) series of designated concrete mixes — a family of mixes intended for applications where a specific structural design is not required and where a standard general-purpose concrete is appropriate. The GEN series runs from GEN0 (the weakest, used for non-structural fill) through GEN1, GEN2, GEN3, to RC (Reinforced Concrete) mixes for structural work.
GEN1 concrete corresponds approximately to a C15 strength class — a characteristic compressive strength of 15 N/mm² at 28 days, as tested on 150mm cubes to BS EN 12390-3. This is a relatively modest strength, reflecting GEN1's intended use in applications where loads are low and the concrete does not need to perform a primary structural role. GEN1 is the standard UK specification for strip foundations and trench fill foundations for domestic buildings, fences, retaining walls, gate posts, and garden structures. It is also used for mass fill, hardcore blinding courses, and general unreinforced concrete work where a higher specification is not required.
The traditional site-mixed equivalent of GEN1 is a 1:2:4 mix by volume (1 part cement : 2 parts sharp sand : 4 parts coarse aggregate), which produces a concrete with workability and strength broadly consistent with the GEN1 designation. In practice, slight variations in this ratio — such as 1:2.5:3.5 or 1:2:3.5 — are also used and produce acceptable GEN1-equivalent concrete. For critical structural applications, a designed mix to BS 8500 from a ready-mix supplier is always preferable to a site-mixed approximation.
GEN1 is specified for a well-defined range of non-structural and lightly loaded applications in UK domestic and light commercial construction:
Understanding where GEN1 sits within the broader family of UK concrete designations is important for choosing the right mix for your project:
GEN1 concrete resources UK 2026: The Concrete Society | Checkatrade – Find a Concrete Contractor | Planning Portal | GOV.UK Building Regulations Approved Documents | British Cement Association
For small domestic projects — fence posts, garden wall footings, small slabs — mixing GEN1 concrete on site using a drum mixer is practical and economical. The standard 1:2:4 mix by volume is prepared as follows:
The table below shows the approximate material quantities required to produce 1 m³ of compacted GEN1 concrete using the standard 1:2:4 mix ratio and common alternative ratios. These figures assume dry aggregates; actual quantities may vary slightly depending on aggregate moisture content and grading.
| Mix Ratio (C:S:A) | Designation | Cement per m³ | Sharp Sand per m³ | Coarse Agg. per m³ | Water per m³ | Typical Use |
|---|---|---|---|---|---|---|
| 1 : 3 : 6 | GEN0 / lean mix | ~180 kg (7 bags) | ~540 kg | ~1,080 kg | ~100 litres | Blinding, mass fill, non-structural |
| 1 : 2 : 4 | GEN1 / C15 | ~250 kg (10 bags) | ~500 kg | ~1,000 kg | ~140 litres | Strip & trench fill foundations, fence posts, garden walls |
| 1 : 2.5 : 3.5 | GEN1 alternative | ~270 kg (11 bags) | ~540 kg | ~945 kg | ~149 litres | Foundations, blinding, general purpose |
| 1 : 2 : 3 | GEN2 / C20 equivalent | ~310 kg (13 bags) | ~620 kg | ~930 kg | ~171 litres | Unreinforced slabs, domestic floor slabs |
| 1 : 1.5 : 3 | GEN3 / C25 equivalent | ~350 kg (14 bags) | ~525 kg | ~1,050 kg | ~193 litres | Reinforced slabs, garage floors, steps |
All quantities are approximate and based on 1 m³ of compacted concrete output, assuming a 10% bulking factor allowance for loose aggregate. A standard 25kg bag of Portland cement contains approximately 18 litres of loose cement by volume.
