Rule of 72 Calculator
Enter a growth rate and see how fast money doubles — or enter a doubling time to find the rate you would need. Shows the classic rule-of-72 estimate and the exact answer side by side.
| Rule of 72 estimate | — |
| Exact answer | — |
| After 2 doublings (4×) | — |
| After 3 doublings (8×) | — |
The rule of 72
Years to double ≈ 72 ÷ interest rate
At 8% growth, money doubles in about 72 ÷ 8 = 9 years (exact: 9.0). The rule works because ln(2) ≈ 0.693, and 72 is close to 69.3 while dividing neatly by 2, 3, 4, 6, 8, 9, and 12 — the reason bankers have used it for centuries of mental math.
Doubling times at common rates
| Rate | Rule of 72 | Exact | Typical of |
|---|---|---|---|
| 2% | 36 yrs | 35.0 | Inflation target |
| 4% | 18 yrs | 17.7 | High-yield savings |
| 7% | 10.3 yrs | 10.2 | Stocks after inflation |
| 10% | 7.2 yrs | 7.3 | Stocks nominal |
| 24% | 3 yrs | 3.2 | Credit card debt — doubling against you |
The dark side of the rule
It works on costs too. Credit card debt at 24% doubles in ~3 years if unpaid. Inflation at 3% halves buying power in ~24 years. A 2% investment fee consumes doublings silently — see the fee drag calculator for that one.
Frequently asked questions
How long does money take to double at 8%?
About 9 years. The rule of 72 gives 72 ÷ 8 = 9; the exact compound-interest answer is 9.01 years.
How accurate is the rule of 72?
Within a few percent for rates between 4% and 15% — the everyday range. At very high rates it overestimates slightly (at 24%, rule says 3 years; exact is 3.2).
Why 72 and not 69.3?
The mathematically pure constant is 100 × ln(2) ≈ 69.3, but 72 divides cleanly by many small numbers, making mental math easy — and it slightly corrects for annual rather than continuous compounding.
Does the rule work for inflation?
Yes, in reverse: divide 72 by the inflation rate to find how long prices take to double (buying power to halve). At 3% inflation, about 24 years.
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Last updated: 2026-07-08