How to Use the Natural Log (LN) in Excel

Use LN to calculate log returns from prices, add them across days and estimate how long a value takes to double.

You will learn to

• Calculate a daily log return
• Calculate a period log return
• Estimate doubling time from an average log return

How it works

LN(number) returns the natural logarithm — the power you raise e to in order to get the number. In finance, =LN(B3/B2) is the log return from one price to the next. Log returns add up neatly: the sum of daily log returns equals LN(last price ÷ first price).

LN also answers “how long?” questions. If something grows at a continuous rate r per period, it doubles in LN(2)/r periods. LN only accepts positive numbers; LN(0) or a negative value returns #NUM!.

=LN(B3/B2)