Rule of 72 Calculator

Rule of 72 calculator for doubling time from an interest rate

The Rule of 72 is a shortcut for answering one specific question: “At this annual interest rate, about how long will it take to double?” If you have a savings account, a long-term investment return assumption, or even a cost that grows each year, doubling time is an easy way to sanity-check whether a rate is meaningfully high or low. This calculator is locked to that decision. You enter a single annual interest rate, and it returns an estimated number of years to double using the Rule of 72, plus an exact compound-interest doubling time so you can see how close the shortcut is.

Use the result as a quick planning signal, not as a guarantee. For example, if a portfolio is assumed to return 9% per year, the Rule of 72 estimate is 72 ÷ 9 = 8 years to double. That is a fast mental check you can do without a spreadsheet. The exact result depends on compounding assumptions (annually, monthly, daily, or continuous compounding). In real life, returns and interest rates vary year to year, fees exist, taxes exist, and contributions or withdrawals change the picture. The reason the Rule of 72 still matters is that it gives you a stable way to compare rates in your head and quickly spot claims that do not pass basic math.

This calculator shows two outputs side by side. The “Rule of 72” figure is the estimate most people mean when they search for a Rule of 72 calculator. The “Exact doubling time” figure solves the compounding equation directly for the selected compounding frequency. The practical difference is usually small for moderate rates, but it becomes noticeable at very low rates (where doubling takes decades) and very high rates (where the shortcut can drift more). If you are comparing options that are close together, rely more on the exact value. If you just need a fast mental model, the Rule of 72 estimate is the point.

Assumptions and how to use this calculator

• The interest rate you enter is an annual nominal rate expressed as a percent (for example, enter 8 for 8%).
• The Rule of 72 estimate assumes doubling time is approximately 72 divided by the annual rate and is meant for quick estimation, not precision.
• The exact doubling time assumes a constant rate and a chosen compounding frequency, with no fees, taxes, or changing rates over time.
• This calculator focuses on doubling a balance from investment growth or interest; it does not model extra deposits, withdrawals, or cash flows.
• If your rate is an average over time (for example, a long-run return assumption), treat the output as an average doubling time, not a schedule.

Common questions

Is the Rule of 72 always accurate?

No. It is a shortcut, and its accuracy depends on the size of the interest rate and what kind of compounding you assume. It is usually “close enough” for many everyday comparisons in the mid-range of interest rates, which is why it is popular. If you need precision for planning or comparing options that are close together, use the exact doubling time shown here. Also remember that real-world returns are not constant, so even an exact formula can be wrong if the input rate is not stable.

What interest rates does this rule work best for?

It works best as a quick estimate for moderate annual rates. If the rate is very low, doubling can take so long that small differences in assumptions matter, and the shortcut becomes less useful as a planning tool. If the rate is very high, the shortcut can drift because the relationship between rate and doubling time is not perfectly linear at extremes. The exact doubling time output is included specifically so you can see the gap and decide whether the shortcut is good enough for your use.

Does compounding frequency change the doubling time?

Yes, but usually by a small amount compared with the size of the rate itself. More frequent compounding (monthly or daily) slightly reduces the exact time to double compared with annual compounding for the same nominal rate, because interest gets applied more often. Continuous compounding is a theoretical limit that gives the shortest doubling time for a given nominal rate. If you are using an advertised bank rate that compounds monthly, selecting monthly makes the exact output line up better with what the institution actually does.

Why does doubling time not require a starting amount?

Doubling time depends on the rate and the compounding rule, not on the initial balance. Whether you start with 100 or 100,000, “doubling” means multiplying by 2, and the time to multiply by 2 is the same under a constant rate. In practice, your contributions, fees, and taxes can change your personal outcome, but those are not part of the Rule of 72 concept. This page is intentionally scoped to the clean doubling-time question so you get a fast answer with minimal inputs.

When should I not use the Rule of 72?

Do not use it when the rate is not annual, when the rate changes substantially over time, or when the growth is not compounding (for example, simple interest or fixed annual increases that do not compound). Also avoid using it as proof that a claim is achievable in the real world. It is a quick estimator, not a promise. If you need to plan cash flows, contributions, withdrawals, or compare investments with fees and taxes, use a dedicated compound interest or investment growth calculator that models those factors explicitly.