Reckonary / Finance / Rule of 72 guide
The Rule of 72, learned by doing
Everyone repeats that money doubles faster at higher returns. Instead of just reading it, drag one slider below and watch the doubling time move. That is when the number actually sticks.
Start with a real number
At an 8% yearly return, the Rule of 72 says your money doubles in about 72 / 8 = 9 years. That is the result below. Now let's see what moves it.
Years to double
$10,000 becomes $20,000 in about 9 years.
What the Rule of 72 really tells you
The Rule of 72 turns a hard question — how long until my money doubles? — into simple division. Take your yearly return and divide 72 by it. At 9% that is 72 / 9 = 8 years. At 6% it is 12. No compounding formula, no spreadsheet — just one number you can work out at a bus stop.
An example you can check by hand
Say you put $10,000 into a fund returning 8% a year and leave it alone. Divide 72 by 8 and you get 9, so in about 9 years you would have roughly $20,000 — and about 9 years after that, $40,000. Each doubling takes the same nine years. That steady repeat is what compounding looks like from the outside.
Doubling time at different returns
| Annual return | 72 / rate | Years to double |
|---|---|---|
| 2% | 72 / 2 | 36 years |
| 4% | 72 / 4 | 18 years |
| 6% | 72 / 6 | 12 years |
| 8% | 72 / 8 | 9 years |
| 10% | 72 / 10 | 7.2 years |
| 12% | 72 / 12 | 6 years |
What it is good for
- Comparing options at a glance: 8% doubles in 9 years, 4% in 18 — twice as slow, no math needed.
- Seeing debt the same way: at 18% APR, an unpaid balance doubles in about 4 years.
- Sanity-checking a pitch: if someone promises to double your money in 2 years, that is 72 / 2 = 36% a year. Ask how.
Where it stops working
The rule assumes one fixed rate and no deposits, withdrawals, taxes, or fees. It drifts at the extremes — above about 20% or below 3% the shortcut gets loose, and the exact compound math is worth using. And real returns do not hold steady: a 7% average can be +20% one year and -10% the next. So treat 72 as a quick estimate, not a promise.
Want the exact doubling time for your own rate?
Open the Rule of 72 calculator →Frequently asked questions
How is this different from a compound interest calculator?
The Rule of 72 is the back-of-the-envelope version — it gives you the doubling time from the rate alone, in your head. A compound interest calculator is the exact tool: it factors in deposits, time, and compounding frequency for a precise balance.
Can I use the Rule of 72 for credit card debt?
Yes, and it is sobering. At 24% APR, divide 72 by 24 and an unpaid balance roughly doubles in 3 years. The rule works on anything that compounds, including debt working against you.
What return should I plug in for the stock market?
A long-run stock average often cited is around 7% after inflation. Running a low case (5%) and a high case (10%) tells you more than a single guess, since real returns swing year to year.
Does it account for inflation, taxes, or fees?
No. It assumes one steady rate and nothing else. Treat the answer as a quick gut check, and run exact numbers for a decision that matters.
Last reviewed July 2026. This guide is for education, not financial advice.