Answer & explanation
The official answer, with Fermi’s reasoning step by step.
Answer & explanation
Official answer
120
inches
How to estimate it
Fermi’s official “Napkin math”
A fingernail grows out completely in about six months, and a nail is roughly three quarters of an inch long. How much is that in a year? Call it an inch and a half.
- Nail growth in a year
- ~1.5 in
- Human lifespan
- × 80 years
- 1.5 × 80
- ≈ 120 inches
A fingernail grows out completely in about six months, and a nail is roughly 2 centimeters long. How much is that in a year? Call it 4 centimeters.
- Nail growth in a year
- ~4 cm
- Human lifespan
- × 80 years
- 4 × 80
- ≈ 320 centimeters
Rounded assumptions may give a slightly different estimate. The official answer above is the game’s reference value.
Source calculation & reference
- Average fingernail growth rate
- 3.47 mm per month
- Over a year
- × 12 = 41.6 mm
- Global life expectancy at birth
- × 73 years
- ≈ result
- 3,040 mm ≈ 120 inches
Reference cited by Fermi: Yaemsiri et al. nail growth study (2010) and UN World Population Prospects 2024
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