Rule of 72 Calculator Formula

Understand the math behind the rule of 72 calculator. Each variable explained with a worked example.

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Formulas Used

Years to Double

years_to_double = 72 / interest_rate

Exact Years (ln2/r)

exact_years = log(2) / log(1 + interest_rate / 100)

Variables

VariableDescriptionDefault
interest_rateAnnual Interest Rate(%)8

How It Works

The Rule of 72

Years to Double ≈ 72 / Interest Rate

This is an approximation. The exact formula is:

Years = ln(2) / ln(1 + r)

The Rule of 72 is most accurate for rates between 6% and 10%.

Worked Example

Your investment earns 8% annually.

interest_rate = 8
  1. 01Rule of 72: 72 / 8 = 9.0 years
  2. 02Exact: ln(2) / ln(1.08) = 0.6931 / 0.07696 = 9.01 years

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