Concrete Volume Calculator
Slab volume in m³ and 40 kg bag count.
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Formula
volume (m³) = length × width × depth; bags = ceil(volume ÷ 0.023)
Multiply the three slab dimensions in metres for cubic metres of concrete. Divide by the yield per 40 kg premix bag (about 0.023 m³) and round up for whole bags.
Worked example
A 4 m × 3 m pad at 10 cm thick is 1.2 m³ of concrete. At ~0.023 m³ per 40 kg bag you'd need 53 bags — always round up; leftover dry mix beats a mid-pour shortage.
Where this can give the wrong answer
- Bag yield varies by mix design and slump — 0.023 m³ is a premix rule of thumb, not a spec sheet.
- Reinforcement, pump loss, and spillage aren't added — add 5–10% waste on site for pours.
- Depth in metres: 10 cm = 0.1 m, not 10.
FAQ
- Above ~1 m³, ready-mix is usually cheaper and stronger per batch consistency. Bags suit small pads, fence posts, and repair patches.