Why do fingernails grow faster than toenails?

A fingernail grows about 0.1 mm a day. Replacing one fingernail completely, from base to tip, takes about six months. A big toenail takes 12 to 18 months.

Participants in that study filed a tiny groove near the base of each nail, then measured how far it had moved one to three months later. Speeds differed even on the same hand: the little fingernail grew slower than the others, and among the toes the big toenail grew fastest.

The most widely accepted explanation is how often you use your fingertips. All day they tap keys, pick things up, and rub against surfaces. Toenails, tucked inside socks and shoes, take far fewer knocks.

The dermatologist Dawber noted that while an injured finger was splinted for three months, its nail grew about 25 percent slower than in the three months after the splint came off. The idea is that places that wear down more often refill faster.

Another explanation is circulation. Hands sit closer to the heart than feet and are usually warmer. Observations that nails grow faster in summer and slow down with age support this idea.

The American physician William Bean measured how much his own nail grew for 35 years, starting in his early thirties, and it slowed little by little as he aged. Still, exactly what sets the speed has not been fully worked out.

Many people say right-handers grow their right-hand nails faster. In the 22-person study, though, the difference between left and right was not statistically significant. It may still vary from person to person.

The claim that nails keep growing after death is false. For a nail to grow, living cells have to build new layers. As the body dries, the skin at the fingertips shrinks back, which only makes the nails look longer.

A white mark left by a bump moves out at the same pace. On a fingernail it reaches the tip within a few months, but on a toenail it can stay for close to a year.