Convert your child's foot length — or just their age — into an estimated US, UK and EU shoe size.
Have your child stand on a piece of paper against a wall, mark the back of the heel and the tip of the longest toe, then measure the distance between the marks in cm or inches. Measure both feet — they're often slightly different sizes — and use the larger one.
About 1cm (roughly a thumb's width) between the longest toe and the end of the shoe is a common guideline — enough room to allow for growth and natural foot swelling during the day, without being so loose the shoe causes rubbing or trips.
Yes, noticeably — sizing charts are standardized in theory but vary in practice between brands and even between styles from the same brand. Use this calculator as a starting point, then check the specific brand's own size chart and try shoes on when possible.
Young children's feet can grow noticeably every 3–4 months in the toddler years, which is why many parents re-measure and re-check shoe fit roughly every season rather than assuming last year's size still fits.
When in doubt, sizing up is generally safer than sizing down for growing feet — a slightly loose shoe with a snug heel and adjustable strap or laces is usually more comfortable and safer than one that's too tight.