# Rule of 72 Calculator

- **URL:** https://codeasystem.com/calculators/finance/rule-of-72-calculator/
- **Category:** finance
- **Description:** Estimate how long money takes to double at a given growth rate; with the exact answer alongside the classic shortcut.
- **Primary output:** Years to double (rule of 72): 9 years

## Inputs
- Annual growth rate (name: `rate`, type: number, example: 8)

## Outputs
- Years to double (rule of 72): 9 years
- Exact doubling time: 9 years

## Formula / methodology
```
Years ≈ 72 / rate     Exact: ln(2) / ln(1 + rate)
```

Dividing 72 by the percentage growth rate approximates the doubling time of compounded money. The exact answer uses natural logarithms; we show both so you can see where the shortcut bends.

## Assumptions & limitations
- Growth compounds annually at a constant rate.
- Most accurate between roughly 4% and 12%.
- Works equally for prices doubling under inflation.

## Example
At an 8% annual return, money doubles in about 9 years by the rule of 72 (exactly 9.0 years with logarithms). At 6% inflation, prices double in about 12.

Result for these inputs:

```
Years to double (rule of 72): 9 years
```

## About this calculator
### A mental math tool with old roots

The rule has guided back-of-envelope finance since before spreadsheets. Its charm is reversibility: 72 ÷ years also tells you the rate needed to double within a deadline.

### Beyond doubling

Chain it: money doubles in D years, so quadruples in 2D. At 8%, $10,000 becomes $40,000 in roughly 27 years without another cent contributed.

## FAQs
### Why 72 specifically?

ln(2) ≈ 0.693, so the constant is near 69.3; 72 works well because it divides cleanly by 2, 3, 4, 6, 8, 9 and 12, and it slightly corrects for annual compounding.

### Does it work for negative rates?

No; halving times need a different mental model. Use the exact logarithm form if rates go negative.

### Rule of 70 or 69.3?

Same idea with different constants; 70 is popular in economics for continuous-ish growth, 72 for clean arithmetic.

## Related calculators
- [Compound Interest Calculator](https://codeasystem.com/calculators/finance/compound-interest-calculator/)
- [CAGR Calculator](https://codeasystem.com/calculators/finance/cagr-calculator/)
- [Inflation Calculator](https://codeasystem.com/calculators/finance/inflation-calculator/)

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Last updated: 2026-08-23 · Version: 1.0.0 · [HTML version](https://codeasystem.com/calculators/finance/rule-of-72-calculator/)