1. Where nails grow from: the nail matrix under the skin
Nails appear to grow from the fingertips, but they are not made there. The place where nail growth begins is the nail matrix, a part hidden under the skin at the base [2]. New nail created here is pushed forward, extending toward the fingertip [1].
The speed is said to average about 0.1 mm per day, or about 3 mm per month for adult fingernails [1]. 0.1 mm is about the thickness of one sheet of copy paper. You can barely notice this movement in a single day, but because it continues every day without rest, it adds up to three ruler marks in one month.
At this speed, it is explained that it takes about 6 months for a fingernail to replace itself from root to tip, and 12 to 18 months for toenails [3]. Three mm per month for six months is about 18 mm. If you compare this to the length of a typical nail, the calculation roughly matches. This means the tip you remove with clippers is a part made about six months ago.
However, 0.1 mm is an average, not a fixed value. Even among healthy people, growth rates vary greatly, ranging from less than 1.8 mm to more than 4.5 mm per month [1].
2. Why nails keep growing without rest, unlike hair
Hair goes through cycles of growing and resting. Nails, however, do not rest. The difference from hair is explained by the fact that cells in the nail matrix do not stop dividing, so nails are produced continuously [1].
The nail plate created this way is made of an average of about 196 layers of cells stacked together [1]. The average thickness is 0.5 mm for women and 0.6 mm for men. It is said that the longer the width (length) of the nail matrix, the thicker the nail plate becomes [1]. Inside the thickness of five to six sheets of copy paper, cell layers are tightly packed. The number 196 is an average and varies by person and finger.
The cells that form the nail plate are already dead cells, so you do not feel pain when cutting them [5]. There is no pain because the plate at the tip of the nail is different from the living parts that transmit sensation.
The white half-moon shape visible at the base of the nail (the lunula) is newly formed nail. It is explained that it looks white because it has not fully hardened into keratin and contains a lot of moisture [5]. This is one place where nail growth is visible.
3. How toenail growth speed compares with fingernail growth
Many people feel they cut fingernails often but do not cut toenails as much. This feeling matches the measured numbers. In a study of 22 healthy young adults in the United States, the average growth per month was 3.47 mm for fingernails and 1.62 mm for toenails. Fingernails grow about twice as fast as toenails [4].
Other sources also state that toenails grow at one-third to one-half the speed of fingernails, and it takes 12 to 18 months for a toenail to fully regrow [3].
Growth speed is also said to change with age. It is said that children grow nails faster than adults because their metabolism is active, and the speed decreases as people get older [5]. However, this is a description of a general trend and does not determine the exact speed for each individual.
4. Why we have nails on our fingertips: from claws to flat nails
What is the purpose of nails? Sources list functions such as protecting the fingertips, aiding touch sensation, making fine tasks like pinching easier, and scratching itches [3][1].
It is widely seen that "nails support the force of the finger pad pressing against objects, allowing for fine work." However, research directly verifying this is not yet sufficient, so it cannot be stated definitively that "having nails always improves touch sensation."
The origin of nails is also not settled. A study of fossils from Wyoming, USA, dating back more than 56 million years, reported that the ancestors of the oldest primates had flat nails as well as grooming claws [6]. The transition from claws to flat nails may have been more complex than previously thought. There is a hypothesis that flat nails changed to suit movements like climbing trees, jumping, and grasping branches, but this is not yet confirmed.
5. Using nail growth to estimate how many days have passed
If nail growth averages 0.1 mm per day, the measured growth since cutting can give a rough estimate of elapsed days. You can check how well this calculation works with your own nails.
Cut your nails with clippers (use only nail clippers, and do this where an adult can see you), and write the date in a notebook. Two weeks later, measure the length from the tip of the finger flesh to the tip of the nail with a ruler. At 0.1 mm per day, it should be about 1.4 mm. Measure again one month later. If it is average, it should be about 3 mm. Measuring the same spot on the same finger reduces errors.
If it is shorter or longer than average, that is simply "that person's speed." Healthy people range from less than 1.8 mm to more than 4.5 mm per month [1].
If you can measure toenails the same way, you can compare them with fingernails. Since toenail growth is about half that of fingernails, you can expect about 1.5 mm in one month.