Feet often swell during long exercise, especially in warm conditions. A shoe that fits perfectly at the start can feel tight an hour later. Allowing for this avoids blisters and black toenails.
How much room
A common guide is roughly a thumb’s width of space between your longest toe and the front of the shoe. Your toes should not touch the end.
Width
The shoe should not squeeze the widest part of your foot. If it does, try a wide version. See wide feet and hybrid racing.
When to try shoes
Try shoes on later in the day, when your feet are larger, and wear the socks you train in.
Heel lock
A shoe with room at the front still needs a secure heel. A slipping heel causes blisters. See lacing techniques.
Mid-race
If your shoes feel tight, loosen the laces in the Roxzone. The row is a good point to do it. See HYROX row pacing.
Socks
Thicker socks take up space. Race in the socks you train in. See socks for hybrid racing.
Related
How to choose a shoe for HYROX and shoe sizing across brands.