Explain it like I’m 5
Imagine you have a magic piggy bank. Every year, it adds a little extra money on top of what’s inside — say 8 cents for every dollar. The extra money then starts earning its own extra money. That’s called compound interest.
The Rule of 72 answers one simple question: “How many years until my money doubles?” You just divide 72 by the growth number. If your piggy bank grows 8% a year, 72 ÷ 8 = 9 years. So $100 becomes $200 in about 9 years, $400 in about 18 years, and $800 in about 27 years.
It works backwards too. Want your money to double in 6 years? 72 ÷ 6 = 12, so you’d need it to grow about 12% a year.
Words you’ll see, in plain English
- Rule of 72
- A shortcut for how long money takes to double: divide 72 by the yearly growth rate. At 8%, money doubles in about 9 years.
- Compound interest
- Interest that earns its own interest. Like a snowball rolling downhill: the bigger it gets, the faster it grows.
- Interest
- Money someone pays you for letting them use your money — or money you pay to borrow someone else’s.
- Inflation
- Prices slowly going up over time, so the same dollar buys a little less each year. A candy bar that cost $1 might cost $1.03 next year.
More money words explained simply in our glossary.
How to use the Rule of 72 calculator
- Pick a question. “How long until it doubles?” if you know the growth rate, or “What return do I need?” if you know how long you want to wait.
- Enter the rate or the number of years. For investments, use the yearly return you expect. For a savings account or CD, use its APY.
- Optionally enter an amount to see your money doubling again and again on the chart.
The math, step by step
Years to double ≈ 72 ÷ yearly growth rate (%)
Growth rate needed ≈ 72 ÷ years
Example with $10,000 growing at 8% a year:
- Divide 72 by 8 → 9. Your money doubles about every 9 years.
- After about 9 years: $10,000 → $20,000.
- After about 18 years it doubles again → $40,000.
- After about 27 years → $80,000. Notice how each doubling adds more dollars than the last one — that’s compounding.
The exact formula is years = ln(2) ÷ ln(1 + rate). The calculator shows both, so you can see how close the shortcut is.
Rule of 72 vs. the exact answer
| Growth rate | Rule of 72 says | Exact answer |
|---|---|---|
| 1% | 72.0 years | 69.66 years |
| 2% | 36.0 years | 35.00 years |
| 3% | 24.0 years | 23.45 years |
| 4% | 18.0 years | 17.67 years |
| 5% | 14.4 years | 14.21 years |
| 6% | 12.0 years | 11.90 years |
| 7% | 10.3 years | 10.24 years |
| 8% | 9.0 years | 9.01 years |
| 9% | 8.0 years | 8.04 years |
| 10% | 7.2 years | 7.27 years |
| 12% | 6.0 years | 6.12 years |
| 15% | 4.8 years | 4.96 years |
| 20% | 3.6 years | 3.80 years |
| 25% | 2.9 years | 3.11 years |
Three surprising things the Rule of 72 shows you
- Small rate differences are huge over time. At 4% money doubles every 18 years; at 8%, every 9 years. Over 36 years, that’s 4× your money versus 16× your money.
- Fees matter. A 1% yearly fee can turn an 8% return into 7%, stretching your doubling time from 9 to about 10.3 years.
- Inflation works against you the same way. At 3% inflation, prices double — and cash loses half its buying power — in about 24 years.
Common mistakes
- Using a decimal instead of a percent. Use 8, not 0.08: 72 ÷ 8 = 9 years.
- Forgetting that returns aren’t guaranteed. Stock returns jump around from year to year; the rule works with the average.
- Ignoring taxes and fees, which lower the real growth rate and stretch the doubling time.
Frequently asked questions
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how many years it takes for money to double: divide 72 by the yearly growth rate. At 6% a year, 72 ÷ 6 = 12 years. It also works backwards: to double your money in 8 years, you need about 72 ÷ 8 = 9% a year.
Why 72?
The exact doubling time is ln(2) ÷ ln(1 + rate), which works out close to 69.3 ÷ rate for small rates. 72 is used instead because it gives better answers for everyday rates between about 6% and 10%, and because 72 divides evenly by lots of numbers (2, 3, 4, 6, 8, 9, 12), which makes mental math easy.
How accurate is the Rule of 72?
Very close for normal rates. At 8% it says 9 years; the exact answer is 9.01 years. It gets less accurate for very high rates — at 25%, it says 2.9 years but the exact answer is 3.11.
What is the difference between the Rule of 72, 70 and 69?
They are all shortcuts for the same thing. The Rule of 69.3 is most accurate for continuous compounding, the Rule of 70 is slightly better for low rates like inflation (1–3%), and the Rule of 72 is best for typical investment returns and easiest to divide.
Can I use the Rule of 72 for inflation and debt?
Yes. Inflation of 3% a year cuts the buying power of your money in half in about 72 ÷ 3 = 24 years. And credit card debt at 24% interest doubles in about 72 ÷ 24 = 3 years if you don’t pay it down.
What is a doubling time calculator?
It is another name for this calculator: you enter a growth rate and it tells you how long something takes to double. The same math works for money, prices, populations or anything else growing by a steady percentage each year.
These results are estimates for education only, based on the assumptions you enter. They are not financial, investment or tax advice. Real returns, rates and taxes vary. See our methodology anddisclaimer.