The Rule of 72: How to Estimate How Fast Your Money Doubles

Ask a financial planner how long it takes your money to double and they will probably answer with a single number: 72. The Rule of 72 is a mental-math shortcut that estimates doubling time without a spreadsheet — and it is accurate enough to drive real planning decisions. Here is how it works, where it shines, and where it falls short.

The Rule in One Sentence

Divide 72 by your annual rate of return, and you get the approximate number of years it takes your money to double.

72 ÷ rate = years to double

At an 8% annual return: 72 ÷ 8 = 9, so an investment doubles roughly every 9 years. At 6%: 72 ÷ 6 = 12 years. You can also run it backwards — if you want to double in 10 years, you need about 72 ÷ 10 = 7.2% per year.

Doubling Time at Common Rates

Annual Rate	Years to Double
4%	18.0
5%	14.4
6%	12.0
7%	10.3
8%	9.0
10%	7.2
12%	6.0

Why 72?

The exact math uses natural logarithms: doubling time = ln(2) / ln(1 + r). The number 72 is a convenience approximation because it is divisible by many rates (2, 3, 4, 6, 8, 9, 12…), and it stays within a fraction of a year of the true answer for the 4%–15% range that most investors care about. For very high rates, 72 slightly overestimates; for low rates it slightly underestimates, but it is never off by enough to matter for planning.

The Rule of 115: Tripling Your Money

The same logic extends beyond doubling. To estimate how long until money triples, use 115. At 7%, 115 ÷ 7 ≈ 16.4 years to triple. If you are comparing long-horizon goals like retirement, the doubling and tripling estimates together give you a quick sense of what a given rate can build over decades.

Use It for Inflation and Fees Too

The Rule of 72 is not just for returns — it works for anything that compounds, including the two silent killers of wealth:

• Inflation: at 3% inflation, 72 ÷ 3 = 24 years until your money buys half of what it buys today. That is why a “safe” 3% return is really just treading water.
• Fees: a 1% annual fee on a 7% portfolio effectively drops the rate to 6% — doubling time stretches from about 10.3 years to 12. Over a 30-year career, that one point of fees costs roughly a quarter of your final balance.

Where the Rule Falls Short

• It assumes a constant rate. Real returns bounce around; the rule is an estimate, not a guarantee.
• It ignores taxes and fees unless you plug in your after-cost rate.
• It ignores volatility — two investments with the same average return can end with very different balances depending on the order of good and bad years.
• It does not account for ongoing contributions, which most real investors make.

For a quick sanity check on any growth question, the Rule of 72 is unbeatable. When you want the precise answer — including contributions, compounding frequency, and year-by-year projections — plug your numbers into our free Rule of 72 calculator at Today Calculator and see exactly how fast your money grows.