How to calculate concrete for a slab
Use length, width and thickness to estimate the concrete volume before ordering ready-mix or planning a small pour.
For a rectangular slab, the calculation is simple: measure the area, convert the slab thickness to meters, multiply and then add a waste allowance. The result is a preliminary volume, not a structural design.
Concrete volume = length x width x thickness. If thickness is in centimeters, divide it by 100 first.
Step-by-step method
1. Measure length and width
Measure the actual slab footprint. For patios, sheds or small pads, use meters and keep two decimals if the slab is not perfectly square.
2. Convert thickness
A 10 cm slab is 0.10 m. A 12 cm slab is 0.12 m. This conversion matters because concrete is ordered in cubic meters.
3. Add waste
Waste covers small over-excavation, uneven ground, spillage and formwork irregularities. For simple slabs, 8 to 10 percent is a practical preliminary allowance.
Worked example
A slab measuring 5 m x 4 m with 10 cm thickness has an area of 20 m2. Thickness is 0.10 m, so the net concrete volume is 20 x 0.10 = 2.00 m3. With 8 percent waste, order or plan around 2.16 m3.
Common mistakes
- Multiplying by 10 instead of 0.10 when using 10 cm thickness.
- Forgetting waste allowance for uneven ground or formwork.
- Not checking whether the slab needs mesh, rebar or joints.
- Using a calculator result as a structural specification.
Use the calculator
Enter your own dimensions in the concrete calculator or estimate gravel and wire mesh with the floor slab calculator.
Frequently asked questions
What is the formula for concrete volume?
Length x width x thickness. Convert thickness to meters first.
How much waste should I add?
For a simple slab, 8 to 10 percent is a common preliminary allowance.
Can I use this for structural slabs?
No. Structural slabs, elevated slabs and slabs carrying heavy loads require professional design and review.
This guide is for preliminary planning only. It does not replace engineering design, soil review, reinforcement detailing or a formal quote.