The Rule of 72 is one of those rare investing shortcuts that takes 5 seconds to learn and stays useful for life.
About this calculator
The Rule of 72 is one of those rare investing shortcuts that takes 5 seconds to learn and stays useful for life. Divide 72 by the annual return rate (in percent) and you get the number of years your money takes to double. At 6%, money doubles in 12 years. At 8%, in 9 years. At 12%, in 6 years. It works because, mathematically, ln(2) ≈ 0.693, and the Taylor expansion of compound interest near typical investment rates gives a clean fraction of 72 (or 69.3 for continuous compounding -- hence the alternative Rule of 69 or 70).
Use it for sanity-checking any investment claim. If a scheme promises to double your money in 4 years, the implied rate is 72/4 = 18% -- aggressive even for equity. If your debt fund yields 7%, expect doubling in roughly 10 years. Pair the rule with a target horizon: a 25-year-old saving for a retirement at 60 has 35 years -- enough for money at 8% to double 3.9 times, growing 14x. The same person at 40 has only 20 years and 2.2 doubles, growing 4.5x. The decade you start in matters more than the absolute amount you start with.
The calculator also shows the exact doubling time using the precise compound-interest formula, the Rule of 70 (more accurate for continuous compounding like rapid inflation models), and the Rule of 114 for tripling. It then projects what your starting amount would grow to in 20 years at the given rate. To go deeper into compound projections, the [[compound-interest]] calculator handles arbitrary tenures with monthly contributions; for SIP-based growth use the [[normal-sip]] or [[lumpsum]] calculator; and for inflation-adjusted real returns use the [[inflation]] calculator.
Common uses
- Mental math at the dinner table when someone quotes a return rate
- Comparing FD/PPF/equity outcomes by doubling-time alone
- Setting realistic retirement-corpus targets given your time horizon
- Teaching kids or beginners how compounding works in 30 seconds
- Sanity-checking
- marketing claims for any investment
Frequently asked questions
How accurate is the Rule of 72?
Within 1% for rates between 6% and 10%, slightly less accurate at the extremes. At 4% it overestimates doubling time by about 0.6 years; at 20% it underestimates by about 0.3 years. For everyday investment decisions the accuracy is more than enough.
Why 72 and not 70 or 69?
Mathematically, the exact formula uses ln(2) ≈ 0.693, so the
Does inflation affect the Rule of 72?
Yes -- if you care about purchasing power, use the real return (nominal return minus inflation). At 12% nominal and 6% inflation, real return is about 6%, so real doubling time is 12 years, not 6.
Can the Rule of 72 work for debt or loan growth?
Yes -- it equally tells you how fast a credit-card balance at 36% APR doubles (just 2 years!) or how fast inflation erodes cash. It is symmetric: the same compounding maths that grows wealth also grows debt.
What is the Rule of 114 for?
Tripling time. 114 / rate gives years for money to triple. So at 12%, money triples in 9.5 years and quadruples (two doubles) in 12 years. The Rule of 144 estimates quadrupling time directly.