How this works
Concrete is sold by volume — cubic yards (yd³) in the US and cubic metres (m³) in most of the rest of the world. To work out how much you need, you compute the geometric volume of the pour and then add a waste allowance because spillage, uneven sub-grade, formwork compression and over-ordering safety all push the real number above the theoretical one. Most pros add 5–10% on top. The geometry is straightforward for the three shapes that cover almost every residential pour: a rectangular slab is length × width × depth; a circular footing or column is π × r² × depth; the rest is just adding a tube, a footing, or a stair tread together. If you're ordering ready-mix concrete from a truck, the supplier wants the answer in m³ or yd³ to one decimal — they'll round up to the nearest delivery increment (usually 0.25 yd³ or 0.5 m³) and you pay for what they pour. If you're mixing from bags, the calculator converts the volume to a count using the standard 80 lb (36 kg) US bag (0.6 ft³ ≈ 0.017 m³) or the 25 kg European bag (~0.012 m³) — but ready-mix is dramatically cheaper per cubic metre once you're past about 1 m³ of concrete, so for anything bigger than a small footing it usually pays to call a supplier. A few practical points. (1) Depth matters more than people expect — a 4-inch slab and a 6-inch slab differ by 50% on volume, not 50% on linear feet. Make sure you're measuring depth at the deepest point if your sub-grade is uneven. (2) Reinforcing mesh and rebar take up negligible volume in residential pours; you don't need to subtract them. (3) The "PSI" (or MPa) rating you'll see on bags is the compressive strength after 28 days of cure: for a basic patio or sidewalk, 3000 PSI / 20 MPa is fine; for footings supporting a house, 3500–4000 PSI / 25–30 MPa. Don't confuse this with how much concrete you're ordering — it's a quality spec, not a quantity one.
The formula
V is the geometric volume in your chosen unit. waste% is your over-order margin (5-10% typical, higher for awkward sub-grades). Bag yields are nominal — actual yield varies ±10% with mix water and aggregate; rules of thumb are 1 m³ ≈ 80 European 25 kg bags, 1 yd³ ≈ 45 US 80 lb bags. The calculator above lets you toggle between metric and imperial.
Example calculation
- Patio slab: 4 m × 3 m × 0.10 m thick.
- Volume = 4 × 3 × 0.10 = 1.2 m³.
- Add 10% waste: 1.2 × 1.10 = 1.32 m³ — order 1.5 m³ (next ready-mix increment).
- Or in bags: 1.32 / 0.012 = 110 × 25 kg bags. (At ~€5/bag that's €550 vs ~€150 ready-mix.)
Frequently asked questions
How much waste should I add?
For a clean rectangular slab on a level sub-grade, 5% is plenty — your geometry is exact and the waste is just spillage and rounding. For footings, sloping sites, awkward shapes or pours where you can't see the bottom (e.g. column footings into a deep hole), bump it to 10%. For trench fills with rough excavation, 12-15% isn't unreasonable. Running short of concrete on the day is much worse than over-ordering — a half-cured slab with a cold joint where you stopped to wait for the next batch will be visibly weaker — so when in doubt, round up.
Bags or ready-mix?
Volume crossover is around 1 m³ / 1.3 yd³. Below that, mixing from bags in a wheelbarrow or cement mixer is cheaper, lower-stakes, and means you don't have to wait for a delivery slot. Above it, ready-mix is dramatically cheaper per cubic metre (often half the price), arrives in one go, and saves hours of mixing — for a 3 m³ patio slab, mixing from bags would mean ~250 × 25 kg bags, which is two days of brutal physical work. Ready-mix needs a clear approach for the truck (most need 3-3.5 m of clearance), and you need to be ready to place and finish before it goes off (about 90 minutes from when it leaves the plant). For very small jobs (< 0.1 m³), pre-mixed pourable concrete in 25 L tubs can be even more convenient than dry bags.
What's the difference between cement, mortar and concrete?
Cement is the binder — a fine grey powder (typically Portland cement) that reacts with water to form a paste that hardens into stone. Mortar is cement + sand + water; it's used as a glue between bricks, blocks or stones — never as a load-bearing element on its own. Concrete is cement + sand + coarse aggregate (gravel) + water; the aggregate is what gives concrete its compressive strength and bulk. This calculator estimates concrete (the structural one). If you're pointing brickwork or laying a course of bricks, you want a mortar calculator instead — the bag yields are very different (mortar bags are sand-heavy, concrete bags are aggregate-heavy).
How long do I have to place the concrete?
About 90 minutes from the moment cement meets water (the "initial set" time for standard Portland cement). For ready-mix, the clock starts when the truck leaves the plant, and most suppliers will charge waiting time after 30 minutes on site — so plan to be ready when the truck arrives. For bag mixing, mix only as much as you can place in 30-40 minutes; if you mix too far ahead, the first batch starts setting before the last batch is poured and you get cold joints. In hot weather (>25 °C), this window can shrink to 60 minutes total. In cold weather, the cement reacts more slowly but the working time doesn't actually extend — and below ~5 °C the reaction can stall entirely, so don't pour without an additive or thermal protection.