United Kingdom · Construction
Concrete CalculatorCalculate cubic metres for slabs, driveways, footings, and pours.
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Example — 4 × 3 m driveway
1.200
cubic metres
A standard 100 mm driveway at 4 × 3 m needs 1.2 m³ of concrete — roughly 100 bags. Always order 10% extra to allow for spillage and waste.
How to calculate concrete in cubic metres
Working in metric makes concrete calculations straightforward. If your length, width, and depth are all in metres, the volume in cubic metres (m³) is simply length × width × depth — no conversion factor needed.
Formula
Volume (m³) = Length (m) × Width (m) × Depth (m)
The one thing that trips people up is that UK building drawings typically show slab depth in millimetres rather than metres. A depth of 100 mm is 0.10 m, 150 mm is 0.15 m, and so on. Always divide your millimetre depth by 1,000 before plugging it into the calculation. A slab that is 4 m × 3 m at 100 mm deep requires 4 × 3 × 0.10 = 1.2 m³.
Worked example: 5 × 3 m garage base
A typical single-garage base in the UK might measure 5 m × 3 m with a 150 mm slab — the depth recommended where vehicles will be driven onto it. Here's how the calculation works:
Convert depth to metres
150 mm ÷ 1000 = 0.15 m
Calculate volume
5 × 3 × 0.15 = 2.25 m³
Add 10% for waste
2.25 × 1.10 = 2.48 m³ to order
Bags if not using ready-mix
2.48 ÷ 0.012 ≈ 207 bags (25 kg)
At 207 bags, this pour is firmly in ready-mix territory. Mixing that by hand would take the best part of two days and risk cold joints in the slab. A half-load of ready-mix (typically 3–4 m³ minimum from most UK suppliers) would cover this comfortably.
Ready-mix concrete vs mixing your own
In the UK there are three practical options for sourcing concrete, each suited to different project sizes:
Ready-mix (volumetric or drum mixer truck)
Delivered pre-mixed and poured directly from the truck. Suppliers specify strength by BS designation — C20 for domestic slabs and patios, C25 for driveways and garage floors, C30 for structural foundations. Most suppliers have a minimum order of 1–3 m³. Best for any pour over 1 m³ where consistency and speed matter.
Ballast and cement (site mix)
Ballast is a blend of coarse aggregate and sharp sand, sold in bulk bags (usually 850 kg or one tonne). You combine it with cement on-site, typically at 6 parts ballast to 1 part cement by weight for a general-purpose concrete equivalent to C20. This approach is cost-effective for 0.5–2 m³ and lets you work at your own pace, but it requires a mixer and more labour.
Bagged all-in-one mix (25 kg bags)
Pre-blended bags such as general-purpose concrete mix or rapid-setting products like Postcrete are best for small jobs — fence posts, gate pillars, steps, or repairs under 0.3 m³. A single 25 kg bag covers approximately 0.012 m³. Convenient, but the most expensive way to buy concrete per m³.
How much does concrete cost per m³ in the UK?
UK concrete prices vary by region, mix specification, and whether you use ready-mix or self-mix. Approximate figures for 2025–2026:
Ready-mix (C20/C25)
£90–140/m³
Plus delivery, typically £50–150
Most cost-effective for jobs over 1 m³. Quality is consistent and controlled.
Ballast + cement
£60–90/m³
Materials only, excludes labour and mixer hire
Cheapest material option. Practical for 0.5–2 m³ with a rented mixer.
Bagged mix (25 kg)
£420–560/m³
Based on 83 bags × £5–7 per bag
Only practical for very small jobs. Expensive at scale.
Common mistakes when ordering concrete in the UK
Confusing millimetres and metres
Drawings show depth as 100 mm or 150 mm, but the formula needs metres. Entering 100 instead of 0.10 gives a result 1,000 times too large — an easy mistake to make and easy to miss.
Skimping on depth for a driveway
100 mm is the minimum for a domestic driveway. If you regularly park a van or 4×4, 125–150 mm is more appropriate. Under-specified slabs crack under load and are expensive to re-pour.
Forgetting the sub-base
UK driveways typically sit on a 100–150 mm compacted hardcore sub-base. This isn't part of your concrete volume, but it affects total excavation depth. Skipping it leads to movement and cracking.
Not checking ready-mix minimums
Many UK ready-mix suppliers have a minimum order of 1–3 m³. If your job needs 0.8 m³, you may be charged for the minimum anyway — worth knowing before deciding between bags and a truck.
Common uses in the UK
Driveways
100–150 mm slab on 100 mm hardcore. A standard single driveway is typically 2–4 m³.
Shed bases
75–100 mm is usually sufficient. A 2.4 × 3.6 m base at 100 mm = 0.86 m³.
Extension bases
Strip or raft foundations vary — always check with an engineer or building control.
Garden walls
Strip footings typically 300 mm deep × 450 mm wide. Calculate per linear metre.
Fence posts
Each post uses a small volume. Bagged Postcrete is practical for 1–10 posts.
Kerbs & edgings
Haunching behind kerb stones. Small volumes but easy to miscalculate — use the calculator.
Frequently asked questions
How do I work out cubic metres of concrete?
Multiply length (m) × width (m) × depth (m). If your depth is in millimetres, divide by 1,000 first. A 4 m × 2 m slab at 100 mm deep = 4 × 2 × 0.10 = 0.8 m³.
How much concrete do I need for a shed base?
A typical 2.4 × 3.6 m shed base at 100 mm depth needs 0.86 m³ — around 72 bags of 25 kg mix. Add 10% waste, so order 80 bags or 0.95 m³ of ready-mix.
What is ballast and when should I use it?
Ballast is a blend of coarse aggregate and sharp sand. Mix it with cement at 6 parts ballast to 1 part cement by weight for a C20-equivalent concrete. It's cost-effective for site-mixed jobs of 0.5–2 m³.
How many 25 kg bags per cubic metre?
Approximately 83–84 bags, as each 25 kg bag covers around 0.012 m³. With 10% waste, budget for 92 bags per m³. For volumes over 0.5 m³, ready-mix or ballast will be cheaper.
What does ready-mix concrete cost in the UK?
Expect to pay £90–140 per m³ for C20/C25 ready-mix, plus a delivery charge of £50–150 depending on your location and supplier. Most suppliers have a minimum order — often 1 m³, sometimes 3 m³.
What concrete mix should I use for a driveway in the UK?
C25 is the standard recommendation for domestic driveways. C20 is acceptable for light use, but C25 handles freeze-thaw cycles better — important in most parts of the UK. For commercial use or HGV access, have an engineer specify the mix.
Disclaimer
This calculator provides estimates only and should be used for planning purposes. Concrete quantities in practice depend on ground conditions, sub-base specification, form accuracy, and material yield — all of which vary between sites and suppliers.
Cost figures are indicative only and based on approximate UK market rates. Prices vary by region, mix specification, and supplier. Always obtain written quotes before placing orders. Worthulator accepts no liability for over- or under-ordering based on these estimates.