Running shoes are one of the few shoes where the size you buy matters for health: too short and you lose toenails, too narrow and you get blisters, too loose and your foot slides. Running brands agree on the basics: running shoes should fit true to size, with room for the foot to expand and about a thumb’s width at the toe. This calculator gives your size from your foot length and the inside length to check on the shoe.
How to use the running shoe size calculator
- Measure your longer foot, heel against a wall, to the longest toe.
- Enter the length in centimetres, millimetres or inches.
- Read your size in US men’s, women’s, UK and EU.
- Try the shoes on with your running socks and check the toe room and the heel.
Running shoe size formula
minimum inside length = foot length + about 1 cm (ASICS: about a thumb’s width)
between sizes → size up
The chart is Nike’s men’s footwear table, which lists the women’s and UK sizes in the same row (retrieved 2026-10-15). Nike’s running guide says running shoes fit true to size with about a thumb’s width between the big toe and the shoe’s end; ASICS says to leave about a centimetre. A foot of 26.5 cm needs shoes at least 27.5 cm long inside.
Worked example
A runner’s longer foot is 26.4 cm (10.4 in).
- Nike chart: the first size whose foot length is at least 10.4 in is men’s 9.5 (women’s 11, UK 8.5, EU 43).
- Inside length needed: 26.4 + 1 = 27.4 cm.
- On try-on, there should be about a thumb’s width at the toe and a snug heel.
Checking the fit
- Toe room: about a thumb’s width, without the toes curling over.
- Heel: snug without slipping.
- Laces: snug, with room for a finger under the knot.
- Afternoon: try shoes on later in the day, when feet are slightly larger.
This is general guidance; a running-specialty store can watch you jog. For width see the Shoe Width Calculator, and for cyclists the Cycling Shoe Size Calculator.
Sources
Every figure above comes from these pages, read directly on the dates shown. Brand names are used only to identify published charts.
- Nike — how should running shoes fit (true to size, thumb’s width) — checked 2026-10-15
- ASICS — how should running shoes fit (about 1 cm, finger under the knot, socks) — checked 2026-10-15
- Nike — men’s and women’s shoe size chart — checked 2026-10-15
Frequently asked questions
Should running shoes be a size bigger?
Nike says running shoes are intended to fit true to size, with about a thumb’s width between the big toe and the end of the shoe. ASICS says to leave about 1 centimetre between the toes and the top of the shoe, because feet expand when you run. Do not simply buy a bigger number: check the room when you try them on.
How much space should there be at the toe?
About a thumb’s width, which is roughly 1 cm. ASICS adds that you should be able to fit a finger under the knot when the laces are snug and that if one foot is bigger you should fit the bigger foot.
What if I prefer a snug fit?
Nike’s guide notes that if you prefer a tighter fit, your toes can touch the end of the shoe, but your toes must not curl over, since that can distort the foot and lead to injury.
Do running socks matter?
Yes. ASICS says to factor in the thickness of your sock and to wear your usual running socks when trying shoes on.
How do I measure my foot?
See the Shoe Size Calculator (foot length): stand on paper with the heel against a wall and measure to the longest toe. For width use the Shoe Width Calculator.