The rule of 72 is a mental shortcut: divide 72 by your annual interest rate (as a percentage) to get the approximate number of years it takes to double your money. At 6% per year, your money doubles in roughly 12 years. At 9%, about 8 years. At 12%, about 6 years. No calculator required — a useful mental anchor for any compound growth discussion.
The rule of 72 is an approximation, not an exact formula. The exact doubling time from compound interest mathematics is ln(2) / ln(1 + r), where r is the annual rate as a decimal. At 7%, the rule of 72 says 72 / 7 ≈ 10.3 years; the exact formula gives ln(2) / ln(1.07) ≈ 10.24 years. Close enough for a napkin calculation, but the calculator above shows both so you can see the gap at any rate.
Rule of 72 estimates vs exact doubling time
The rule of 72 is most accurate at rates between 6% and 10%. At 6%, the estimate is 12.00 years and the exact answer is 11.90 years — an error of 0.10 years, or about 5 weeks. At 10%, the estimate is 7.20 years and exact is 7.27 years — an error of about 3.5 weeks. At very high rates (above 20%) or very low rates (below 2%) the rule's accuracy degrades.
The rule works because 72 is close to 69.3 — which is 100 × ln(2) ≈ 69.3 — but 72 is divisible by more whole numbers (2, 3, 4, 6, 8, 9, 12) making it easier to work with mentally. Some texts suggest using 69.3 or 70 for more accuracy at very low rates, but 72 is the version you'll see in virtually all financial contexts.
The rule of 72 in reverse: solving for the rate
The rule works in both directions. If you want to double your money in a specific number of years, divide 72 by the number of years to get the required rate. To double in 8 years, you need roughly 72 / 8 = 9% per year. To double in 12 years, roughly 6%. This makes the rule useful for evaluating investment pitches: "guaranteed to double in 5 years" implies a ~14.4% annual return, which should raise questions about risk.
It also works for any doubling-time question — not just money. Inflation that erodes purchasing power, population growth, energy consumption, any exponentially growing or shrinking quantity. If something grows at 3% per year, it doubles in about 24 years. If inflation runs at 4%, prices double in about 18 years.
Using the compound interest calculator with the rule of 72
The calculator above is pre-filled with 7% and 11 years — close to the exact doubling time at 7% (about 10.24 years). You can verify the rule directly: set your principal to any amount, set contributions to zero, and adjust the years until the ending balance is close to double the start. At 7%, you'll find the balance reaches exactly double at about 10.24 years.
When you add contributions, the "time to double" concept no longer applies cleanly — the ending balance depends on how much you add, not just the rate and time. For contribution-heavy scenarios, the future value calculator frames the question more naturally: what does a given contribution rate produce over a given horizon?
Frequently asked questions
What is the rule of 72?
The rule of 72 says that dividing 72 by an annual interest rate gives the approximate number of years it takes to double an investment. At 8%: 72 / 8 = 9 years. The exact formula is ln(2) / ln(1 + r), where r is the rate as a decimal, which gives 9.006 years at 8% — very close to the rule's estimate.
How accurate is the rule of 72?
Very accurate between about 6% and 10% annual rates. At 6%, the estimate is 12.00 years and the exact answer is 11.90 years (error: 0.10 yr). At 9%, estimate is 8.00 and exact is 8.04 (error: 0.04 yr). At extreme rates — below 1% or above 25% — the error grows and the rule becomes a rougher approximation.
What rate doubles money in 10 years?
Using the rule of 72: 72 / 10 = 7.2% per year. The exact rate is: 2^(1/10) − 1 ≈ 7.18% per year with annual compounding. With monthly compounding the required nominal rate is slightly lower (about 6.96%) since monthly compounding adds a bit of extra effective yield.
Does the rule of 72 work for inflation?
Yes — any constant exponential growth rate. At 4% inflation, prices double in roughly 72 / 4 = 18 years. At 7% inflation, prices double in about 10.3 years. The rule applies to any quantity growing or shrinking at a constant percentage rate.