# Rule of 72 Calculator

> Estimate doubling time and compare it with the exact compound-growth result.

Page: https://askcalculators.com/calculator/rule-of-72-calculator/
Definition: https://askcalculators.com/calculator/rule-of-72-calculator/definition.json
JSON Schema: https://askcalculators.com/calculator/rule-of-72-calculator/schema.json
Version: 1.0.0
Maintained by: AskCalculators (software project; not a credentialed reviewer)
Qualified review: pending. Automated checks do not establish professional approval.
Page status: Live
Review status: Pending

## Use
Open the calculator page and enter values. Query-string parameters can restore and bookmark a calculation.
Example: https://askcalculators.com/calculator/rule-of-72-calculator/?annualRate=8

## Inputs
- annualRate: Annual growth rate; type=percent; required; default=8; min=0.01; max=100

## Outputs
- ruleOf72Years: Rule of 72 doubling time; format=number; primary
- exactYears: Exact doubling time; format=number
- difference: Approximation difference; format=number

## Formula
Estimated doubling years = 72 ÷ annual rate

The exact result is ln(2) ÷ ln(1 + rate).

## Rules
- Empty optional inputs are undefined, not zero.
- Hidden conditional inputs are ignored.
- Do not invent missing required inputs.