Rule of 72 Calculator
How many years it takes to double (and triple) money at a given return.
Result
Years to double
9.0
Years to triple
14.3
How it works
The rule of 72 is mental arithmetic for compound growth: divide 72 by the annual rate and you get roughly the years needed to double. At 6% a year, money doubles in about 12 years; at 9%, in about 8. Its value is that it needs no calculator and turns abstract percentages into a felt timescale. An 8% return sounds modest until you see it doubles capital every nine years — four doublings, or sixteen times the money, over a working career. The rule is an approximation that is most accurate between roughly 5% and 12%. Outside that band it drifts: at 1% it slightly underestimates the time, at 25% it overestimates. For inflation the same trick works with 70 instead of 72, and it applies to anything compounding — savings, debt, population, subscriber counts.
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Frequently asked questions
How accurate is the rule of 72?
Very close for rates between about 4% and 12%; outside that range it drifts a little.
Why 72?
72 has many divisors and matches the compound-interest math closely for typical rates.
Can I use it backwards, to find the rate I need?
Yes — divide 72 by the years you have. To double your money in 9 years you need about 8% a year; in 6 years, about 12%. It is the fastest reality check on any promised return.
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