Home & DIY
How much concrete do you need for a slab (m³)?
Cubic meters of concrete for a slab or footing from length, width and thickness — with a waste allowance.
Quick answer
Ordering concrete by guesswork is expensive both ways: too little stops the pour mid-slab; too much pays for cubic meters you cannot return. For a rectangular slab or footing, volume is length × width × thickness — then add 5–10% for spillage, uneven subgrade and the fact that trucks measure what they deliver, not what you hoped to use.
Concrete volume
1.8 m³
- Volume with waste
- 1.89 m³
- Approx. weight
- ~4.5 t (2,400 kg/m³)
For rectangular slabs and footings: length × width × thickness. Ready-mix is ordered in m³; add waste for spillage and uneven subgrade. Structural work needs engineered specs — this is a quantity estimate only.
How it works
Ready-mix is sold by the cubic meter (yard in the US); weight is roughly 2.4 tonnes per m³ for standard mix. The calculator gives net volume, volume with waste, and indicative weight so you can compare quotes. For anything structural — load-bearing slabs, columns, foundations under walls — get engineered specs; this tool sizes quantity for sheds, paths, fence posts and DIY pads. Pair the pour with a concrete mixer or wheelbarrow if you are batching on site instead of ordering a truck.
Frequently asked questions
How much waste should I add for a concrete slab?+
5% is enough on a flat, well-prepared base; 10% is safer for irregular trenches, steep access or your first pour. Structural contractors often pad more on complex formwork — DIY rectangular slabs usually sit at 5–7%.
Ready-mix truck vs bags — when does each make sense?+
Order a truck from roughly 2 m³ upward — below that, minimum load fees hurt. Bags (25–30 kg) suit fence posts, small pads under 0.5 m³ and repairs. A mixer pays off between those extremes if you have power and help on site.
What thickness for a garden shed slab?+
Light sheds on good ground: 8–10 cm of concrete over compacted hardcore is common. Heavier workshops or soft soil want 12–15 cm and sometimes mesh reinforcement. Always check local building rules for permanent structures.
How long do I have to work the concrete after delivery?+
Ready-mix typically stays workable 90–120 minutes depending on temperature, admixtures and slump. Hot days set faster — schedule pour, level and finish before ordering, and have enough hands for the volume you calculated.
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