myUSFinance

1. Enter your Annual Interest Rate (%) — This value represents your annual interest rate
2. Enter your Years to Double — This value represents your years to double
3. Click Calculate — Review your results in the output section below the form. The calculator instantly computes all values based on your inputs.
4. Adjust and Compare — Modify any input to see how changes affect the result. Try different scenarios to find the optimal approach for your situation.

All calculations are performed instantly in your browser. Your data is never sent to any server or stored anywhere — your financial information remains completely private.

Where:

• 72 — The constant used in this approximation (works best for rates between 2% and 15%)
• Annual Interest Rate — The expected annual rate of return or growth (as a whole number, not decimal)
• Years to Double — The number of years it takes for money to double

Worked Example

At 8% return: 72 ÷ 8 = 9 years to double. Starting with $10,000: $20,000 in 9 years, $40,000 in 18 years, $80,000 in 27 years, $160,000 in 36 years. Each doubling creates more absolute wealth than the previous one.

Limitations and Assumptions

The Rule of 72 is an approximation of the exact formula: Years = ln(2) / ln(1 + r). It is most accurate for rates between 6% and 10%. For rates below 6%, the Rule of 69.3 is more precise. For rates above 12%, use the Rule of 78. The rule also works in reverse for inflation: at 3% inflation, purchasing power halves every 24 years (72 ÷ 3).

Understanding the following key concepts will help you interpret your results and make better financial decisions:

• Principal — The initial amount of money involved in the calculation, whether it is a starting balance, loan amount, or investment.
• Interest Rate — The percentage charged or earned on the principal amount, typically expressed as an annual rate (APR). This rate determines how quickly your money grows or how much borrowing costs.
• Compounding — The process of earning interest on previously earned interest. More frequent compounding (daily vs. monthly vs. annually) results in higher effective returns or costs.
• Time Horizon — The length of time over which the calculation applies. Longer time horizons amplify the effects of compounding and small differences in rates.
• Present Value vs. Future Value — Present value is what money is worth today; future value is what it will be worth at a specific point in the future, accounting for growth or inflation.

These concepts form the foundation of virtually all financial calculations. Understanding how they interact helps you evaluate any financial product or decision with confidence.

Let's walk through a practical example using this calculator.

Scenario: Consider a typical situation where you need to evaluate different financial options. By entering your specific numbers into the calculator, you can compare scenarios side by side.

Example inputs: Using representative values for an average American household, the calculator produces results that highlight the impact of each variable. Small changes in one input — such as increasing a contribution amount by $100 per month or adjusting a rate by 0.5% — can lead to significantly different outcomes over time.

Key takeaway: The most valuable insight from running calculations is understanding sensitivity — which variables have the greatest impact on your results. Focus your optimization efforts on those high-impact factors first, as they provide the greatest return on effort. Run multiple scenarios with different assumptions to build a range of outcomes rather than relying on a single projection.