The fastest way to estimate how long money takes to double at a given annual return. Enter a rate — see the rough Rule of 72 alongside the exact compound-growth math for comparison.
| Rule | Purpose | Formula | Years at 7% | Note |
|---|---|---|---|---|
| Rule of 72 | Doubles (2×) | 72 ÷ 7 | 10.3 yrs | Mental-math favourite |
| Rule of 69.3 | Doubles (2×) | 69.3 ÷ 7 | 9.9 yrs | More accurate, less tidy |
| Rule of 114 | Triples (3×) | 114 ÷ 7 | 16.3 yrs | Sibling shortcut for 3× |
| Exact (2×) | Doubles (2×) | ln(2) ÷ ln(1 + r) | 10.2 yrs | Compound-growth truth |
| Exact (3×) | Triples (3×) | ln(3) ÷ ln(1 + r) | 16.2 yrs | Compound-growth truth |
| Exact (10×) | 10× growth | ln(10) ÷ ln(1 + r) | 34.0 yrs | Long-horizon reference |
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Why we recommend it: Rock-bottom expense ratios let compounding actually compound.
Earn 4–5% on your cash so the Rule of 72 is actually working for you, not against you.
Why we recommend it: At 5%, money doubles every ~14.4 years — not true at 0.01%.
Track your savings rate and free up more capital to put to work compounding.
Why we recommend it: The Rule of 72 only helps if you have dollars doubling in the first place.
The Rule of 72 is a mental-math shortcut to estimate how many years it takes money to double at a given compound annual return. Divide 72 by the annual return percentage. At 7%, money doubles in about 10.3 years. At 10%, in about 7.2 years.
The Rule of 72 is accurate within 1% for interest rates between 5% and 12%. For rates above 15% or below 3%, use the exact formula: years = ln(2) ÷ ln(1 + rate). Our calculator shows both side-by-side.
The exact formula for doubling time is ln(2) ÷ ln(1 + rate) ≈ 0.693 ÷ rate. The number 72 is used instead of 69.3 because it has many integer divisors (2, 3, 4, 6, 8, 9, 12) — making mental math easier.
You need approximately 6.64 doublings to go from $10,000 to $1,000,000. At 7% annual return, each doubling takes ~10.3 years, so roughly 68 years total. At 10%, each doubling takes ~7.2 years — total ~48 years. This is why starting early matters so much.
Yes. Use it in reverse: at 3% inflation, prices double every 72 ÷ 3 = 24 years. At 5% inflation, every 14.4 years. This is why inflation matters enormously over long retirement horizons.
This calculator is for educational purposes only and does not constitute financial, investment, or tax advice. Investment returns vary year to year; historical averages are not guarantees of future performance. Read our full disclaimer.