Rule of 72 Calculator
Quickly estimate how many years it takes to double your money at a given interest rate.
Built and maintained by Jefferson Almeida · Last updated
Your numbers
Time to double your money
What the Rule of 72 does
The Rule of 72 estimates how long an investment takes to double: divide 72 by the annual rate of return.
At 8%, doubling takes roughly 72 ÷ 8 = 9 years. At 6%, twelve years. At 12%, six years.
It's a mental shortcut, not a formula - the exact answer requires logarithms - but it's accurate enough to be genuinely useful, and it's precise in the range that matters most.
How accurate is it?
| Rate | Rule of 72 | Actual | Error |
|---|---|---|---|
| 2% | 36.0 yrs | 35.0 yrs | +1.0 |
| 4% | 18.0 yrs | 17.7 yrs | +0.3 |
| 6% | 12.0 yrs | 11.9 yrs | +0.1 |
| 8% | 9.0 yrs | 9.0 yrs | 0.0 |
| 10% | 7.2 yrs | 7.3 yrs | −0.1 |
| 15% | 4.8 yrs | 5.0 yrs | −0.2 |
| 20% | 3.6 yrs | 3.8 yrs | −0.2 |
It's most accurate between 6% and 10% - conveniently, the range covering most long-term investment assumptions. Outside roughly 2–20% it drifts, and for very low rates the Rule of 69.3 is closer.
Why 72? The mathematically exact constant is ln(2) ≈ 0.693, giving a "Rule of 69.3." But 72 is divisible by 2, 3, 4, 6, 8, 9, and 12, which makes it far easier to compute mentally, and it happens to correct slightly for annual rather than continuous compounding. It's a deliberate trade of precision for usability.
Where it's actually useful
Estimating retirement growth. A 35-year-old with $100,000 invested at 7% doubles roughly every 10.3 years - about $200,000 at 45, $400,000 at 55, $800,000 at 65. Three doublings, no calculator required. For a projection that also accounts for what you keep adding each year, use the retirement calculator.
Understanding inflation. Run it on the inflation rate and it tells you how fast prices double. At 3%, prices double in 24 years - meaning today's $50 grocery run costs $100 by the time a newborn finishes college. This is the clearest way to see why cash held long-term loses purchasing power.
Evaluating debt. At 24% APR, unpaid debt doubles in three years.
Sanity-checking claims. Anyone promising to double your money in two years is claiming a 36% annual return. Applying the rule in reverse - 72 ÷ years = required rate - is a fast fraud filter.
Related rules
- Rule of 114 - tripling time
- Rule of 144 - quadrupling time
Same method, different constants.
FAQ
How accurate is the Rule of 72?
Very close for rates between about 6% and 10%, usually within a few weeks of the exact answer. At much higher or lower rates it drifts further off - for very precise numbers at unusual rates, use the "exact doubling time" figure above, which comes from the actual compound interest formula rather than the shortcut.
Does the Rule of 72 work for debt, not just investments?
Yes - the same math applies in reverse. It estimates how quickly a balance doubles at a given interest rate, which is just as relevant for high-interest debt as it is for investments. See the debt payoff calculator for a full payoff timeline instead of just a doubling estimate.
How is this different from the compound interest calculator?
This tool answers one specific question fast - "how long until this doubles?" - using a well-known shortcut. The compound interest calculator is for full projections with contributions, compounding frequency, and a custom timeframe.
Can I use it for monthly returns?
Yes, but the answer comes back in months. At 1% monthly, doubling takes about 72 months - six years.
Does it account for taxes, fees, or contributions?
No. It answers one narrow question: how long a lump sum takes to double at a constant rate. Fees reduce your effective rate - subtract them before dividing. A 7% return with a 1% expense ratio should be run as 6%.
💡 Did you know?
The Rule of 72 traces back to 1494, when Italian mathematician Luca Pacioli documented it in his book Summa de Arithmetica. He didn't claim to invent it - he described it as a shortcut merchants already used, suggesting it was common knowledge among Italian traders long before he wrote it down.
The math behind it comes from the natural logarithm of 2 (≈0.693), which is the "true" constant for continuous compounding - that would make 69.3 the mathematically precise number to use. 72 was chosen instead because it divides evenly by more numbers (2, 3, 4, 6, 8, 9, 12), trading a little precision for much easier mental math.