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 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.