Rule of 72 Calculator
The rule of 72 is the fastest mental math in finance: divide 72 by your annual return and you get the years to double. This calculator shows the shortcut and the exact math.
How to use this calculator
- Enter annual return.
- Your results appear instantly below and update as you change the numbers.
How this calculator works
The rule of 72 approximates doubling time as 72 ÷ annual rate. At 8%, money doubles in about 9 years; at 6%, about 12. The exact answer uses logarithms — ln(2) ÷ ln(1 + rate) — which the calculator shows alongside the shortcut so you can see how close the rule is.
Frequently asked questions
How accurate is the rule of 72?
Within about 1% for returns between 6% and 10%, the range most investors care about. Below 4% it overstates the time slightly; above 20% it understates it. For quick comparisons it's close enough — use the exact figure for final decisions.
Does this account for taxes and inflation?
No — it's nominal, pre-tax math. After 24% federal tax on a 8% nominal return, the after-tax return is about 6.1%, which takes roughly 11.8 years to double. Subtract your effective tax and inflation rate for a real-world number.
Can I use it for debt too?
Yes — the rule works both ways. Credit card debt at 24% doubles in 3 years if unpaid; a 7% car loan doubles the interest cost in about 10 years. It's a fast way to feel the cost of high-rate borrowing.
Disclaimer: Results are estimates for general information only and do not constitute financial, tax, or legal advice. Figures reflect 2026 rules and may change. Always confirm current limits and consult a qualified professional before making decisions. Official figures: IRS.gov · 2026 limits per IRS tax inflation adjustments.