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Financial Planning

Rule of 72 Calculator

Estimate how long money takes to double with the Rule of 72, compared with the exact doubling time and the Rules of 70 and 69.3.

Your inputs

Results

Rule of 72 estimate

9 years

Exact doubling time (annual compounding)
9.006 years
Rule of 70 estimate
8.75 years
Exact with continuous compounding (rule of 69.3)
8.664 years

≈Estimate based on the stated model and assumptions

Inputs are never added to links for financial, salary or health calculators.

What this calculator does

The Rule of 72 is a mental shortcut: divide 72 by the annual percentage rate to estimate the doubling time. It also works in reverse to find the rate needed to double in a given time.

Formula and method

Doubling time ≈ 72 ÷ rate %; exact (annual compounding) = ln 2 ÷ ln(1 + r); continuous = 69.3 ÷ rate %

Worked example

Calculated by this tool with its default inputs:

Inputs

  • Solve for: Years to double from a rate
  • Annual growth rate: 8 %

Results

  • Rule of 72 estimate: 9 years
  • Exact doubling time (annual compounding): 9.006 years
  • Rule of 70 estimate: 8.75 years
  • Exact with continuous compounding (rule of 69.3): 8.664 years

How to read the results

The rule is most accurate around 8 %; the exact value is shown for comparison.

Assumptions and limitations

  • It also estimates how fast inflation halves purchasing power.

Financial results are estimates based on the inputs and assumptions shown. They are not financial advice; confirm figures with your lender, provider or a qualified adviser before making decisions.

Questions

Why 72?

72 has many divisors and gives a good approximation for typical interest rates under annual compounding.

Methodology and verification

Result type
Estimate based on the stated model and assumptions
Formula version
1.0
Automated verification
3 reference and validation test cases, checked against independently computed values. The site cannot be built if any of them fail.
Editorial review
Awaiting human editorial review. The explanation was drafted with AI assistance; the formula and examples are verified by the automated tests above.

Privacy

Everything you enter is calculated in this browser and is not sent to Numerith or anyone else. Because this calculator can involve personal financial or health details, your inputs are never added to share links or saved to history. Privacy policy