Rule of 72 Calculator: Quick Doubling Estimator
Our Rule of 72 Calculator approximates doubling time: Years ≈ 72 / rate. Compares to exact compound formula.
What is the Rule of 72?
Quick mental math: Divide 72 by annual rate for years to double investment (or halve value with inflation).
Accuracy by Rate
| Rate (%) | Rule 72 | Exact (Annual) | Error |
|---|---|---|---|
| 1% | 72 | 69.66 | High |
| 8% | 9 | 9.01 | Low |
| 18% | 4 | 4.19 | Medium |
Variations
- Rule of 69.3: More exact (ln(2)*100)
- Rule of 72: Best for 6-10%
- Rule of 78: Higher rates
Example: 8% Return, $10,000 Initial, Monthly Compound
- Rule 72: 9 years
- Exact: 8.69 years
- Doubled: $20,000
- 10x: ~29 years
Formula
Estimate: t = 72 / r
Exact: t = ln(2) / (n * ln(1 + r/n)) years
Applications
- Investment growth
- Inflation impact
- Debt compounding
- Link to Compound Interest
Pro Tips
- Use 72 for quick calc
- Adjust for compounding (more frequent = slightly faster)
- Inflation: Higher rate = faster erosion
- Combine with CAGR
Limitations
- Approximation only
- Assumes constant rate
- No fees/taxes
- Best 6-12%
Conclusion
Master compounding with our Rule of 72 Calculator. Explore Finance tools.