Rule of 72 — How Fast Will You Double Your Money?
Rule of 72 is a simple way to calculate in how many years your investment will double. Learn how it works and how to apply it in practice.
What is the Rule of 72?
Rule of 72 is a quick way to estimate in how many years an investment will double. Simply divide 72 by the annual rate of return.
Formula: 72 ÷ rate of return (%) = years to double
Quick Answer
The Rule of 72 is a mental-math shortcut that estimates how many years an investment takes to double: simply divide 72 by the annual rate of return. At 2% (a savings account) money doubles in 36 years, while at ~10% (a historical S&P 500 ETF) it doubles in about 7 years. Reversed, dividing 72 by the inflation rate shows how fast purchasing power halves. It is accurate for 5–15% rates but ignores taxes and fees — Polish investors can multiply the rate by 0.81 to approximate the 19% Belka tax (zero inside IKE).
Examples
| Rate of Return | Time to Double |
|---|---|
| 2% (savings account) | 36 years |
| 4% (bonds) | 18 years |
| 7% (global ETF) | ~10 years |
| 10% (growth stocks) | ~7 years |
| 12% | 6 years |
Practical Example
You have 50,000 PLN in a savings account (2% annually). Doubling to 100,000 PLN will take 36 years.
The same 50,000 PLN in an S&P 500 ETF (historically ~10% annually) will double in ~7 years. And then double again in another 7 years → 200,000 PLN after 14 years.
Why is the Rule of 72 So Useful?
- Mental calculator — you don't need a spreadsheet
- Comparing options — quickly see the difference between savings account and ETF
- Motivation — visualizing money doubling encourages investing
- Inflation works the other way — with 6% inflation your money loses half its value in 12 years
Rule of 72 & Inflation
You can reverse the Rule of 72: 72 ÷ inflation = years to lose half purchasing power.
With 4% inflation: 72 ÷ 4 = 18 years — in 18 years your 100,000 PLN will buy what 50,000 PLN buys today.
This is why keeping money "under the mattress" or in non-interest accounts is risky — inflation eats it away.
Limitations of Rule of 72
- Approximation — accurate for 5-15% rates, less precise for extreme values
- Assumes constant return rate — in reality rates change
- Doesn't account for taxes — real after-tax return is lower
- Doesn't account for fees — fund's TER reduces effective rate
Tax Correction
For Polish investors: real rate = gross rate × 0.81 (after 19% Belka tax). ETF with 7% gross = 5.67% net → doubling in ~13 years instead of 10. Unless you invest through IKE — then tax = 0%.
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FAQ
What exactly is the Rule of 72?
The Rule of 72 is a mental-math shortcut that estimates how many years it takes to double a sum at a given annual compound rate. Divide 72 by the rate expressed as a whole number, and the result is the doubling period in years. At 8% annual returns, money doubles in roughly nine years.
Why 72 and not some other number?
72 is chosen because it has many small divisors (2, 3, 4, 6, 8, 9, 12) which makes mental division easy. The mathematically exact constant is around 69.3 (the natural log of 2 times 100), but 72 is a close approximation for the 5%–15% range most investors care about. For very low or very high rates the rule loses precision.
Can the Rule of 72 be used for inflation?
Yes, and it is one of the most useful applications. Dividing 72 by the annual inflation rate tells you how many years it takes for purchasing power to be cut in half. At 4% inflation, money under the mattress loses half its value in roughly 18 years.
Does the Rule of 72 account for taxes and fees?
No — it uses the gross rate, so taxes, fund fees, and trading costs are ignored. For Polish investors a quick adjustment is to multiply the rate by 0.81 to approximate the 19% Belka tax on capital gains. Inside IKE or IKZE, where capital gains tax is deferred or eliminated, the unadjusted rate is closer to reality.
Is there a similar rule for tripling money?
Yes — the "Rule of 114" estimates the years needed to triple a sum, and the "Rule of 144" estimates quadrupling. They use the same logic with different constants derived from natural logarithms. They are less commonly used but follow identical assumptions about constant compound returns.
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