How to measure a rectangular concrete slab

Rectangular slab showing length, width and depth; volume equals length times width times depth. — Convert all three dimensions to one length unit before multiplying. Diagram is not to scale.

Measure the planned pour, not the space around it. Record length, width and thickness separately, with a unit beside every number. Thickness is often measured in millimetres or inches while the other dimensions are in metres or feet. Convert those numbers before multiplying.

A worked example

Item	Example or check
Length	5 m
Width	4 m
Thickness	100 mm = 0.1 m
Net volume	5 × 4 × 0.1 = 2 m³

Check the assumptions

Measure depth at several points if the base is uneven. A single reading may understate the fill. The calculator assumes a flat rectangular shape; divide an L-shaped area into rectangles and calculate each section separately without overlapping them. A drawing with labelled rectangles is easier to audit than one unexplained total.

If you choose a 10% extra allowance, the order target becomes 2.2 m³. That is your purchasing assumption, not a recommendation about slab thickness. Ask the supplier how they round deliveries and whether the order has a minimum quantity or small-load charge.

Before using the estimate

Do not infer the required structural thickness from a quantity calculator. Loads, ground conditions, reinforcement and local rules belong in a separate design decision.

Open the Concrete Calculator and compare your measurements with the example.

Further reference: Publisher or manufacturer guidance. Product specifications can change; use the current label.