Choose your size guide

Three guides, one for each footwear line

Each Biomecanics line is built on a different last, so each one has its own chart. Check the guide for the line of the model you're interested in, measure the foot (always the foot, never the insole or the outsole of the shoe) and find that measurement in centimetres. Each model has a single guide, the one for its line, and that's the one that gives its size.

Before you check the chart

What should I measure, the foot or the shoe?

The bare foot. The charts in all three guides are based on the measurement of the foot, not on that of the insole or the outsole. Both are larger than the foot that goes inside, so measuring them leads to a size bigger than the one needed.
What's the best way to measure it?

A sheet of paper on the floor against a wall, with the heel touching the wall and the foot completely flat. Mark where the longest toe ends (it isn't always the big toe) and measure from the edge of the paper to the mark. Each guide explains it step by step.
Can my size be in two guides at once?

Yes, and it isn't a mistake. The three lines share part of their size range: children's footwear goes up to size 41, the adult line starts at 36 and the barefoot line covers 19 to 36. Because each line has its own last, the same size can appear in two guides with slightly different measurements. What doesn't change is that each model has a single guide, the one for its line: that's the one that gives its size.
Are the UK and US equivalents exact?

They're approximate, and they help if you're used to buying shoes in another sizing system. The reliable reference is the centimetre: measure the foot, find its measurement in the chart and take the EU size from that row.
When is the best time to measure?

At the end of the day, when the foot is at its largest. And measure both feet: they don't usually measure exactly the same, so the size is set by the larger of the two.