Rule of 72 Calculator
Estimate how long it takes to double your money at a given return — or the rate needed to double in a set time.
At 8.0% a year, your money doubles in about 9.0 years, quadruples (4×) in ~18.0 years, and grows 8× in ~27.0 years.
This is an estimate for general information only and is not financial, tax or investment advice. Figures may differ from a lender or advisor.
TThe Crypto Tools69+ free crypto calculators — profit, fees, DCA and more.Our networkVisit →The Rule of 72 is a fast mental shortcut for compound growth: divide 72 by an annual rate of return and you get the approximate number of years it takes for money to double. At 8% a year, 72 ÷ 8 = 9 years to double. This calculator does it both ways — enter a rate to get the doubling time, or enter a number of years to get the rate you'd need.
It's an estimate, not an exact formula, but it's remarkably close for the rates most investors and savers deal with (roughly 4%–15%). Alongside the Rule-of-72 answer, this tool also shows the mathematically exact figure — from the compound-growth formula — so you can see how small the gap is and when it starts to matter.
How to use it
- 1Choose a mode: Time to double (from a rate) or Rate to double (from a number of years).
- 2In Time-to-double mode, enter the annual interest or return rate as a percentage.
- 3In Rate-to-double mode, enter how many years you want your money to double in.
- 4Read the Rule-of-72 estimate next to the mathematically exact value.
- 5Use the summary line to see when your money would 4× and 8× at the same rate.
Frequently asked questions
Why 72 and not another number?+
72 is chosen because it's close to the mathematically 'correct' constant (about 69.3, from 100 × ln 2) while dividing evenly by many common rates — 2, 3, 4, 6, 8, 9, 12 — which makes the mental math easy. For rates near 8% it's very accurate; for very low or very high rates, 69 or 70 (or the exact formula) is closer.
How accurate is the Rule of 72?+
It's most accurate for rates between about 4% and 15%. At 8% the rule gives 9 years and the exact figure is about 9.0 years — nearly identical. At 2% the rule says 36 years while the exact answer is about 35 years; at 20% the gap widens a little more. This calculator shows both so you can judge.
Does it work for any kind of growth?+
Yes — the Rule of 72 applies to anything that grows (or shrinks) at a compounding percentage: investment returns, savings interest, inflation eroding buying power, even population or revenue growth. For inflation, 72 ÷ inflation rate tells you how fast prices double, i.e. how fast money loses half its value.
What rate should I use for investments?+
Use the average annual return you realistically expect, after fees. Historically, broad stock-market returns have averaged roughly 7%–10% a year over long periods, but results vary widely year to year and past performance doesn't guarantee future returns. This is an estimate for planning, not financial advice.
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