Science Puzzle
How Long Would It Take to Count Your Cells?
Scientists estimate that an adult human body is made of roughly 37 trillion cells. Estimates vary a little, but that's a good working figure: 37 followed by twelve zeros.
Suppose you could count one cell every second, day and night, without ever stopping to eat or sleep. How long would it take you to count all of your own cells?
The Answer
About 1.2 million years.
A year holds about 31.5 million seconds (60 × 60 × 24 × 365). Divide 37,000,000,000,000 by 31,500,000 and you get roughly 1,170,000 years. Our own species has only existed for about 300,000 years, so you'd have needed to start counting long before there were any humans to do it.
There's a worse problem. Your body is constantly replacing worn-out cells, especially red blood cells and the cells lining your gut. A 2021 study estimated that the body makes around 330 billion new cells every day. You'd count 86,400 cells a day while your body swapped hundreds of billions; the number you were trying to reach would keep changing under you, and you could never finish.
Huge numbers are hard to feel. A million seconds is about eleven and a half days. A billion seconds is nearly 32 years. A trillion seconds is about 31,700 years. Turning a number into time like this is one of the simplest ways to see how big it really is.
The principle: Orders of magnitude. Each step from thousand to million to billion to trillion is a thousand times bigger, and converting a huge count into time makes its real size easier to grasp.