Rule of 72

One Formula. Four Ways to See It.

Divide 72 by your interest rate. That's it. That's the Rule of 72 — a mental shortcut that's been used by investors and economists for centuries to instantly estimate how quickly money grows, debt compounds, or inflation erodes.

But the real power comes from applying it in all directions. Your 7% portfolio doubles every decade. Your credit card at 20% APR doubles your balance in under four years if you ignore it. Inflation at 3% quietly cuts your purchasing power in half every 24 years. And your real return — what you actually earn after inflation — tells a more honest story than any nominal number. This tool lets you run all four scenarios, flip the calculation around, and connect it to your specific retirement timeline.

Rule of 72

A mental math shortcut for doubling time - works for investments, debt, inflation, and real returns

Investment growth
Rule of 72Divide 72 by an annual rate to get the approximate doubling time. Divide 72 by years to get the rate needed to double in that time. Accurate within 1-2% for rates between 2% and 20%.
Modes: Investment / Debt / Inflation / Real ReturnThe same math applies to any compounding rate. Debt at 20% APR doubles in 3.6 years. Inflation at 3% halves purchasing power in 24 years. Real return subtracts inflation from nominal return.
What are you calculating?
Solve for
Annual return rate 7.0%
Starting amount $10,000
Retirement context
Doublings to retirementDivide your years to retirement by the doubling time to see how many times your money can double. Each doubling is a 2x multiplier - 3 doublings turns $10,000 into $80,000.
Starting amountAffects the absolute dollar values shown but not the doubling time. The compounding math is the same regardless of starting balance.
Your current age 35
Retirement age 65

Rule of 72 Calculator - Results

10.3
years to double
72 / 7% = 10.3 years
Starting amount
$10,000
Doubled to
$20,000
2x in
10.3 yrs
4x in
20.6 yrs
8x in
30.9 yrs
16x in
41.1 yrs
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Compound growth
Doubling milestones
Disclaimer: The Rule of 72 is an approximation for educational purposes only. Actual doubling times depend on compounding frequency and other factors. This calculator does not constitute financial advice. Consult a qualified financial advisor before making investment decisions. Past returns do not guarantee future results.