Compound Interest Doubling Time Calculator
The Rule of 72 gives a quick estimate, but the actual doubling time depends on your exact interest rate and compounding frequency. This calculator uses the precise logarithmic formula to show you exactly how long your money takes to double.
About this calculator
Calculate how long it takes for an investment to double at a given interest rate using the precise logarithmic compound interest formula, compared to the Rule of 72 estimate.
Frequently Asked Questions
What is the Rule of 72?
The Rule of 72 is a quick mental-math shortcut that divides 72 by your interest rate to estimate doubling time, and it's fairly accurate for rates between about 6% and 10%.
Why does compounding frequency matter?
More frequent compounding (like daily versus annually) applies interest to your balance more often, which slightly shortens the actual time needed to double your money at the same nominal rate.
Is the exact formula always better than the Rule of 72?
Yes, the logarithmic formula is mathematically precise for any rate and compounding frequency, while the Rule of 72 is just an approximation that becomes less accurate at very high or very low rates.