Fingernails and toenails do grow at a fairly consistent rate, even if it doesn't always seem like it. However, changes in growth rate and how it happens can be signs of various health conditions.
Speed of Nail Growth
Nails normally grow at a rate of 1 mm every 10 days, or .1 mm per day.
How Nails Grow
Toenails and fingernails, which are made mostly of the hardened protein keratin, grow from the matrix, the hidden part of the nail unit under the cuticle. When new cells grow in the matrix, they push old cells out. The old cells are compacted as they form the nail.
What Nail Growth Can Signify
Shape and irregularity in nails can indicate health issues. Clubbed nails, which make the tips of digits take a round, swollen shape, can indicate pulmonary or bowel problems. Spoon-shaped nails can indicate anemia and trauma. Painless separation of the nail from the bed can indicate psoriasis, infection or hyperthyroidism. Pitting, or small depressions in the nail, can be a sign of psoriasis or Reiter's syndrome. Horizontal lines across the nail are often signs of trauma.