| Concrete Type | Designation | Price per m³ (excl. VAT) – SE England 2026 | Minimum Load | Notes |
|---|---|---|---|---|
| GEN0 / ST1 | C10 equivalent | £90–£115/m³ | Typically 4–6 m³ | Blinding, fill, non-structural |
| GEN1 / C15 | C15 / 1:2:4 equivalent | £100–£125/m³ | Typically 4–6 m³ | Strip foundations, trench fill, fence posts |
| GEN2 / C20 | C20 equivalent | £108–£132/m³ | Typically 4–6 m³ | Unreinforced slabs, domestic ground floors |
| GEN3 / RC25 | C25 equivalent | £115–£140/m³ | Typically 4–6 m³ | Reinforced slabs, garage floors |
| RC30 / C30 | C30 structural | £122–£148/m³ | Typically 4–6 m³ | Driveways, structural slabs, reinforced work |
| Volumetric / Mini-Mix (any grade) | Any | £130–£170/m³ (higher unit cost, no minimum) | No minimum load | Ideal for small volumes; mixes on site; mix grade adjusted as required |
| FND2 / FND3 (sulphate resisting) | C25–C30 equivalent | £125–£155/m³ | Typically 4–6 m³ | Aggressive ground conditions; sulphate-resisting cement |
All prices are indicative 2026 ex-works/delivered South East England estimates, excluding VAT and pump hire. Deduct 8–15% for North England, Scotland, and Northern Ireland. Add 10–20% for Greater London. Ready-mix suppliers typically apply small-load surcharges for orders below their minimum load (usually 4–6 m³); volumetric / mini-mix trucks have no minimum load but charge a higher unit rate. Always obtain quotes from at least two ready-mix suppliers before ordering.
The following additional costs are commonly encountered on domestic concrete projects in the UK in 2026 and should be budgeted for in addition to the concrete material cost.
| Item | Typical Cost Range (2026) | Notes |
|---|---|---|
| Drum mixer hire (per day) | £40–£80/day | 130–150 litre drum; sufficient for most domestic foundation pours |
| Concrete pump hire (boom pump) | £400–£900 per pour | For difficult access sites; minimum 6–8 m³ usually required |
| Concrete pump hire (line pump) | £250–£600 per pour | For pumping over longer distances; smaller minimum volumes |
| Ready-mix small load surcharge | £100–£300 | Applied when order is below supplier's minimum load (typically 4–6 m³) |
| Formwork / shuttering (timber, per m²) | £8–£20/m² material cost | 19mm or 25mm structural plywood; stakes and bracing additional |
| Steel reinforcement mesh (A142, per m²) | £4–£9/m² | A142 mesh (6mm bars at 200mm centres) for lightly reinforced slabs |
| Steel reinforcement mesh (A193, per m²) | £5–£11/m² | A193 mesh (7mm bars at 200mm centres); heavier-duty slabs |
| Rebar (12mm T12, per metre) | £1.80–£3.50/m | For foundation reinforcement cages and beam reinforcement |
| Hardcore / MOT Type 1 (per tonne) | £30–£60/tonne delivered | Sub-base for slabs; typically 150mm compacted depth; 2.0–2.2 tonnes/m³ |
| Sand blinding (25kg bag) | £3–£6 per bag | 50mm sharp sand layer over hardcore before DPM; smooths surface |
| DPM (damp-proof membrane, 250 micron, per m²) | £0.80–£1.80/m² | Heavy-duty polythene; laid over blinding before slab concrete |
| Labour – concrete laying (per m³) | £80–£180/m³ | Placing, compacting, and levelling; excludes excavation and formwork |
| Excavation – mini digger hire (per day) | £200–£400/day | 1.5–3 tonne machine; typically includes driver; most domestic foundation excavation |
| Concrete float hire (power float, per day) | £50–£90/day | For smooth surface finish on slabs; not required for foundations |
| Curing compound (per litre) | £6–£14/litre | Sprayed onto fresh concrete to retain moisture and aid curing; covers ~10–15 m²/litre |
GEN1 concrete corresponds to a C15 compressive strength class — a characteristic compressive strength of 15 N/mm² at 28 days when tested on 150mm cubes to BS EN 12390-3. This is adequate for strip and trench fill foundations under normal domestic loading conditions, but not for reinforced structural concrete, driveways, or heavily loaded slabs.
The traditional site-mixed equivalent of GEN1 is a 1:2:4 mix by volume — 1 part Portland cement, 2 parts sharp (concreting) sand, 4 parts 20mm coarse aggregate. This ratio produces approximately 1 m³ of compacted concrete from 250 kg (10 bags) of cement, 500 kg of sand, and 1,000 kg of aggregate. Never substitute building sand for sharp sand in concrete.
