Concrete calculator

How to calculate how much concrete you need

The fundamental calculation is straightforward: multiply length × width × depth (in metres) to get your volume in cubic metres. The catch is that depth is usually measured in millimetres on site, so you need to divide by 1,000 to convert. A 4m × 3m garden slab at 100mm depth is 4 × 3 × 0.1 = 1.2m³. Add a 10% wastage allowance on top — concrete is not perfectly pourable and some will inevitably be lost to spillage, over-ordering, and surface finishing. For awkward shapes or where multiple pours are needed, calculate each section separately and sum them.

Concrete grades explained: C20, C25, C30, C35

The "C" in C20, C25, C30, and C35 stands for compressive strength — specifically the characteristic compressive strength in Newtons per square millimetre (N/mm²) measured at 28 days. Higher numbers mean stronger, denser concrete that resists more load.

C20 is suitable for mass concrete fill, blinding layers, garden walls, and low-traffic domestic paths. It is not recommended where ground conditions are poor or where frost exposure is significant.

C25 is the workhorse grade for UK domestic construction. Strip and trench fill foundations, domestic floor slabs, garage bases, shed bases, and steps all typically specify C25. It is what most ready-mix suppliers deliver as their standard domestic mix. Building Regulations compliance for foundations generally requires C25 as a minimum.

C30 is used for exposed driveways, suspended slabs, beam and block floors, and where reinforcement is required. If you are laying a driveway for a heavy van or machinery, C30 with a reinforcement mesh is the right specification. It is also used for retaining walls that see soil pressure.

C35 covers commercial applications, heavily loaded ground beams, exposed outdoor structures in aggressive environments, and pre-cast elements. For most domestic tradespeople, C35 is overkill except in specific structural situations where an engineer has specified it.

Mix ratios for hand mixing

When hand mixing or using a site mixer, the mix ratio describes the proportions of cement : fine aggregate (sharp sand) : coarse aggregate (gravel), measured by volume using a bucket or a gauge box. Water is added to achieve a workable consistency — the goal is the minimum water needed to make the mix manageable, since excess water weakens concrete.

• C20 (1:2:3) — one bucket cement, two buckets sharp sand, three buckets 20mm gravel
• C25 (1:1.5:2.5) — one bucket cement, one and a half buckets sand, two and a half buckets gravel
• C30 (1:1.5:2.5) — same volumetric ratio as C25, but with a lower water/cement ratio to achieve higher strength
• C35 (1:1:2) — one bucket cement, one bucket sand, two buckets gravel

Water/cement ratio is the key variable that site-mixed concrete is hardest to control accurately, which is why ready-mix concrete is the professional choice for anything structural. On a typical job, aim for a slump of 75–100mm for general work and 50mm for drier, stiffer mixes used in foundations.

Foundations

Strip foundations for a standard two-storey domestic extension in good ground (bearing capacity of 100 kN/m²) are typically 600mm wide and 225mm deep under a 200mm cavity wall, giving a volume of 0.6 × 0.225 = 0.135m³ per linear metre. A rear extension with a 10m perimeter might need 1.35m³ just for the foundations — plus the internal load-bearing lines. Always add a 10–15% overpour allowance; foundation trenches are rarely perfectly rectangular once excavated.

Trench fill foundations, increasingly common because they reduce bricklaying below DPC, typically fill the trench to within 150mm of ground level. On a 900mm deep trench at 600mm wide, that is 0.54m³ per metre run — significantly more concrete but faster overall. Your structural engineer or Building Control surveyor will confirm the required specification for your site.

Slabs: driveways, patios, and garage floors

A standard domestic slab on a prepared sub-base is typically 100mm thick for pedestrian use (patios, paths) and 150mm for vehicular traffic (driveways, garage floors). A double garage at 6m × 6m × 150mm requires 6 × 6 × 0.15 = 5.4m³ — a decent-sized ready-mix order. Specify C30 for driveways or where a car sits regularly, and consider a 200mm width with A142 reinforcement mesh if the sub-base is uncertain.

Expansion joints at roughly 3–4m intervals are essential to prevent cracking as the slab expands and contracts seasonally. A 5mm compressible foam strip at each joint costs almost nothing and prevents ugly random cracking later. Leave the joint in place rather than filling it.

Ready-mix concrete: ordering and logistics

Ordering ready-mix requires some forward planning. Most batch plants need 24–48 hours' notice and will ask for: grade (e.g. C25), aggregate size (typically 20mm for foundations and slabs, 10mm for thinner sections), slump (workability — 75mm is standard, 125mm for pump-mix), and delivery address with access notes.

