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Prepare to Build Building Control Conservation Areas Tree Preservation Orders (TPOs) Listed Buildings The Party Wall etc. Act 1996 Basement Development Advice HETAS NICEIC Gas Safe Construction Management Waste Management Interior DecorationWork out exactly how much concrete you need for slabs, posts, footings, and driveways — with ready-mix volumes, bag counts, waste factors, and cost benchmarks for UK projects.
Multiply length × width × depth (all in metres) to get the volume in cubic metres (m³). For a typical 100 mm thick garden slab, a 3 m × 3 m area needs 0.9 m³ of concrete; add 10% waste, so order 1.0 m³ from a ready-mix supplier or buy roughly 100 × 25 kg bags.
Concrete volume is always calculated in cubic metres (m³). The formula is simple: Volume (m³) = Length (m) × Width (m) × Depth (m). Convert all measurements to metres before you multiply — so 150 mm depth becomes 0.15 m. Always add a 10% waste allowance on top of your calculated figure before ordering.
| Project | Typical Size | Depth | Net Volume | +10% Waste | Approx. 25 kg Bags |
|---|---|---|---|---|---|
| Garden patio slab | 3 m × 3 m | 100 mm | 0.90 m³ | 1.00 m³ | ~100 bags |
| Driveway slab | 5 m × 3 m | 150 mm | 2.25 m³ | 2.48 m³ | ~248 bags |
| Shed base | 2.4 m × 1.8 m | 100 mm | 0.43 m³ | 0.47 m³ | ~47 bags |
| Fence post (per post) | Ø 300 mm × 600 mm deep | — | 0.042 m³ | 0.046 m³ | ~2 bags |
| Strip footing (per metre) | 600 mm wide × 200 mm deep | — | 0.12 m³/m | 0.13 m³/m | ~13 bags/m |
| Garage floor | 6 m × 3 m | 150 mm | 2.70 m³ | 2.97 m³ | ~297 bags |
Use a tape measure and record both dimensions in metres to two decimal places (e.g. 4.80 m × 2.40 m).
Pedestrian slabs: 100 mm (0.10 m). Driveways for cars: 150 mm (0.15 m). Driveways for HGVs/heavy loads: 200 mm (0.20 m). Strip footings: at least 200 mm (0.20 m) deep and typically 3× the wall width wide.
Net m³ = Length × Width × Depth. Example: 4.80 × 2.40 × 0.15 = 1.728 m³.
Multiply net volume by 1.10. Example: 1.728 × 1.10 = 1.90 m³. This covers spillage, uneven sub-base, and formwork gaps.
Ready-mix is cost-effective above ~0.5 m³. Below that, bagged is more practical. One 25 kg bag yields approximately 0.01 m³ (10 litres) of mixed concrete.
Specify the right mix class to your supplier — see the mix-class table below. For most domestic slabs, RC35 (C35/45) is the standard; for footings, RC25 (C25/30) is widely used.
Concrete mixes for domestic use in the UK are specified under BS 8500-2:2015+A2:2019 (Concrete — Complementary British Standard to BS EN 206). The table below lists the most common designated mixes for residential projects.
