Sizes are not standardised. A 42 from one maker is a 43 from another, and the same brand can vary between models. The only number that is yours is the length of your foot in millimetres — measure that once and every size chart becomes usable.
This matters to us directly: the waiting-list form asks for your usual size to plan a first size run, and a measured answer is worth ten guessed ones.
Measure in the evening, in socks
Feet swell over the day; a morning measurement will buy you a shoe that is tight by four o'clock. Stand on a sheet of paper against a wall, heel touching the wall, wearing the socks you will wear with the shoe. Have someone mark the tip of the longest toe — it is not always the big toe — then measure wall to mark in millimetres. Do both feet and use the longer one.
Add about 10 mm for a shoe you will walk in. That is the length you are shopping for.
Width is the number nobody gives you
Two people with the same foot length can need different shoes. If you have ever sized up to stop a shoe pinching across the ball of the foot, you have a wide foot and you have been wearing shoes that are too long. Measure the widest point of the foot and note whether it is above or below about 100 mm for a men's 42 or 90 mm for a women's 38 — rough thresholds, but enough to know which reviews to trust.
Reading a size chart
Find the chart's millimetre or centimetre column and ignore the rest. If it only lists EU/US/UK, look up that brand's conversion — the table below is a common one, but treat it as a starting point rather than a rule. Where you fall between sizes, go up in a shoe you will wear thick socks with and down in a shoe with a soft, stretchy upper.
EU 39 ≈ US men's 6.5 / women's 8 ≈ UK 6 ≈ 250 mm. EU 42 ≈ US men's 9 / women's 10.5 ≈ UK 8 ≈ 270 mm. EU 44 ≈ US men's 10.5 / women's 12 ≈ UK 9.5 ≈ 280 mm.
Adjusting the fit you already have
A pair that is half a size loose is fixable; a pair that is half a size tight is not. A trim-to-fit insole takes up volume without changing the length, and a heel-lock lacing pattern stops the slip. Keep the paper with your measurement in the drawer with the spare laces; you will use it again.