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How long until it doubles?
Your rate — the live lookup next to the chart.
REFERENCE CHART · MONEY
Rule of 72 Chart
Divide 72 by your interest rate and you get the years it takes money to double. It’s the most useful piece of mental math in finance — and it’s accurate within a fraction of a year for the rates real people actually earn. This chart pairs the rule with the exact doubling time for rates 1% to 20%, plus a live lookup below. At 8%, your money doubles in 9.00 years by the rule, 9.01 exactly.
The chart: years to double, by rate
The exact doubling time is ln(2) ÷ ln(1 + r) — the rule of 72 is the shortcut. Print it, pin it, and never wonder again.
| Rate | Rule of 72 (yrs) | Exact (yrs) | Rule error |
|---|---|---|---|
| 1% | 72.00 | 69.66 | +3.36% |
| 2% | 36.00 | 35.00 | +2.85% |
| 3% | 24.00 | 23.45 | +2.35% |
| 4% | 18.00 | 17.67 | +1.85% |
| 5% | 14.40 | 14.21 | +1.36% |
| 6% | 12.00 | 11.90 | +0.88% |
| 7% | 10.29 | 10.24 | +0.40% |
| 8% | 9.00 | 9.01 | -0.07% |
| 9% | 8.00 | 8.04 | -0.54% |
| 10% | 7.20 | 7.27 | -1.00% |
| 11% | 6.55 | 6.64 | -1.45% |
| 12% | 6.00 | 6.12 | -1.90% |
| 13% | 5.54 | 5.67 | -2.34% |
| 14% | 5.14 | 5.29 | -2.78% |
| 15% | 4.80 | 4.96 | -3.22% |
| 16% | 4.50 | 4.67 | -3.64% |
| 17% | 4.24 | 4.41 | -4.07% |
| 18% | 4.00 | 4.19 | -4.49% |
| 19% | 3.79 | 3.98 | -4.90% |
| 20% | 3.60 | 3.80 | -5.31% |
The rule is closest where it matters: between 6% and 12%, the error stays under half a percent. It drifts at the extremes — at 1% it’s off by +4%, at 20% by −5.6% — because 72 was chosen for divisibility (by 2, 3, 4, 6, 8, 9, 12), not perfection. For continuously compounded money, 69.3 is the exact constant; the Rule of 69 is the purist’s version, but nobody can divide 69 by 8 in their head.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Rule-of-72 doubling time | 72 ÷ rate | The shortcut: 9.00 years at 8%. |
| Exact doubling time | ln(2) ÷ ln(1 + r) | The truth: 9.01 years at 8%. |
| Rule error | (rule − exact) ÷ exact | How far the shortcut drifts: −0.07% at 8% — two days. |
| Tripling bonus | 114 ÷ rate | The Rule of 114 does the same job for tripling: 8% triples money in ~14.3 years. |
Who uses this
- Retirement savers: 7% returns double your money every ~10.3 years — three doublings in a 30-year career turns $1 into $8.
- Debt fighters: 24% credit-card interest doubles what you owe every 3 years — the rule makes the urgency visceral.
- Negotiators & shoppers: sanity-checking any “guaranteed return” pitch in five seconds flat.
- Students & teachers: the cleanest intuition for exponential growth there is — cite it with the block below.
Accuracy & limitations
The rule assumes a constant rate with annual compounding. Real investments bounce around; the doubling time of a volatile 8% portfolio is longer than 9 years because losses hurt more than gains help (volatility drag). The rule also works in reverse for debt, inflation, and anything decaying: at 3% inflation, your money’s purchasing power halves every 24 years. For rates above ~20% or sub-1% rates, use the exact formula — the shortcut’s charm fades there.
Cite this calculatorBright Side Kit. “Rule of 72 Chart.” brightoolbox.com, 2026.
Bright Side Kit. "Rule of 72 Chart." https://brightoolbox.com/rule-of-72-chart/ Accessed October 22, 2026.Frequently asked questions
How long does it take money to double at 7 percent?
About 10.3 years — 72 ÷ 7 = 10.29 by the rule, 10.24 exactly. That’s why 7% is the magic number of retirement planning: a 30-year career holds almost three doublings, turning each dollar into roughly eight.
How long does it take money to double at 10 percent?
7.2 years by the rule (72 ÷ 10), 7.27 exactly. The error is under 1% — this is the rule’s sweet spot.
What is the rule of 114?
The tripling version of the rule of 72: divide 114 by your rate to get the years until money triples. At 8%, that’s about 14.3 years. (The exact constant for continuous compounding is 109.9; 114 wins for the same divisibility reasons 72 does.)
Does the rule of 72 work for debt?
Yes — and it’s sobering. At 24% credit-card interest, an unpaid balance doubles every 3 years (72 ÷ 24). At 20%, every 3.6 years. The rule is rate-agnostic: it measures any compounding, good or bad.
Why 72 and not 70 or 100?
72 has more divisors (2, 3, 4, 6, 8, 9, 12) than its neighbors, so the mental math comes out clean for the most common rates. 70 would be slightly more accurate for small rates, 69.3 is exact for continuous compounding — but 72 divides by 8 and 12 without a calculator, which is the whole point.