Concrete Volume Calculator UK
Calculate m³, bag counts, and ready-mix tonnes for slabs, footings, columns, and steps — UK units, updated 2025.
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Builder's Advice
My standing rule on concrete pours: anything under 1 m³, use bags; anything over 2 m³, call the batching plant. In between, it comes down to access and deadline. What I have learned the hard way is that you need to think through the entire pour before you commit to anything. Where does the lorry park? How long is the chute? Do you have enough people to place and tamp before it stiffens? In summer that window can be as short as 45 minutes from the first discharge — not long if you're dealing with a larger slab or a complicated shape.
If you are ordering ready-mix, phone the plant the day before to confirm the order and again on the morning of the pour to confirm arrival time. Delays are common, particularly in busy periods, and a transit mixer sitting on the road while your team scrambles to get the formwork right costs money. Specify the slump when you order — 75mm for most domestic work, 100mm if you are pumping. And one thing that catches people out: the concrete in the drum continues to be worked as the lorry drives to site. In hot weather, a long journey can mean a stiffer-than-expected load arrives. Make this clear to the plant when ordering, and they'll adjust accordingly.
Concrete on Site: What the Calculator Can't Tell You
Ready Mix vs Bags — The Real Decision
The crossover point where ready-mix becomes the sensible choice is roughly 0.5–1 m³. Below that, the minimum order charges and short-load surcharges from batching plants (typically £50–150 for loads under 3–4 m³) make bags cheaper and more flexible. Above 1 m³, you're paying a significant premium for bags, your labour cost for mixing them is considerable, and the consistency of a batched mix is far better than hand-mixed or even mixer-mixed bags on site.
When ordering ready-mix, decide between a transit mixer (the standard rotating drum lorry) and a volumetric mixer. Transit mix is cheaper and works fine when access is good and you can pour quickly. A volumetric mixer mixes to order at the point of delivery — you only pay for what you use and can vary the mix class mid-pour. On tight urban sites or awkward access, a volumetric is worth the small premium because the lorry can park further away with a long chute, and if your shuttering takes longer than expected, you're not watching £400 of concrete go off in the drum.
Preparing the Ground — Ignored Until It Goes Wrong
More concrete slabs fail because of inadequate ground preparation than incorrect mix specification. For domestic slabs and driveways, lay a minimum of 100mm compacted Type 1 MOT (crushed stone sub-base) over a prepared formation level. If the ground is soft, over-excavate and fill with more sub-base rather than trying to concrete over soft spots — soft ground will cause the slab to settle unevenly and crack.
UK clay soils deserve particular attention. They shrink in dry summers and expand when wet — far more than most people appreciate. Large-diameter trees within 5–10m of a slab can draw moisture from the ground and cause heave or settlement. On sites with heavy clay and established trees, get a structural engineer's opinion before pouring. The extra cost of a thicker slab, reinforcement mesh, or a void-forming system is invariably less than cutting out and re-laying a cracked floor two years later.
For driveways: form a cross-fall or channel drain to carry water away — Building Regulations Part H and the permitted development rules for front drives both require surface water to be managed. A minimum 1:60 fall across the slab (roughly 17mm per metre) is sufficient for domestic driveways.
Placing, Finishing, and Curing
For slabs over roughly 15 m², plan the pour sequence before the lorry arrives. You need to fill the formwork from one end and work progressively, tamping and screeding as you go, before the concrete stiffens. In summer that window can be as short as 45 minutes. In winter you have more time but a different problem: cold concrete (below 5°C) cures very slowly and is vulnerable to frost damage in the first 24–48 hours — cover with insulating blankets and, if frost is forecast, protect carefully.
Once poured and tamped, resist the temptation to over-work the surface. Excessive trowelling brings fine cement particles (laitance) to the surface, creating a weak layer that dusts and flakes. A light tamping, a bull-float or long-board screed, and then a final steel-float pass once the bleed water has evaporated is the correct sequence. For a non-slip finish on paths and steps, drag a stiff broom across the surface while the concrete is still green. Cover with polythene for a minimum of 3 days, 7 in cold weather, to retain moisture for curing.
How Much Does a Concrete Slab Cost in the UK? (2025)
Ready-mix concrete in 2025 typically costs £95–135 per m³ for C20/C25 mix from a UK batching plant, excluding short-load surcharges (usually £50–120 for orders under 3–4 m³). Bagged concrete (25kg) costs approximately £5–8 per bag from builders merchants. The fully installed cost of a plain concrete slab — including sub-base, formwork, placing, tamping, and a brush finish — ranges from £70–120 per m² for a standard 100mm domestic path or patio slab, to £110–160 per m² for a reinforced 150mm garage floor. Prices are significantly higher in London and the South East.
A concrete driveway fully installed (Type 1 sub-base, concrete, shuttering, brushed or exposed aggregate finish) typically costs £100–170 per m² in most of England — making it comparable to block paving but requiring less long-term maintenance. For a standard double driveway of 40 m², expect a total cost of £4,000–7,000 depending on ground conditions and finish. Searches like "concrete driveway cost UK 2025" and "how much does a concrete slab cost per m²" are among the highest-traffic queries in the domestic construction space, reflecting just how many homeowners and tradespeople need this information.
UK Concrete Mix Reference
C20 — general purpose concrete for paths, steps, domestic floors and non-structural work.
C25 — suitable for foundations, reinforced ground floors, and garage floors.
C30 — structural work, exposed locations, driveways subject to heavy vehicles.
Ready-mix — ordered by the m³ from a batching plant. Minimum order is typically 0.5 m³. Allow 5–10% extra.
25kg bags: Approx 40 bags per m³ for C20 mix. Bags are convenient for small pours; ready-mix is more economical above ~0.5 m³.