Outdoor fit depends on the product and the trip. Always use the size chart on the product page when one is available.
Hiking footwear
Measure both feet late in the day while wearing the socks you expect to use. Leave enough toe room for descents, but do not accept heel lift or side-to-side movement. Different brands use different lasts, so a familiar numerical size is only a starting point.
Outdoor layers
Choose a base layer that stays close without restricting movement. Insulation and shell layers need enough room for the layers beneath them while still allowing cuffs, hems, and hoods to seal properly.
Backpacks
Torso length matters more than body height. Position the hip belt over the hip bones, then adjust shoulder straps and load lifters. Check the fit with realistic weight inside the pack.
Gloves and accessories
Measure around the widest part of the hand. Insulated gloves should not compress the fingers, because compressed insulation provides less warmth.
FAQ
Should hiking boots be one size larger?
Not automatically. Use measured foot length, the brand chart, your intended socks, and a downhill fit check rather than adding a fixed amount to every size.