A standard drum truck carries 6–8m³. For smaller orders, a mini-mix truck (typically 1–3m³ capacity, with a narrower body) can access tighter sites but will cost more per m³. If the truck cannot get the chute within 3–4 metres of the pour, you will need a site pump or wheelbarrow gangs — factor in the extra time and labour.

Do not order more than you can pour and finish in around 90 minutes. Concrete begins to stiffen as it hydrates and once it starts to go off, adding water to loosen it (retempering) weakens the finished strength. Have your tools ready — tamping bar, float, straight-edge, and any reinforcement already placed — before the truck arrives.

How much does concrete cost in the UK in 2026?

Ready-mix concrete prices in the UK currently sit at roughly £110–145 per cubic metre for domestic grades (C20–C30), including delivery within a standard radius. Prices at the higher end reflect premium grades (C35+), urban areas with higher delivery costs, small-batch surcharges, and Saturday or urgent deliveries. Regional variation is significant — London and the South East tend to be 15–25% above Midlands and Northern England prices.

For hand mixing, material costs in 2026 run to approximately £6.50 per 25kg bag of cement, £40–50 per tonne of sharp sand (depending on delivery), and £35–42 per tonne of 20mm gravel. A cubic metre of C25 hand-mixed concrete requires roughly 7 bags of cement, 0.6 tonnes of sand, and 1.0 tonne of aggregate — a materials cost of around £130–160, broadly comparable to ready-mix before you account for the considerable labour of mixing it yourself.

Pump hire adds £300–500 for a half-day on a typical domestic job. This is worth it for inaccessible back gardens or upper-floor pours where wheeling concrete by barrow would be impractical. Some ready-mix suppliers offer a combined supply and pump service.

Pouring concrete steps

Concrete steps require more formwork preparation than a flat slab. Each tread (the horizontal surface) has a standard going of 250–300mm and each riser (the vertical face) is typically 150–175mm. Steps must be formed in timber shuttering that can withstand the weight of wet concrete — use 18mm shuttering ply braced with timber stakes and coach bolts. The volume calculation stacks each step as an additional rectangular prism on top of the previous one: a four-step flight 1.2m wide with a 175mm rise and 260mm going needs roughly 0.35–0.4m³ depending on the base construction.

Compact the concrete well into the corners with a rod or vibrator, particularly in the nosings where cracking is most common. Finish the treads with a slightly coarse texture (dragged brush finish) to improve grip when wet. Remove formwork after at least 48 hours and cure with wet hessian or a spray-applied curing compound.

Common mistakes to avoid

Pouring too thin is the most frequent error on domestic jobs. A 75mm slab will crack and shift; always go to at least 100mm for any surface that sees foot traffic. Equally, a soft or uncompacted sub-base causes differential settlement — spend the time to lay and compact a minimum of 100mm hardcore or MOT Type 1 before you pour anything structural.

Adding too much water is the other common mistake, particularly when hand mixing. A sloppy mix is easier to pour but the excess water evaporates as the concrete cures, leaving voids that reduce compressive strength. If the mix is too stiff to work, use a plasticiser admixture rather than extra water.

Never pour concrete onto waterlogged ground or standing water. The water dilutes the concrete at the base of the pour, creating a weak layer exactly where you need the most strength. If the trench or excavation has flooded, pump it out and allow it to drain before pouring.

Reinforcement

Plain concrete handles compressive loads well but is weak in tension — it cracks when pulled or bent. Steel reinforcement (rebar or mesh) handles the tensile forces that concrete cannot. For domestic slabs over 3m in any direction, A142 reinforcement mesh (6mm wire at 200mm centres) laid at mid-depth of the slab significantly reduces cracking risk. For structural elements — beams, lintels, cantilevered slabs — get a structural engineer to specify the reinforcement properly. Under-reinforcing a structural element is dangerous; over-reinforcing it is expensive and unnecessary.

Ensure steel has adequate concrete cover: minimum 40mm to any exposed face in a foundation, 25mm in an internal slab. Use plastic spacer chairs to hold the mesh at the correct depth — resting it on bits of broken brick or stones is not reliable and risks the steel sitting at the bottom where it corrodes.

Quoting concrete work as a tradesperson

When pricing a concrete job, the materials are only part of the cost. Excavation and disposal of spoil, sub-base supply and compaction, formwork fabrication, concrete supply, placing, compacting and finishing, and curing all add to the total. A typical domestic rear extension foundation might be quoted at £80–120 per linear metre for supply and lay, while a garage slab including sub-base, reinforcement, and C30 concrete is often quoted at £90–140 per m² depending on access and specification.

Always build in time for unexpected over-excavation, wet ground conditions, and the time spent organising ready-mix deliveries. A quote that only accounts for the nominal concrete volume will leave you short once the job is done. Use this calculator to get the volume right, then build your labour, plant, and waste disposal costs on top.