Half a size up or down? What to do when you're between sizes
When you're stuck between two sizes, size up. You can always tighten a shoe that's a touch loose — lace it snugger, drop in an insole, wear a thicker sock — but you can't add length to one that's too short, and a cramped toe box is what actually ruins a shoe. Size down only when the model runs large or the bigger size gaps at the heel.
When to size up half a size
- Wide feet or a high instep — a half size adds a little length and girth, buying room when the standard D width is snug.
- Thick or orthotic insoles — aftermarket footbeds and winter socks eat volume; a half size up gives it back.
- The model runs small — most narrow, performance-derived running lasts fit short. Check which brands run small or large.
- You're shopping late in the day — feet swell up to half a size by evening, which is when you'll often be wearing the shoe.
- Toe room — aim for about a thumb's width (≈10 mm) between your longest toe and the end of the shoe.
When to size down half a size
- The brand runs large — roomy lasts (many lifestyle and trail brands) fit long; the smaller size is the true fit.
- Heel slip — if your heel lifts or the collar gaps when you stand, the shoe is too long, not too loose.
- Unlined leather — leather uppers stretch and mould to the foot, so a snug-but-not-tight start is correct.
How much is half a size, really?
On the US and UK scale, a full size is one barleycorn — about 8.5 mm of foot length — so half a size is roughly 4.2 mm of internal length. That's small, which is why a half size fixes length, not width: if a shoe pinches across the ball of your foot, a half size up barely helps — you need a wider width (2E/4E) or a different last. See the wide-fit guide for width.
It depends on the brand
"Size up" is the safe default, but the right call is brand-specific: a Nike runner and a New Balance lifestyle shoe in the same label fit very differently. Use these to decide before you buy:
Common questions
Should I size up or down if I am between sizes?
Default to sizing up. A slightly loose shoe can be tightened with the lacing, an insole, or a thicker sock, but a shoe that is too short cannot be made longer and will cramp your toes. Size down only if the brand or model is known to run large, or the larger size gaps badly at the heel.
How much length is half a shoe size?
About 4.2 mm (roughly 1/6 inch) of internal length per half size on the US/UK scale — one barleycorn is a full size, so half a size is half of that. Width barely changes between half sizes, so half-sizing is not a fix for a too-narrow or too-wide shoe.
When should I size up half a size?
Size up if you have wide feet, a high instep, wear thick or orthotic insoles, shop late in the day when feet swell, or the shoe runs small (most narrow performance running lasts). You generally want about a thumb's width of room ahead of your longest toe.
When should I size down half a size?
Size down if the brand or model runs large, the heel slips or gaps when you stand, or the upper is unlined leather that will stretch and mould to the foot with wear.