Paper, a pencil and a ruler beat guessing from your last pair.
Set up
Tape a sheet of paper to a hard floor against a wall. Wear the socks you intend to use with the shoes. Measure late in the day, when feet are at their largest.
Trace and measure
Stand with your heel against the wall and your weight evenly distributed. Mark the tip of the longest toe, which is not always the big toe. Measure from the wall edge of the paper to the mark in centimetres.
Measure both feet
Most people have one foot slightly longer than the other. Always buy for the larger foot and use insoles or lacing adjustments to fine-tune the other side.
Add wearing room
Add around 0.5 cm for dress shoes and up to 1.0 cm for running shoes. Enter the final figure into a foot-length converter to see US, UK and EU sizes side by side.
Frequently Asked Questions
Should I measure standing or sitting?
Standing. The foot lengthens under body weight.
What about width?
Measure the widest part of the forefoot and compare to brands that publish width fittings such as D or EE.