Rule of 72 Calculator
Use the Rule of 72 to quickly estimate how long it takes to double your money, or what rate you need.
The Rule of 72
The Rule of 72 is a simple shortcut to estimate how long it takes for an investment to double at a given annual return. Divide 72 by the interest rate to get the approximate number of years. You can also work backwards: divide 72 by your target years to find the required rate.
The formula works in both directions: Years to Double = 72 / Rate, and Required Rate = 72 / Years.
Frequently Asked Questions
What is the Rule of 72?
The Rule of 72 is a quick mental math trick for estimating how long it takes an investment to double in value. You divide 72 by the annual rate of return to get the approximate number of years. For example, at 8% annual return, your money roughly doubles in 9 years (72/8 = 9).
How accurate is the Rule of 72?
The Rule of 72 is most accurate for rates between 6% and 10%. At very low rates (below 4%) or very high rates (above 20%), it becomes less precise. For everyday investment planning, it is accurate enough to give you a useful ballpark figure without needing a calculator.
When should I use the Rule of 72?
Use it whenever you want a quick estimate of compounding effects. It is great for comparing investment options, understanding the impact of different interest rates, or explaining compound growth in simple terms. It works for savings accounts, investments, inflation, and even debt growth.
What are the limitations?
The Rule of 72 assumes a constant rate of return, which rarely happens in practice. It does not account for taxes, fees, or additional contributions. It only tells you about doubling, not about the exact value at any other time. For precise projections, use a full compound interest calculator.
Does the Rule of 72 work for all interest rates?
It works reasonably well for rates between about 2% and 20%. Outside that range, the approximation drifts. For very high rates, the Rule of 69.3 is slightly more accurate. For most practical investment scenarios, 72 is the standard because it divides evenly by many common numbers (2, 3, 4, 6, 8, 9, 12).