Rule of 72 Calculator
Divide 72 by your return to see how long it takes to double your money, and check the shortcut against the exact answer.
72 ÷ 8 = 9.00. Exact answer: 9.01 years (off by 0.01).
| After | $10,000 becomes |
|---|---|
| 9.0 years | $20,000 |
| 18.0 years | $40,000 |
| 27.0 years | $80,000 |
| 36.0 years | $160,000 |
| 45.0 years | $320,000 |
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Get a plain-English read of the result above: what drives it, what to try changing. It sends only the numbers in this calculator and your question. Educational only, not financial advice.
Where the shortcut drifts
The exact doubling time is ln 2 ÷ ln(1 + r). 72 is chosen because it’s close to the true constant around 8% and has lots of divisors (2, 3, 4, 6, 8, 9, 12). The table shows how close it stays.
| Rate | Rule of 72 | Exact | Error |
|---|---|---|---|
| 2% | 36.00 yrs | 35.00 yrs | +1.00 |
| 3% | 24.00 yrs | 23.45 yrs | +0.55 |
| 4% | 18.00 yrs | 17.67 yrs | +0.33 |
| 5% | 14.40 yrs | 14.21 yrs | +0.19 |
| 6% | 12.00 yrs | 11.90 yrs | +0.10 |
| 7% | 10.29 yrs | 10.24 yrs | +0.04 |
| 8% | 9.00 yrs | 9.01 yrs | -0.01 |
| 9% | 8.00 yrs | 8.04 yrs | -0.04 |
| 10% | 7.20 yrs | 7.27 yrs | -0.07 |
| 12% | 6.00 yrs | 6.12 yrs | -0.12 |
| 15% | 4.80 yrs | 4.96 yrs | -0.16 |
| 20% | 3.60 yrs | 3.80 yrs | -0.20 |
Using the Rule of 72
- Investments: a portfolio earning 7% doubles roughly every 10 years, so $50,000 at 35 could be about $200,000 at 55 and $400,000 at 65, before any new deposits.
- Fees: a 1% fee cuts 7% to 6% and stretches doubling from 10.3 to 11.9 years.
- Debt: a 24% credit card balance doubles in about 3 years if unpaid.
- Inflation: at 3% the purchasing power of cash halves in about 24 years.
For a full projection with deposits use the compound interest calculator.
Questions people ask
What is the Rule of 72?
A mental shortcut: divide 72 by the annual interest rate to estimate how many years it takes money to double. At 8%, 72 ÷ 8 = 9 years. The exact answer is ln(2) ÷ ln(1.08) = 9.01 years.
How accurate is the Rule of 72?
Very accurate between roughly 6% and 10%, where the error is well under a quarter of a year. It overstates the time at low rates and understates it at high rates. The Rule of 70 is slightly better for low rates and the Rule of 69.3 is exact for continuous compounding.
Can I use it backwards?
Yes. Divide 72 by the number of years to find the rate needed to double in that time. To double in 6 years you need about 12% a year.
Does it work for inflation?
Yes, it shows how fast prices double or money loses half its value. At 3% inflation prices double in about 24 years; at 6% in about 12.