Concrete Calculator
This concrete calculator works out how much concrete you need for a slab, floor, footing or path, and estimates how many bags of premixed concrete that represents. Enter the length and width of the area in metres and the thickness in centimetres, and it instantly returns the volume in cubic metres along with an approximate bag count. It is ideal for planning small building and DIY projects where you want to buy the right amount of material without over-ordering or running short.
Concrete volume
Premixed concrete bags (approx.)
A 25 kg bag of premixed concrete yields roughly 0.011 m³ of concrete; the actual yield depends on the mix design (the ratio of cement, sand, gravel and water) and the type of mix.
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How it works
The volume of a rectangular slab is simply its length times its width times its thickness, with every measurement expressed in the same unit. Because the length and width are entered in metres and the thickness in centimetres, the calculator first divides the thickness by 100 to convert it to metres, then multiplies the three values together to get the volume in cubic metres (m³). To estimate the number of premixed concrete bags, it divides the volume by the yield of a single bag — about 0.011 m³ of concrete per 25 kg bag — and rounds up to the next whole bag, since you cannot buy a fraction of one. The bag figure is an approximation: real yield varies with the mix design and how much sand, gravel and water are used.
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Preguntas frecuentes
- How do I calculate the volume of concrete I need?
- Multiply the length by the width by the thickness of the slab, using the same unit for all three. For a floor 4 m long, 3 m wide and 10 cm (0.10 m) thick, the volume is 4 × 3 × 0.10 = 1.2 m³. This calculator does the unit conversion for you: you enter length and width in metres and thickness in centimetres, and it returns the volume in cubic metres.
- How many bags of premixed concrete do I need per cubic metre?
- As a rough guide, a 25 kg bag of premixed concrete yields about 0.011 m³ of concrete, so one cubic metre needs roughly 90 bags. The exact number depends on the mix ratio (how much sand, gravel and water you add). This calculator estimates the count by dividing the volume by 0.011 and rounding up.
- Why does the bag count round up?
- You cannot buy part of a bag, so any leftover volume still requires a full extra bag. The calculator always rounds up to the next whole bag to make sure you have enough material. For example, a volume needing 81.8 bags is shown as 82.
- Is the estimate for concrete or just cement?
- The volume figure is the total volume of finished concrete — cement plus sand, gravel and water combined. The bag figure assumes bags of premixed (ready-mix) concrete, which already contain cement, sand and gravel, so the bags cover the whole volume and you only add water. If you instead mix your own concrete from loose cement, you will need far fewer cement bags plus sand and gravel bought separately.
- Should I add extra for waste?
- Yes. It is common to add about 5–10% to the calculated volume to account for spillage, uneven ground and slight over-excavation. Ordering a little extra is usually cheaper and faster than running out mid-pour and having to mix a second, smaller batch.