How Long Does It Take to Double Money?

The exact answer

Years = ln(2) / ln(1 + r)

Where r is the annual rate as a decimal. This is the log form of the equation (1 + r) ^ Years = 2.

A table for intuition

 4%   ≈ 17.7 years
 6%   ≈ 11.9 years
 8%   ≈  9.0 years
10%   ≈  7.3 years
12%   ≈  6.1 years
15%   ≈  5.0 years
20%   ≈  3.8 years
25%   ≈  3.1 years

The Rule of 72 shortcut

Years ≈ 72 / r%

At 8%, that gives 9 years — almost identical to the exact 9.01. In the 5–12% range the rule is within about 1% of the truth.

What it does not tell you

Doubling time assumes a constant return. Real markets do not deliver constant returns; they deliver an uneven series of years whose geometric mean is the rate you assumed. Plan for the long-run mean, but expect the path to be lumpy.