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What does an animal dose mean in a person?
The conversion people reach for most and get wrong most. A rat dose is not a human dose divided by nothing — the FDA scales it by body surface area, and the number comes out smaller than the one you started with.
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Why the number goes down
A rat is not a small human. It clears most compounds faster per kilogram, because metabolic rate tracks body surface area rather than mass. The FDA's 2005 guidance corrects for that with a table of Km values — body weight in kg divided by surface area in m² — and the conversion is the ratio of the two.
| Species | Km | Multiply a mg/kg dose by |
|---|---|---|
| Mouse | 3 | 0.081 |
| Hamster | 5 | 0.135 |
| Rat | 6 | 0.162 |
| Guinea pig | 8 | 0.216 |
| Rabbit | 12 | 0.324 |
| Monkey | 12 | 0.324 |
| Dog | 20 | 0.541 |
| Mini-pig | 27 | 0.73 |
| Human | 37 | 1 |
So a rat dose divides by about six, and a mouse dose by about twelve. Someone who skips this step and multiplies a mouse dose by their own body weight lands an order of magnitude high.