Rule of 72 Calculator
Doubling time · Exact compound formula · Rule variants · Inflation-adjusted
Time to reach each multiple, at the reference rate
| Multiple | Rule estimate | Exact time | Value |
|---|
Different rounded constants have been used for doubling-time math over the years. Here's how each compares at your reference rate.
Compare your rate against up to three alternatives to see how much faster (or slower) your money grows.
Independent verification of the Rule of 72 estimate and the exact compound-growth formula.
What Is the Rule of 72, and Why Use It?
The Rule of 72 is a mental-math shortcut that estimates how many years it takes to double an investment: divide 72 by the annual interest rate. This Rule of 72 calculator does that instantly, and also shows the exact compound-growth answer alongside it, so you can see precisely how close the shortcut gets.
In practice, the rule works because compound growth is logarithmic, and 72 happens to be a convenient stand-in for the exact constant, 100 times the natural log of 2, roughly 69.3. It's most useful as a quick sanity check when comparing investments, not as a substitute for a precise projection.
When to Use the Exact Formula Instead
At very low or very high rates, or when compounding happens more often than once a year, the Rule of 72 estimate can drift noticeably from reality. Switch to "Rate needed" mode, adjust the compounding frequency, or just read the exact figure shown next to the estimate whenever precision matters.
Tips for using the Rule of 72
Use it for quick comparisons, not final decisions
The Rule of 72 is designed for fast mental math, comparing a 6% bond to an 9% stock fund in your head. For an actual financial plan, lean on the exact figure this calculator shows next to the estimate.
Remember it's most accurate near 6–10%
The 72 constant was chosen to fit typical long-term investment returns. At a 2% savings rate or a 30% speculative return, the gap between the estimate and the exact answer grows — check the Rule Accuracy card.
Try the generalized rules for tripling or more
Doubling isn't the only milestone that matters. Switch the Target multiple to 3x or 4x to use the Rule of 114 or Rule of 144, or check the Milestones tab for a full table up to 100x.
Account for inflation to see real growth
A nominal 8% return during 5% inflation doesn't double your purchasing power nearly as fast as the headline rate suggests. Add an inflation rate in Advanced options to see the real doubling time.
Check how compounding frequency shifts the exact answer
Monthly or daily compounding reaches the same target slightly faster than annual compounding at the same stated rate. The Compounding frequency option shows exactly how much that shaves off.
Flip it around to find the rate you need
Have a deadline instead of a rate, like doubling your money in 10 years for retirement? Switch to "Rate needed" mode and enter your time horizon to see the required annual return.
Frequently asked questions
What is the Rule of 72?
A quick mental-math shortcut for estimating how many years it takes an investment to double. Divide 72 by the annual interest rate, and the result is the approximate number of years to double your money.
How accurate is the Rule of 72?
Most accurate for annual rates roughly between 6% and 10%. Outside that range the approximation drifts further from the exact logarithmic answer, so it's better to rely on the exact figure shown here.
What's the difference between Rule of 72, 70, and 69.3?
69.3, which is 100 times the natural log of 2, is the mathematically exact constant for continuous compounding. 72 is used for periodic compounding since it divides evenly by more small numbers. 70 is sometimes preferred at lower rates.
Can it be used for tripling or quadrupling money?
Yes — generalized versions exist. The Rule of 114 approximates tripling time, and the Rule of 144 approximates quadrupling time, since quadrupling is equivalent to doubling twice, giving 2 times 72.
Does the Rule of 72 account for inflation or taxes?
No, the basic rule works on a nominal rate of return only. Enter an inflation rate in Advanced options to see the real, inflation-adjusted doubling time alongside the nominal figure.
How does compounding frequency affect it?
The classic rule assumes annual compounding. Compounding more often, such as monthly or daily, shortens the exact doubling time slightly for the same stated annual rate.
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Sources & further reading
Investopedia – Rule of 72
A reference explainer on the origin, formula, and accuracy limits of the Rule of 72.
Investor.gov – Compound Interest
The U.S. SEC's plain-language resource on how compound interest works over time.
CFPB – Saving and Building Wealth
Consumer Financial Protection Bureau guidance on savings strategies and understanding growth over time.
Disclaimer
This calculator is provided for general informational and educational purposes only and does not constitute financial or investment advice. It is not affiliated with, endorsed by, or a substitute for consultation with a licensed financial advisor.
The Rule of 72 (and its variants, such as 70, 69.3, 114, and 144) are simplified approximations of compound growth. All figures, including the exact time or rate, milestone table, and inflation-adjusted results, are based solely on the numbers you enter and standard compound-growth formulas. They do not account for market volatility, fees, taxes, or variable rates of return that apply to real investments.
Past or assumed rates of return are not guarantees of future performance. FinToku does not manage investments or guarantee any outcome. You are solely responsible for decisions made using these estimates.
Free tool by FinToku · Results are indicative. Actual investment returns vary.