GEN1 should not be used for: vehicle-trafficked driveways or parking areas (use RC30 / PAV1); reinforced structural slabs, beams, or columns (use RC25–RC40); garage floors (use GEN3 or RC30); aggressive or sulphate-rich ground conditions (use FND2–FND4); or exposed copings and parapets subject to freeze-thaw cycling (use PAV1 or equivalent air-entrained mix).
The target water:cement (W/C) ratio for GEN1 is approximately 0.55–0.60, equating to approximately 14–15 litres of water per 25kg bag of cement. Adding too much water is the most common site-mixing mistake — excess water weakens concrete significantly (every 0.1 increase in W/C ratio reduces 28-day strength by approximately 5–8 N/mm²) and increases shrinkage cracking.
GEN1 concrete begins to set approximately 1–2 hours after mixing (faster in warm or dry weather). It reaches approximately 70% of its 28-day strength after 7 days and full design strength after 28 days. Curing — keeping the concrete moist — is critical for the first 3–7 days. Cover with polythene or damp hessian and avoid loading or trafficking until at least 7 days have elapsed.
Do not place GEN1 concrete when air temperatures are at or below 2°C or falling, or when the ground is frozen. Concrete damaged by frost before it has cured (typically within the first 24–48 hours) loses strength permanently and cannot be recovered. In cold weather, protect fresh concrete with insulating blankets and consider using warm mixing water. In hot weather, use chilled water and avoid pouring in direct sunlight.
A standard 130-litre drum mixer produces approximately 85–95 litres of compacted concrete per load (approximately 0.09 m³). To produce 1 m³ of GEN1 concrete requires approximately 11–12 mixer loads. For volumes above 1–2 m³, ready-mix or volumetric concrete is typically more economical and produces more consistent results than site mixing.
For volumes below approximately 0.5–1 m³, site mixing in a drum mixer is usually most practical and economical, avoiding ready-mix minimum load charges. For volumes above 2–3 m³, ready-mix or volumetric concrete is generally more cost-effective per m³, faster, and produces a more consistent mix. For volumes of 0.5–3 m³, a volumetric / mini-mix truck (which mixes on site and charges only for what you use, with no minimum load) is often the best option.
Building Regulations Approved Document A requires strip foundations to be at least 1 m deep in most soil conditions (deeper in shrinkable clay or near trees), at least 450mm wide under a single-leaf wall, and at least 600mm wide under a two-leaf cavity wall. Minimum foundation thickness is the greater of 150mm or the projection beyond the wall face. Always consult a structural engineer for foundations on non-standard ground.
GEN1 concrete uses standard Portland cement (CEM I or similar), which can be attacked by sulphates in certain soils and groundwater. Where a ground investigation or site experience indicates sulphates are present (identified by the Brownfield, clay, or slag conditions), a sulphate-resisting designated mix (FND2, FND3, or FND4) must be used instead of GEN1. Using GEN1 in sulphate-bearing ground can result in foundation deterioration and structural failure over time.
Always use sharp (concreting) sand, not building (soft) sand, in concrete. Sharp sand has angular particles of consistent grading that interlock and bond with cement to provide strength. Building sand has finer, rounded particles that produce a weaker, more workable (but not stronger) mix. Using building sand in concrete significantly reduces compressive strength and increases shrinkage cracking. Sharp sand is available from all builders merchants in 25kg bags or bulk bags.
GEN1 (or even the weaker GEN0) is entirely adequate for concrete around fence posts and gate posts. The concrete provides lateral restraint, not compressive load-bearing. Mix a stiff (low-water) 1:2:4 mix, fill the hole around the post to within 50mm of the surface, and mound the concrete slightly above ground level to shed water away from the post. Allow to cure for at least 48 hours before fitting gates or panel loading.
