Convert a single period without mixing units
Use the natural logarithm, not base 10. For this price-only calculation, assume positive prices, no distributions and no trading costs. R = P₁ / P₀ - 1; r = ln(1 + R). A percentage label means 100 times the decimal value. These identities follow the return definitions in Eric Zivot's textbook linked below.
- Calculate ln(1 + 0.10) for a 10% simple gain. Feeding 10 into that formula means a 1,000% gain.
- For small changes, ln(1 + R) is close to R. At a 1% simple return, the log return is 0.9950%; at a 50% simple return, it is 40.5465%. Use the exact conversion when reconciling results.
- Log returns are a representation of growth. Calling a return continuously compounded does not establish that a trading strategy reinvested every profit.
| Hypothetical price move | Simple return R | Log return r | Reverse conversion |
|---|---|---|---|
| 100 to 110 | 0.10 = 10.0000% | 0.095310 = 9.5310% | exp(0.0953101798) - 1 = 0.10 |
| 100 to 90 | -0.10 = -10.0000% | -0.105361 = -10.5361% | exp(-0.1053605157) - 1 = -0.10 |
| 100 to 100 | 0 = 0% | 0 = 0% | exp(0) - 1 = 0 |