What is the rule of 72 calculator — how long to double money?
In short
At 7% annual return, money doubles in 72÷7 = 10.3 years. At 10%, it doubles in 7.2 years. The Rule of 72 is a mental math shortcut — the exact formula uses logarithms and gives nearly identical results.
Uses the Rule of 72 to estimate how many years it takes to double your money at a given rate. Also shows how long until your money triples or grows 10×, with real dollar milestones at 10, 20, and 30 years.
How to use this calculator
- 1Enter your expected annual return rate (interest rate, investment return, or inflation rate).
- 2Enter your starting amount to see real dollar milestones.
- 3Review the doubling/tripling years and the growth milestone table.
The formula
- r
- — Annual growth rate as a decimal (e.g., 0.07 for 7%)
- ln
- — Natural logarithm
- 72
- — Rule of 72 approximation constant (works best for rates 6–10%)
Worked example
The scenario
At 8% annual return with $25,000 starting amount.
The result
Rule of 72: 72÷8=9 years to double. Exact: 9.01 years. Reaches $50,000 in 9 years, $200,000 in 27 years.
Common use cases
- Comparing investment return rates quickly
- Understanding the cost of inflation on savings
- Teaching the power of compound interest
- Estimating how fast debt grows if unpaid
Limitations & assumptions
- Most accurate for rates between 6% and 10% — less precise at very high or very low rates.
- Assumes a constant annual rate with no contributions or withdrawals.
- The actual doubling formula (ln(2)/ln(1+r)) gives exact results for any rate.
Frequently asked questions
What is the Rule of 72?
How accurate is the Rule of 72?
Can the Rule of 72 apply to debt?
What rate do I need to double my money in 10 years?
What is the Rule of 70 vs Rule of 72?
Disclaimer: KalkWise calculators are provided for general informational and educational purposes only and do not constitute financial, investment, tax, or legal advice. Results are estimates based on the figures you enter and the assumptions described above. Actual outcomes will vary. Consult a qualified professional before making financial decisions.