Doubling Time Calculator
From an annual return, get both the rule-of-72 estimate and the exact years to double.
Results
Fill in the inputs and click Calculate.
- Exact Years
- –
- Rule of 72
- –
Risk disclaimer
Fixed-rate compounding math; real returns vary.
How accurate is the rule of 72?
72 ÷ annual % is the classic mental shortcut; in the common 5–20% range it stays very close to exact.
Exact = ln2 ÷ ln(1 + annual), more rigorous but needs a calculator.
How to use
- Enter the annual return (e.g. 10%).
- Click Calculate to compare the estimate with the exact value.
Formula
- Exact = ln(2) ÷ ln(1 + annual %)
- Rule of 72 = 72 ÷ annual %
Example
At 10%: rule of 72 says 7.2 years; exact ≈ 7.27.
At 5%: rule of 72 says 14.4; exact ≈ 14.21.
FAQ
Why 72 and not another constant?
72 divides evenly and stays accurate at low-to-mid returns; error grows at very high rates.