Concrete Calculator
Work out how much concrete a slab, footing or path needs — in cubic metres and cubic yards, plus the count of pre-mixed bags. Includes a waste allowance because concrete jobs can't pause mid-pour to buy more.
How to use this calculator
- Enter the slab's length and width in metres and thickness in centimetres.
- Choose your bag size, or select volume-only for ready-mix ordering.
- Keep the waste allowance at 10% — spillage and uneven subgrade eat concrete.
Formula used
Keep units consistent — thickness in metres (10 cm = 0.10 m). A 40 kg pre-mix bag yields roughly 0.018 m³; a 20 kg bag about 0.0095 m³. Concrete weighs ~2,400 kg per cubic metre.
Example calculation
A 5 m × 3 m slab at 10 cm thick = 5 × 3 × 0.10 = 1.5 m³. With 10% waste, order 1.65 m³ (~2.16 cubic yards).
That's ~92 of the 40 kg bags — well past the point where ready-mix delivery makes more sense.
Ordering concrete without a second trip
Concrete is unforgiving of shortfalls: once a pour starts it must finish continuously, or you get a cold joint — a weak seam where fresh concrete meets partly-set concrete. That's why the waste allowance matters more here than for most materials, and why running slightly over is far safer than running short.
The bags-versus-ready-mix crossover sits around 1 m³. Below it, bags are convenient; above it, the sheer number (dozens of 40 kg bags to mix by hand) makes ready-mix delivery cheaper, faster and more consistent. The calculator's bag count makes that crossover obvious.
Why use this calculator?
- Get volume and bag count together, so you know when ready-mix wins.
- The waste allowance guards against cold joints from running short.
- Cubic-yard conversion for ordering from US-style suppliers.
Frequently asked questions
How many bags of concrete are in a cubic metre?
Roughly 55 bags of 40 kg pre-mix, or about 108 bags of 20 kg. It's a lot — which is why anything above ~1 m³ is usually ordered as ready-mix delivery instead.
How thick should a concrete slab be?
Typical minimums: 10 cm (4") for patios and paths, 15 cm (6") for driveways and areas bearing vehicles, more for heavy loads. Always check local building codes and add reinforcement as specified.
Why add waste to concrete?
Uneven ground, spillage, and slight over-excavation mean actual usage exceeds the theoretical volume. 10% is standard; a rough or sloping subgrade may warrant more. Running out mid-pour causes a weak cold joint.