The Rule of 72: Quickly Estimate How Long to Double Your Money
The Rule of 72 is a mental-math trick: divide 72 by the interest rate % to find how long money takes to double. We explain how to use it, why it works, and the powerful lesson about compounding.
A mental-math trick that changes how you see money
You do not need a financial calculator to answer: "At this rate, how long until my money doubles?" Just the Rule of 72 — a simple mental-math trick that reveals the power of compound interest.
What the Rule of 72 is
The formula is extremely simple:
Years to double ≈ 72 / interest rate (%)
You just divide 72 by the annual rate of return (as a percentage).
Examples:
- 8% per year → 72 / 8 = 9 years to double.
- 12% per year → 72 / 12 = 6 years.
- 6% per year → 72 / 6 = 12 years.
- 2% per year (low savings rate) → 72 / 2 = 36 years.
One division, and you have a fairly accurate estimate.
Why it works
The Rule of 72 is an approximation of the compound interest formula. It is not exact to the day, but accurate enough for typical interest rates (around 5–15%) to estimate quickly in your head. The beauty: you need no tools, just the number 72.
The powerful lesson behind the number
The Rule of 72 is more than a trick — it reveals two important truths:
1. A few percentage points make a huge difference
Look at the examples: 6% takes 12 years to double, 12% takes only 6 years — twice as fast. A small difference in rate creates a huge difference over time. This is why fees and rates matter so much.
2. Low rates struggle to beat inflation
Saving at 2% takes 36 years to double. Meanwhile, if inflation erodes real value each year, your "safe" money may actually be shrinking in purchasing power. The Rule of 72 shows why holding only cash is a quietly expensive choice.
Use it in reverse: how fast inflation erodes money
You can apply the Rule of 72 to inflation to find how long until your money''s purchasing power halves:
- 4% inflation per year → 72 / 4 = 18 years to lose half its purchasing power.
- 6% inflation per year → 72 / 6 = 12 years.
This is a sobering view: money that does nothing loses value, and the Rule of 72 shows you how fast.
Conclusion
The Rule of 72 is a mental-math trick: 72 divided by the interest rate gives the years to double your money. Simple but powerful — it shows that a few percentage points create a huge gap over time, and why leaving money idle at low rates is a steep cost. Use it to remind yourself: time and compounding are an investor''s greatest allies.
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