| Designated Mix | Strength Class | Typical Use | Min. Cement Content |
|---|---|---|---|
| GEN 1 | C10/12.5 | Blinding layers, mass fill | 180 kg/m³ |
| GEN 3 | C20/25 | Unreinforced foundations in low-risk ground | 240 kg/m³ |
| RC25 | C25/30 | Reinforced strip/pad footings | 260 kg/m³ |
| RC35 | C35/45 | Driveways, garage floors, reinforced slabs | 300 kg/m³ |
| PAV1 | C30/37 | Unreinforced concrete paving, pedestrian | 300 kg/m³ |
| PAV2 | C35/45 | Concrete paving subject to de-icing salts | 320 kg/m³ |
| Factor | Bagged (25 kg) | Ready-Mix (truck) |
|---|---|---|
| Best for volume | Up to ~0.5 m³ | 0.5 m³ and above |
| Minimum order (ready-mix) | N/A | Typically 0.5–1.0 m³ (varies by supplier) |
| Approx. cost per m³ (2024) | £130–£180 (materials only) | £90–£130 delivered (excluding pour) |
| Labour intensity | High — mix by hand or mixer | Low — pour direct from chute |
| Time to pour 1 m³ | 2–4 hours (1 person) | 15–30 minutes |
| Flexibility | Mix in batches, pause anytime | Must be poured within 90 minutes of batching |
| Waste risk | Low — buy exact quantity | Over-order risk; unused concrete is wasted |
| Supply Method | Unit | Approx. Cost (2024) |
|---|---|---|
| 25 kg bagged general purpose | Per bag | £4–£7 |
| 25 kg bagged (rapid-set) | Per bag | £7–£12 |
| Ready-mix, GEN 3 / RC25 | Per m³ delivered | £90–£115 |
| Ready-mix, RC35 / PAV1 | Per m³ delivered | £100–£130 |
| Ready-mix short-load surcharge | Per order <3 m³ | £50–£150 extra |
| Pumped concrete (pump hire add-on) | Per day | £350–£600 extra |
Concrete needs a stable, compacted sub-base to prevent cracking and settlement. The sub-base itself is not included in your concrete volume calculation — budget for it separately.
Volume calculated (L × W × D) in m³
10% waste added to net volume
Correct depth chosen for the project type
Correct BS 8500 mix class specified (e.g. RC35 for driveway)
Sub-base compacted and formwork in place before truck arrives
Access route confirmed — ready-mix truck needs 3 m clear width and 4 m height clearance
Chute reach checked — standard chute reaches ~3–4 m; order pump if further
Discharge time noted — concrete must be poured within 90 minutes of batching
Short-load surcharge queried with supplier if ordering under 3 m³
Curing plan ready — cover with polythene or curing compound immediately after finishing
One 25 kg bag of concrete yields approximately 0.01 m³. To cover 1 m² at 100 mm deep, you need 10 bags. To cover a 3 m × 3 m slab at 100 mm deep, you need roughly 100 bags — or just 1 m³ of ready-mix.
Multiply length × width × depth, all in metres, to get the volume in cubic metres (m³). Then add 10% for waste. For example, a 4 m × 3 m slab at 150 mm deep = 4 × 3 × 0.15 = 1.80 m³, plus 10% = 1.98 m³ — so order 2.0 m³.
At 100 mm depth, you need approximately 10 × 25 kg bags per square metre. At 150 mm depth, you need about 15 bags per square metre. Each 25 kg bag yields roughly 0.01 m³ (10 litres) of mixed concrete.
Most ready-mix plants have a minimum order of 0.5 m³ to 1.0 m³, but they often charge a short-load surcharge of £50–£150 for orders under 3 m³. For volumes between 0.5 m³ and 3 m³, a volumetric (mix-on-site) lorry is usually cheaper as you only pay for what is poured.
A domestic car driveway should be at least 150 mm thick, laid over a 150 mm compacted MOT Type 1 sub-base. For regular van or light-commercial traffic, increase the concrete to 175–200 mm. Thinner slabs risk cracking under vehicle loads.
For a domestic driveway or garage floor, specify RC35 (C35/45) under BS 8500-2. For a pedestrian patio, PAV1 (C30/37) is sufficient. If your patio or driveway may be treated with de-icing salts in winter, upgrade to PAV2 (C35/45) which has a higher cement content for freeze-thaw resistance.
In 2024, ready-mix concrete costs approximately £90–£115 per m³ for GEN 3 / RC25 mixes, and £100–£130 per m³ for RC35 / PAV1 mixes, delivered. Short-load surcharges of £50–£150 apply to orders under 3 m³. Bagged concrete works out at £130–£180 per m³ in materials alone.
Concrete is typically walkable after 24–48 hours at normal UK temperatures (10–20 °C). It reaches approximately 70% of its design strength at 7 days and full strength at 28 days. Do not load a fresh slab with vehicles for at least 7 days, and keep it covered with polythene for the first 3–7 days to aid curing.