01
Measure one rectangular section
A rectangular section needs three measurements: length, width, and thickness. Volume is length × width × thickness, so all three must use a consistent unit before they are multiplied together.
Measure length and width in the same unit—feet or metres—across the longest and widest usable points. Record thickness separately, because it is usually a much smaller number.
02
Split irregular slabs into rectangles
An L-shaped or T-shaped slab does not need one clever measurement. Split the shape into the fewest rectangular sections, measure each one, then add the volumes together later.
| Section | Length | Width | Thickness |
|---|---|---|---|
| A | 10 ft | 6 ft | 4 in |
| B | 4 ft | 3 ft | 4 in |
Keep one row per section so the numbers stay traceable. A rough sketch with the section letters written on it makes the record easier to check back at the jobsite.
03
Record thickness in one unit
Thickness is the most common source of unit mistakes. In US units, thickness is usually written in inches while length and width are in feet: 4 inches is 4 ÷ 12 = 0.333 feet. In metric, thickness is often millimetres or centimetres: 100 mm is 0.10 metres.
Convert the thickness to match the length and width unit before multiplying. Do not mix feet and inches, or metres and millimetres, in the same volume calculation.
04
Measuring checklist
- Same unit everywhere: confirm length, width, and converted thickness share one unit.
- Longest usable span: measure the actual pour footprint, not a rough wall-to-wall guess.
- Thickness at the right places: check the form depth where the slab will actually be poured.
- One row per section: keep irregular slabs split into labelled rectangles.
- Recheck the numbers: a second pass catches transposed digits before they reach an order.
05
Worked example: a 10 × 6 ft slab
A single 10 ft × 6 ft section with 4 inches of thickness has an exact volume of 20 ft³. After a 10% planning allowance, the planned volume is 22 ft³.
The measuring work is complete once the three dimensions are recorded in one unit. Feed those numbers into a calculator to turn volume into bags or ready-mix quantities—measuring and estimating stay separate steps.
06
Sources and limitations
This guide explains how to measure slab dimensions, not how to size a slab. It does not recommend slab thickness, reinforcement, mix strength, soil preparation, drainage, or structural suitability.
- NIST volume conversion factors — reference for foot, inch, and metre conversion.
Last reviewed: September 1, 2026
Once your measurements are recorded, combine several slab sections, open the Concrete Calculator to estimate volume, or compare whole-bag counts for a known volume.
Open the Concrete Calculator