Rule of 72 Calculator
Instantly estimate how long it takes an investment to double — or what rate you need to double in a target time — using the Rule of 72.
Did this tool work for you?
How to use this calculator
Divide 72 by the annual interest rate to get approximate years to double. Divide 72 by years to find the required annual rate.
- 1
Choose whether you want to find the years to double or the rate needed.
- 2
Enter the annual interest rate (%) if finding years, or the target years if finding the rate.
- 3
The calculator shows the Rule of 72 approximation alongside the mathematically exact answer.
- 4
Compare the two results — they are usually within 1–2% of each other for rates between 3–15%.
Frequently asked questions
Why is 72 used instead of the mathematically exact 69.3?
72 is divisible by more integers (2, 3, 4, 6, 8, 9, 12) making mental math easier. The approximation error is small for rates between 3% and 15%, which covers most real-world scenarios.
Does the Rule of 72 work for inflation and debt too?
Yes. At 4% inflation, your purchasing power halves in roughly 72/4 = 18 years. For debt at 18% APR (a typical credit card), your balance doubles in just 72/18 = 4 years if you make no payments.
How accurate is the rule for very high or very low rates?
At rates below 3% or above 25%, the approximation drifts. At 1% the rule gives 72 years but the exact answer is 69.7 years. At 30% the rule gives 2.4 years but the exact answer is 2.64 years. Use the exact formula shown for precision at extreme rates.
Can I use it for non-annual compounding?
Yes — use the effective annual rate (EAR). If an account compounds monthly at 6% nominal, the EAR is about 6.17%. Use 6.17 in the Rule of 72 for a more accurate estimate.
The Rule of 72 explained
A mental-math shortcut for investors
The Rule of 72 is a quick way to estimate how long compound interest takes to double money. Divide 72 by your annual return percentage. At 6% annually your money doubles in about 12 years; at 9% in about 8 years; at 12% in about 6 years. It works equally well for GDP growth, population growth, and debt accumulation.
Rule of 72 vs Rule of 69 vs exact formula
The mathematically exact "doubling rule" uses ln(2)/ln(1+r) ≈ 0.693/r, which gives the Rule of 69.3. The Rule of 72 intentionally trades a tiny bit of accuracy for far easier arithmetic. For continuous compounding (as in some bond instruments) use 69.3. For annual or monthly compounding, 72 is the standard.
Inflation: the silent doubler working against you
The same rule quantifies the threat of inflation. At 3% inflation your real purchasing power halves in 24 years. At 7% (high-inflation periods) it halves in just over 10 years. This underscores the importance of investing in assets that grow faster than inflation rather than leaving cash in a zero-rate account.
Learn more from an authoritative source:
InvestopediaCompound Interest Calculator
Calculate how your investment or savings grows over time with the power of compounding.
Simple Interest Calculator
Calculate simple interest, total amount, and interest earned using principal, rate, and time.
ROI Calculator
Calculate return on investment, net profit, and annualised ROI for any investment.
CAGR Calculator
Calculate Compound Annual Growth Rate (CAGR) for investments, revenue, or any metric over time.
Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →