Concrete Calculator

Concrete volume is calculated by multiplying length × width × depth for rectangular slabs, or πr² × depth for circular areas such as post holes. The result in cubic metres tells you how much ready-mix to order, or how many bags of dry mix you need. A standard 25 kg bag of post-mix makes approximately 0.011 m³ of concrete. A slab of 5m × 3m × 0.10m therefore comes to 1.5 m³, while a shed base of 3 m × 2.4 m × 0.1 m works out at 0.72 m³, rising to 0.79 m³ once 10% wastage is added, which is roughly 72 bags.

For small jobs (under 1 m³), bagged dry-mix is practical. For larger pours, ready-mix delivery is more economical and gives consistent quality. Most UK suppliers sell ready-mix in minimum loads of 1 m³ for mini-mix and 6 m³ for standard trucks, although some firms will bring 0.5 m³ at a premium. Add 5-10% for wastage, spillage and uneven ground. Typical UK pricing in 2026 runs at £100-£140 per cubic metre delivered. Enter your dimensions and the calculator shows the volume in cubic metres, the number of 25 kg bags needed and an approximate cost estimate.

Mix ratios are quoted by volume as cement, sand and aggregate. C7.5, used for footpaths and light foundations, is 1:4:6. C20/25 for residential slabs and driveways is 1:2:4. C25/30 for structural work and beams is 1:1.5:3, and C30/40 for heavy industrial use is 1:1:2. A domestic driveway normally takes C25 at £110-£130 per cubic metre, while garage and shed foundations manage on C20 at £100-£115. For DIY mixing under 0.5 m³, bagged general purpose ballast, which is sand and aggregate already combined, at 1 part cement to 5 parts ballast is the simplest route.

Building Regulations set minimum depths that catch people out. Foundations should go at least 1m below ground level in clay soils because of heave risk, or 600mm in sandy and gravel soils. Trees change that picture. Within one tree height of a mature tree, 1.5m or more is recommended, and thirsty species such as oak and willow can demand 2.5m or more. Slab thickness has its own minimums, at 100mm for a domestic patio, 125mm for a driveway carrying cars, and 150mm or more for vans and heavy use. A142 mesh is standard reinforcement for driveways, with A193 where vehicles load the slab. Any structural foundation work needs a Building Notice or Full Plans, so speak to local Building Control first.

Concrete gains strength slowly rather than all at once. It reaches 70% of its strength at 7 days and 95% at 28 days, the industry standard for testing. Light foot traffic is fine after 24-48 hours, cars after 7 days, and full loading after 28 days. Curing conditions matter more than most people expect, with 10-25°C and 60-90% humidity being ideal. Below 5°C the concrete will not cure properly, so use an accelerator additive or wait for milder weather. Above 30°C it dries too fast and cracks, so mist the surface with water for the first 3 days. Covering the pour with plastic sheeting or wet hessian gives a slower, stronger cure, and in a UK winter it is worth avoiding any pour where the ground temperature is below 5°C.

Project costs put the concrete itself in perspective. A patio of 20 m² at 100mm needs 2 m³, so £220-£280 of ready-mix sits alongside £150 of base materials such as Type 1 sub-base and weed membrane, plus £500-£800 of labour if you hire it, giving £870-£1,230 in total. A driveway of 50 m² at 125mm takes £325-£420 of concrete, £400 of base, £150 of edging and £1,500-£2,500 of labour, so £2,400-£3,500 overall. A shed base of 3m × 2m at 100mm needs only 0.6 m³, and because the minimum ready-mix order would force you to buy a full cubic metre, roughly 40 bags at £6 each, about £240 plus 4 hours of work, often makes better sense.