Rule of 72 Calculator
The Rule of 72 estimates how long it takes money to double at a given interest rate. Enter your rate, starting amount, and projection years to see exact doubling time, number of doublings, and a growth curve chart.
How to use this tool
- Enter annual return rate, starting amount and projection years in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your years to double (rule of 72) and the full breakdown beneath it.
The Rule of 72 is a mental math shortcut: divide 72 by your annual return rate to estimate how many years it takes to double your money. At 6%, money doubles in 12 years. At 12%, in just 6.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Doubling time (years): T = 72 / rate where rate is the annual return in percent (e.g., 6 for 6%).
Exact balance at any year y: Balance = start × (1 + rate/100)y. Number of doublings in the projection period = years / T.
How it works
The Rule of 72 is a mental math shortcut: dividing 72 by the annual percentage return approximates the number of years it takes an investment to double. This calculator reports that estimate alongside the mathematically exact balance computed with annual compounding for each year of the projection period.
The Rule of 72 is most accurate for rates between roughly 5% and 12%; at very low or very high rates the approximation diverges from the exact log-based doubling time. The chart uses exact compounding, not the Rule of 72, so it will always be precise.
Worked example
- Inputs: 6% annual return, $1,000 starting amount, 12-year projection.
- Rule of 72 doubling time = 72 / 6 = 12 years.
- Exact balance after 12 years = $1,000 × (1.06)12 ≈ $2,012.
- Number of doublings in 12 years = 12 / 12 = 1.
Years to double (Rule of 72): 12 | (Exact: $1,000 doubles approximately once in the 12-year period)
Common mistakes to avoid
- Applying the rule to monthly rates instead of annual rates — dividing 72 by a 1% monthly rate gives 72 months, but 1% monthly is 12.7% annual with a correct doubling time of about 5.8 years.
- Confusing the output (years to double) with years to triple or quadruple — each additional doubling requires another full T = 72 / rate period.
- Relying on the rule at very high rates (above 25%) where the approximation breaks down; the exact formula shows a shorter doubling time than 72 / rate implies.
Key terms
- Rule of 72
- A quick mental calculation shortcut: divide 72 by the annual return percentage to estimate the years required for an investment to double in value.
- Doubling time
- The number of years it takes for an investment to grow to twice its initial value at a given constant rate of return.
- Compound growth
- Growth calculated on both the original principal and all previously accumulated gains, causing the balance to accelerate over time.
- Annual compounding
- Interest or returns are credited once per year; the Rule of 72 was derived assuming this compounding frequency.
- Number of doublings
- How many times the initial investment doubles within the projection period; computed as total years divided by the doubling time.
Frequently asked questions
- How accurate is the Rule of 72?
- It's a good approximation for rates between 6% and 10%. For exact doubling time, use ln(2) / ln(1 + r) ≈ 0.693 / r.
- Does it work for debt too?
- Yes — at 18% APR your credit card debt doubles in roughly 4 years if you make no payments. The rule is a powerful reminder of compounding in reverse.