| Designation | Strength Class | Max Agg. Size | Typical Applications | Site Mix Equivalent |
|---|---|---|---|---|
| GEN0 | C10 | 20mm | Blinding, non-structural fill, temporary works | 1 : 3 : 6 |
| GEN1 | C15 | 20mm | Strip foundations, trench fill, fence posts, garden walls, mass fill | 1 : 2 : 4 |
| GEN2 | C20 | 20mm | Unreinforced slabs, domestic ground floors (pedestrian only) | 1 : 2 : 3 |
| GEN3 | C20–C25 | 20mm | Reinforced ground floor slabs, garage floors (light vehicles), steps | 1 : 1.5 : 3 |
| RC25 | C25 | 20mm | Reinforced slabs, beams, lightly loaded structural elements | Designed mix only |
| RC30 | C30 | 20mm | Driveways, reinforced slabs, garage floors, structural concrete | Designed mix only |
| RC35 | C35 | 20mm | Heavily reinforced structural elements, ground-bearing slabs | Designed mix only |
| PAV1 | C30 (air entrained) | 20mm | Concrete driveways, parking areas, pavements (residential) | Designed mix only |
| PAV2 | C35 (air entrained) | 20mm | Heavily trafficked concrete pavements | Designed mix only |
| FND2 | C25 (sulphate resisting) | 20mm | Foundations in DS-2 sulphate ground conditions | SRPC mix only |
| FND3 | C25 (sulphate resisting) | 20mm | Foundations in DS-3 sulphate ground conditions | SRPC mix only |
| FND4 | C32/40 (sulphate resisting) | 20mm | Foundations in DS-4 / DS-5 aggressive sulphate conditions | Designed mix only |
GEN1 concrete is a general-purpose mix designated under BS 8500-2, equivalent to approximately C15 compressive strength. It is the standard UK specification for strip foundations and trench fill foundations for domestic buildings, fence and gate post footings, garden wall foundations, mass concrete fill, and blinding courses over compacted hardcore. It is not suitable for reinforced structural slabs, driveways, or heavily trafficked areas — for those applications, GEN3, RC30, or PAV1 are the appropriate specifications.
The standard site-mixed equivalent of GEN1 concrete is a 1:2:4 mix by volume — 1 part Portland cement (CEM I or CEM II), 2 parts sharp (concreting) sand, and 4 parts 20mm coarse aggregate (crushed stone or gravel). This ratio produces approximately 1 m³ of compacted concrete from roughly 250 kg (10 × 25kg bags) of cement, 500 kg of sharp sand, and 1,000 kg of coarse aggregate. The water:cement ratio should be kept to approximately 0.55–0.60 — equivalent to approximately 14–15 litres of water per 25kg bag of cement. Never use more water than necessary to achieve a workable mix.
To calculate the volume of GEN1 concrete needed for a strip foundation, multiply the length × width × depth of the foundation trench. For example, a typical strip foundation for a two-storey house might be: 20m perimeter length × 0.6m wide × 0.3m deep = 3.6 m³ of concrete. Add a 10–15% wastage allowance to account for irregular trench sides and over-excavation — so order approximately 4.0–4.1 m³. For trench fill foundations (which fill the trench to near-ground level), the depth might be 0.9–1.5m, significantly increasing the volume required. Use the GEN1 calculator above to calculate volumes automatically.
For a standard 1:2:4 GEN1 mix, you need approximately 10 bags (250 kg) of 25kg Portland cement per m³ of compacted concrete output. This is based on the standard material proportions: approximately 250 kg cement, 500 kg sharp sand, and 1,000 kg coarse aggregate per cubic metre. If you are using the slightly richer 1:2:3 mix (GEN2 equivalent), you will need approximately 12–13 bags of cement per m³. The GEN1 calculator above calculates the exact number of bags for your project dimensions and chosen mix ratio.
GEN1 and C15 refer to the same concrete mix described under different standards. C15 is the European strength class designation under BS EN 206, indicating a characteristic compressive strength of 15 N/mm² at 28 days (150mm cube). GEN1 is the UK designated mix classification under BS 8500-2, which specifies the mix in terms of its constituent materials and intended use rather than just its strength. In practice, GEN1 and C15 are interchangeable terms for the same general-purpose low-strength concrete — ready-mix suppliers and builders merchants will understand both designations. When ordering ready-mix concrete, you may ask for either "GEN1" or "C15" and receive the same product.
