How to measure your foot at home

Almost every shoe size in the world is a foot length wearing a disguise. Japan prints the length in centimetres and calls it the size; Korea prints the same number in millimetres; the European system counts Paris points — two thirds of a centimetre each — and the US and UK count barleycorns, a third of an inch. Different units, one measurement underneath.

Which is why measuring your foot beats guessing your size. A men's US 9 is a foot of 27 cm — and that is exactly what Japan calls 27 and Korea calls 270. Get the length right and every other number follows.

Measure in the evening. Feet swell over the course of a day by a noticeable amount, and a shoe fitted to a morning foot is the shoe that hurts at dinner. If you land between two rows in the table below, take the longer one — you can fill a roomy shoe with a thicker sock, but you cannot lengthen a short one.

One continuous scale, 0C – 6Y — the same ladder the whole way up, so a baby's first shoes and a ten-year-old's trainers are read off one table.

Children's feet grow in bursts, so a measurement has a shelf life of about three months for a toddler and six for a school-age child. Leave roughly a thumb's width of room beyond the longest toe when you buy — but buy from the length, not from the age on the label.

• Measuring sitting down. An unloaded foot is shorter than a standing one. Stand up.
• Tracing around the foot. The pen tilts outward and adds several millimetres. Mark the two ends only.
• Assuming both feet match. Most people have one foot noticeably longer. Fit the bigger one.
• Trusting the last size you bought. Adult feet do change — after pregnancy, after years of running, with age. Re-measure every couple of years.
• Reading a US number on a European shoe. A "38" on a box is not a US 38. See the shoe size chart for the full ladder in all six systems.