The Rule of 72: How to Calculate When Your Money Doubles

Want to know how long it will take for your investment to double without pulling out a spreadsheet? The Rule of 72 is one of the most useful mental shortcuts in finance. Whether you're looking at a savings account or a stock market index, this rule gives you a quick answer in seconds.

What is the Rule of 72?

The Rule of 72 is a simple formula used to estimate the number of years required to double the invested money at a given annual rate of return. It's surprisingly accurate for interest rates between 5% and 20%.

The Formula

For example, if your investment earns 8% per year, it will take approximately 9 years to double (72 / 8 = 9).

Doubling Times at Common Rates

Interest Rate	Years to Double
4%	18 years
6%	12 years
8%	9 years
10%	7.2 years
12%	6 years

Connection to Milestones

On our main calculator, you'll see "Milestone Badges" that highlight when you've reached 2×, 5×, or 10× your initial investment. The Rule of 72 is exactly what determines that first 2× badge! By increasing your rate of return by just 2%, you can often shave years off your doubling time.