Why the conversion matters
Area and volume formulas only work when the units are compatible. Using 100 instead of 0.1 for a 100 mm depth makes a cubic-metre calculation wildly wrong.
The rule
Divide millimetres by 1,000 to get metres. 1 mm = 0.001 m, 10 mm = 0.01 m and 100 mm = 0.1 m.
Common depth examples
50 mm = 0.05 m; 75 mm = 0.075 m; 100 mm = 0.1 m; 150 mm = 0.15 m; 200 mm = 0.2 m; 300 mm = 0.3 m.
Keep the raw measurement too
Record both the site dimension and the converted value so the calculation can be checked later.
Use the numbers in a calculator
Accuracy note
Where a figure depends on a product, supplier, installation method or project specification, use the current information for the exact material being purchased. The reference pages explain the maths and estimating logic; they are not a substitute for current technical data.