How many cells are in a human body?

Question

Estimate the number of cells in an adult human body. Work from the body's mass and a reasonable guess at the mass or size of a typical cell, and say why the answer is uncertain by a factor of a few.

Accepted Answer

A few tens of trillions: about 3 × 10 to the 13. By volume. A typical cell is roughly a sphere 10 to 20 micrometres across, say 15 micrometres = 1.5 × 10 to the minus 5 m. Its volume is about (1.5 × 10 to the minus 5) cubed, roughly 3 × 10 to the minus 15 cubic metres, and at the density of water its mass is about 3 × 10 to the minus 12 kg. An adult of 70 kg is then made of about 70 / (3 × 10 to the minus 12) = 2 × 10 to the 13 cells, 20 trillion, with a factor-of-several uncertainty from the cell size guess (halving the diameter multiplies the count by eight). The refinement biologists use. …

How many cells are in a human body?

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