Compound Interest Calculator · Practical guide
How Long to Double Money at 7 Percent?
The Rule of 72 shortcut and the exact compounding answer, side by side.
At a glance
The Short Answer
At a 7% annual return, money doubles in about 10.3 years by the Rule of 72 (72 ÷ 7) — and the exact compounding answer is 10.24 years (ln 2 ÷ ln 1.07). Compounded annually, $10,000 passes $20,000 early in year 11.
Interactive tool
Compound Interest Calculator
Enter your own values, then compare the result with the examples below.
Growth Summary
Year-by-Year Breakdown
| Year | Start Balance | Contributions | Interest | End Balance |
|---|
The Rule of 72
Divide 72 by the annual rate of return to estimate the doubling time in years. The shortcut is most accurate near 8% and slightly optimistic at low rates, slightly pessimistic at high ones — but always close enough for mental math.
Doubling Times at Common Rates
| Annual Return | Rule of 72 | Exact Doubling Time |
|---|---|---|
| 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 |
| 10% | 7.2 years | 7.27 years |
Doublings Compound Too
Each doubling restarts the clock on a larger balance. At 7%, one doubling takes ~10.3 years, so over about 41 years your money doubles four times — turning $10,000 into roughly $160,000 with no additional contributions. That is why the starting date of an investment matters more than almost any other decision.
See Your Own Doubling Point
Enter your principal and rate in the calculator above and scan the year-by-year table: the year your end balance passes twice your total contributions is your effective doubling point, contributions included.
Frequently Asked Questions
Why 72 and not 70 or 100?
72 approximates 100 × ln 2 ≈ 69.3, adjusted upward for accuracy at typical rates and because 72 divides cleanly by 2, 3, 4, 6, 8, 9, and 12 — handy for mental math.
Does the Rule of 72 work for tripling?
Use 114 instead of 72 for tripling (from ln 3 ≈ 1.0986). At 7%, tripling takes about 114 ÷ 7 ≈ 16.3 years.
Does inflation halve my purchasing power the same way?
Yes, in reverse: divide 72 by the inflation rate. At 3% inflation, a dollar's purchasing power halves in about 24 years.
Is the doubling time the same regardless of the amount?
Yes. Doubling time depends only on the rate, not the principal — $1,000 and $1,000,000 both double in 10.24 years at 7%.