Concrete Volume Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/5/2026
The volume of concrete needed for a slab is calculated with length × width × thickness (converted to meters). For a 5 × 4 m slab and a 10 cm thickness, you need 2 m³ of concrete.
Explanation
This calculator estimates the volume of concrete needed to pour a rectangular slab (patio, garage, light foundation), from its three dimensions. Thickness is entered in centimeters, the most natural unit for this kind of measurement on a job site, then converted to meters before calculating the volume. This result corresponds to the theoretical concrete volume needed, with no margin for waste or ground irregularities: in practice, plan for an extra 5-10% safety margin to account for variations in ground level. For the area of a room rather than the concrete volume of an outdoor slab, see our area calculator.
Example: a 5 m × 4 m slab, 10 cm thick
Inputs
Length: 5 m. Width: 4 m. Thickness: 10 cm.
Calculation
10 cm = 0.10 m. Volume = 5 × 4 × 0.10 = 2 m³, or 2,000 liters.
Result
This slab needs 2 m³ of concrete.
Frequently asked questions
Should I add a margin on top of this calculated volume?
Yes, this calculator gives an exact theoretical volume for a perfectly flat slab. In practice, the ground is never perfectly even: it's common to plan for 5-10% extra concrete to compensate for ground irregularities and avoid running out mid-pour.
What slab thickness should I choose?
It depends on the use: about 10 cm is enough for a pedestrian patio or a walkway, 12-15 cm for a garage with a light vehicle, and more for heavier loads or professional use. When in doubt, ask a building professional for advice.
How do I convert this volume into a number of ready-mix concrete bags?
A bag's yield depends on its exact formulation (shown on the packaging by the manufacturer), so this calculator doesn't estimate it, to avoid showing a potentially wrong number of bags. Check the liters-per-bag yield printed on your chosen product's packaging to convert the total volume obtained here.