Yes — for very small volumes (typically less than 0.1–0.2 m³, such as a single fence post or a small pad footing), mixing GEN1 concrete by hand on a board or in a barrow is practical. Use a spade or shovel to combine the dry ingredients first, then gradually add water while turning the mix until it is evenly combined and workable. However, hand mixing is physically demanding and difficult to achieve consistently for larger volumes — for anything above 2–3 fence posts, hiring a drum mixer (from approximately £40/day) will save significant effort and produce a more consistent result. For volumes above 0.5–1 m³, ready-mix or volumetric concrete is usually more practical.
GEN1 concrete begins its initial set within approximately 1–2 hours of mixing at normal temperatures (10–20°C), during which time it must be placed and compacted. It achieves sufficient strength to walk on after approximately 24–48 hours and reaches approximately 70% of its 28-day design strength after 7 days. Full characteristic compressive strength (15 N/mm²) is achieved after 28 days. Curing — maintaining moisture — for the first 3–7 days is critical for strength development. Setting and curing are significantly slower in cold weather (below 5°C) and faster in hot, dry weather (above 25°C). Never load or stress GEN1 concrete before at least 7 days have elapsed.
No — GEN1 concrete should not be used for garage floors or driveways. Its characteristic strength of 15 N/mm² is insufficient for vehicle loading, and GEN1 does not incorporate the freeze-thaw resistance required for external trafficked surfaces. For a garage floor, the minimum specification is GEN3 or RC30 (C30), typically 100mm thick on a compacted hardcore sub-base with A142 steel mesh reinforcement. For a concrete driveway, the correct specification is RC30 or PAV1 (which is air-entrained for freeze-thaw resistance), minimum 100mm thick on a compacted sub-base. Using GEN1 for these applications will result in surface dusting, cracking, and structural failure under vehicle loading.
In 2026, ready-mix GEN1 / C15 concrete costs approximately £100–£125 per m³ (excluding VAT and delivery) in South East England, or £88–£108 in North England and Scotland. However, most ready-mix suppliers have a minimum load of 4–6 m³ — orders below this attract a small-load surcharge of £100–£300. For small volumes (below 2–3 m³), a volumetric mini-mix truck (which mixes on site and charges only for the exact volume used, with no minimum load) is usually the most economical option at approximately £130–£170/m³. For very small volumes (below 0.5 m³), site mixing from bags is typically most economical when delivery charges are factored in.
Concrete foundations and footings are typically covered by the Building Regulations application for the structure they support — the foundations themselves do not require a separate application, but the Building Control inspector will inspect and approve them as part of the main project. For a new dwelling or extension, Building Regulations approval is required, and the inspector will visit to check foundation depth, width, concrete specification, and bearing conditions before the concrete is poured. For minor garden structures (walls under 2m, sheds, outbuildings within Permitted Development limits), Building Regulations may not apply, but a competent foundation design is still important for stability and longevity. Planning permission is not required specifically for concrete foundations — it is required (or not) based on the structure being built, not the foundations.
The standard aggregate size for GEN1 concrete is 20mm nominal maximum aggregate size (20mm crushed stone or gravel), which is the most widely available coarse aggregate size from UK builders merchants and is appropriate for all standard domestic applications. For narrow-section work (such as fence post holes less than 150mm diameter), a 10mm aggregate may be more practical as it flows more easily around posts and reinforcement. The sand fraction should always be sharp (concreting) sand — never building (soft) sand — with a particle size range of approximately 0–4mm. Both aggregates are available from all major builders merchants in 25kg bags, 850kg bulk bags, and loose tipped loads.
To calculate the volume of GEN1 concrete needed, multiply the dimensions of the area to be filled: Length (m) × Width (m) × Depth (m) = Volume (m³). For a rectangular strip foundation: 10m × 0.6m × 0.3m = 1.8 m³. For a circular pad: π × radius² × depth = 3.14 × 0.3² × 0.5 = 0.14 m³. Always add a 10–15% wastage allowance to the calculated volume to account for irregular excavation profiles and over-break. Use the GEN1 Concrete Mix Calculator at the top of this page to calculate volumes, material quantities, mixer loads, and costs automatically for your specific project dimensions.
Additional concrete resources UK 2026: The Concrete Society | Checkatrade – Concrete Contractors | British Cement Association | GOV.UK Building Regulations Approved Documents | TrustATrader