ToolnozaConcrete volume formula for a slab
For a rectangular slab, volume = length × width × thickness. Measure all three dimensions in the same unit first. The result is a cubic measure: feet give cubic feet, metres give cubic metres. Convert the finished volume only after multiplying.
A slab that is 12 ft long, 10 ft wide and 4 in thick has 4 in ÷ 12 = 0.3333 ft of thickness. Its geometric volume is 12 × 10 × 0.3333 = about 40 ft³.
Convert cubic feet to cubic yards
In the US, divide cubic feet by 27 to get cubic yards because one cubic yard contains 27 cubic feet. The example above is 40 ÷ 27 = about 1.48 yd³ before any allowance. For metric dimensions, keep the volume in m³; 1 m³ = 1,000 litres. Do not mix feet, inches and metres in the same multiplication.
Account for the actual slab shape
For an L-shaped or stepped area, split the plan into rectangles, calculate each section and add the volumes. Subtract openings only when they pass through the full slab thickness. A slope or variable depth needs more than one thickness measurement; divide it into sections or use an appropriate site takeoff.
Plan an ordering allowance carefully
The geometric volume is not automatically the delivery quantity. Subgrade irregularities, spillage, form movement and placing method can change what is needed. Check the supplier’s batch minimum, delivery rounding and product yield. Choose an allowance based on measured site conditions and the contractor or supplier’s guidance instead of treating one percentage as universal.
Use the calculator and check the result
- Enter the slab length, width and thickness using matching units.
- Read the calculated volume, then convert it to the unit your supplier requests.
- Compare the dimensions and units with your drawing before ordering; get a site professional to confirm unusual shapes or structural work.
The result estimates volume from the dimensions entered. It does not design slab thickness, reinforcement, joints, base preparation or concrete strength.