Concrete Calculator

What this calculator does. It works out how much concrete you need to pour a rectangular slab — in cubic metres, with a waste allowance — and translates that into the number of standard 25 kg bags of pre-mix you'd need to buy if you're doing the job by hand instead of ordering ready-mix.

The formula in plain English. Volume = length × width × depth, all in metres. We add a waste percentage on top, then divide by 0.012 m³ — the typical yield of one 25 kg bag of pre-mixed concrete once water has been added — to get the bag count. Bags = ⌈(L × W × D × (1 + waste%)) ÷ 0.012⌉.

A worked example. A 4 m × 3 m patio at 100 mm thick is 4 × 3 × 0.1 = 1.2 m³. With a 10% waste allowance that becomes 1.32 m³. As 25 kg bags, that's 1.32 ÷ 0.012 = 110 bags — clearly impractical to mix by hand. At this volume, a ready-mix delivery of 1.5 m³ is far cheaper, faster and gives a more consistent slab. A small shed base of 2 m × 2 m × 75 mm, by contrast, is 0.3 m³ — around 28 bags, which is comfortably a one-day job for two people with a mixer.

Why this matters. Concrete is unforgiving: once you've started a pour, you have to finish it before it sets, or you'll end up with a visible cold joint and a structurally weaker slab. Knowing the exact volume in advance lets you place a ready-mix order at the right size (suppliers price by 0.5 m³ increments), book the right number of helpers, and avoid the classic disaster of running out of mix halfway across the slab. For a typical UK domestic driveway, getting the volume right saves £100–£200 in over-ordering or aborted pours.

When to use it. Planning a patio, shed base, garage floor, garden path, fence post footing or driveway; deciding between bags and ready-mix; sizing a hire mixer for the job; or pricing a project so you can compare quotes from different ready-mix suppliers.

When not to rely on it alone. Foundations and load-bearing slabs for extensions or garages should follow a structural engineer's specification — the mix grade (C20, C25, C30 etc.), reinforcement and depth all matter. Sloped or uneven ground also needs site-specific judgement on depth; the calculator assumes a flat, level slab.

Common mistakes. Entering depth in centimetres without converting to metres (the input expects cm and converts for you, but doing the maths by hand in mixed units gives wildly wrong answers); under-ordering with no waste allowance on a job you can't pause; and forgetting to factor in the time and cost of the hardcore sub-base, which is often the bigger material cost on a small slab. If you're laying the slab as a base for a patio, the tile calculator will tell you what goes on top; for a driveway or path, pair this with the paint calculator if you're finishing exposed walls.