Answer & explanation
The official answer, with Fermi’s reasoning step by step.
Answer & explanation
Official answer
8
liters
How to estimate it
Fermi’s official “Napkin math”
A vampire bat is tiny, maybe 30 grams. How much does it drink in a night? About its own weight, so much it can barely take off after. And it needs that meal almost every night.
- A vampire bat
- ≈ 30 g
- Drinks its weight a night
- ≈ 30 ml
- 30 ml × 365
- ≈ 11 liters
A vampire bat is tiny, maybe an ounce. How much does it drink in a night? About its own weight, so much it can barely take off after. Call it an ounce of blood, almost every night.
- A vampire bat
- ≈ 1 oz
- 1 oz × 365 nights
- = 365 oz
- 365 ÷ 128 oz a gallon
- ≈ 3 gallons
Rounded assumptions may give a slightly different estimate. The official answer above is the game’s reference value.
Source calculation & reference
- Measured intake, Philadelphia Zoo vampire bat colony
- ≈ 23 ml a day
- 23 ml × 365 days
- ≈ 8,400 ml
- ≈
- 8 liters
Reference cited by Fermi: Philadelphia Zoo colony feeding data (AZA Nutrition Advisory Group)
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