Rule of 72 Calculator
Estimate how long it takes an investment to double.
Results update as you type.
How it works
The Rule of 72 is a mental-math shortcut that estimates how many years it takes an investment to double at a steady annual rate of return. It matters because doubling time is an intuitive way to grasp the power of compounding without a spreadsheet: divide 72 by your expected annual return, and the answer is roughly the number of years your money needs to double. This Rule of 72 calculator does exactly that division and shows the result. The number 72 is used because it is close to the mathematically exact figure (about 69.3, from the natural logarithm of 2) yet divides cleanly by many common return rates like 6, 8, 9 and 12, which is why generations of investors have kept it in their heads. Savers and investors use the rule to compare opportunities at a glance — an 8% return doubles money in about nine years, while a 4% return takes roughly eighteen — and to appreciate why even a couple of extra percentage points of return dramatically shortens the wait. It is an approximation, most accurate for rates between about 6% and 10%, and it assumes a constant rate with no withdrawals, so treat it as a quick sanity check rather than a precise projection. For exact figures, use a full compound-interest calculation, but for fast intuition the Rule of 72 is hard to beat.
Years to double ≈ 72 ÷ annual rate of return
Compounding growth is exponential, so the time to double is governed by the natural logarithm of 2 divided by the growth rate. That exact constant is about 69.3, but 72 is used instead because it is close enough and divides evenly by many everyday return rates, making the arithmetic something you can do in your head. Dividing 72 by the annual percentage return gives the approximate number of years for the balance to double, which is why higher returns shrink the doubling time so sharply.
Source:Rule of 72 (compound-growth heuristic)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter annual rate of return.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Years to double updates instantly as you type — no button to press.
Frequently asked questions
How accurate is the Rule of 72?
It is a close approximation, most accurate for annual returns between about 6% and 10%. In that band it comes within a fraction of a year of the exact answer. For very low rates it slightly underestimates the time, and for very high rates it overestimates. Some people use 70 or 69.3 for continuous compounding, or 73 for higher rates. For everyday planning the standard 72 gives a reliable quick estimate; use a full compound-interest formula when you need precision.
Why is the number 72 used?
The mathematically exact constant for doubling time is the natural logarithm of 2, roughly 69.3, divided by the growth rate. But 69.3 is awkward to divide by common return figures. The number 72 is very close to it and divides evenly by 2, 3, 4, 6, 8, 9 and 12 — many of the rates investors actually encounter — so it makes the mental arithmetic clean while staying accurate enough for practical use. That balance of simplicity and precision is why 72 became the convention.
Can I use the Rule of 72 for inflation?
Yes. Applied to an inflation rate, the rule estimates how long it takes for prices to double and, equivalently, for money to lose half its purchasing power. At 3% inflation, 72 ÷ 3 = 24 years for prices to double. This makes it a handy way to see how inflation quietly erodes savings over time. The same shortcut works for any steady rate of growth or decay, not just investment returns.
Does it account for taxes and fees?
No. The Rule of 72 works on a single net rate of return, so it only reflects taxes and fees if you plug in an after-tax, after-fee rate. Using a gross return will make the doubling look faster than it really is. To be realistic, subtract expected fund fees and any tax drag from your assumed return before dividing, so the doubling time reflects what actually lands in your account rather than the headline rate.
What return should I assume?
Use a rate that matches the investment. Broad stock market indexes have historically averaged roughly 7–10% a year before inflation over long periods, bonds considerably less, and cash savings less still. Because the rule assumes a constant rate and real markets fluctuate, lean toward a conservative figure for planning. Remember the result is an estimate of average doubling time, not a guarantee that your money will double on any particular schedule.