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 (63.6 cu ft)
- Volume with waste
- 1.89 m³ (66.7 cu ft)
- 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
Quick thickness guide for DIY, non-structural work:
| Job | Thickness |
|---|---|
| Garden path | 8–10 cm |
| Shed pad on good ground | 10 cm |
| Workshop, or soft soil | 12–15 cm (mesh often wise) |
Fence post holes are a different shape: volume ≈ π × (diameter ÷ 2)² × depth × number of holes, and still add waste. On quantity, 1 m³ ≈ 40 bags of 25 kg of dry mix if you batch on site — check your bag yield, it varies. Ready-mix delivered usually beats bags from about 2 m³ upward; below that, bags and a concrete mixer win.
Ready-mix is sold by the cubic meter (yard in the US); weight is roughly 2.4 t/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.
Concrete does not set and finish — it sets and then keeps gaining strength for weeks, and it can only do that while it stays damp. A slab poured in sun and left to dry loses a large part of its surface strength and will dust and chalk for the rest of its life, which is the real reason so many garage floors shed powder. Cover it and keep it wet for about a week. Temperature runs the same way: below roughly 5 °C the reaction crawls and the pour stays weak for days, and if it freezes before it has set the damage is permanent. If the mixer is the sticking point, the buy or rent comparison gives you the break-even in days before you decide.
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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%.
How many 25 kg bags per m³ of concrete?+
Roughly 40 bags of typical dry-mix per cubic metre once mixed with water — always read the bag's stated yield. For 0.3 m³ (small pad) that is about 12 bags; for 2 m³, order a truck instead.
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.
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