What is the Rule of 72?

The Rule of 72 is a mental math shortcut: divide 72 by the annual interest rate to estimate years to double your money. At 6%, 72 ÷ 6 = 12 years. At 9%, 72 ÷ 9 = 8 years. It works well for rates between 6% and 10%.

Can I use the Rule of 72 for inflation?

Yes — at 3% inflation, purchasing power halves in 72 ÷ 3 = 24 years. At 6% inflation it halves in 12 years. It works for any exponential decay or growth, including debt, GDP, or population growth.

What about the Rule of 69 or Rule of 70?

Rule of 69.3 is mathematically exact for continuous compounding. Rule of 70 works well for low rates. Rule of 72 is easiest to divide mentally for rates like 6, 8, 9, 12% and is most commonly used in finance.

Does the Rule of 72 apply to debt?

Absolutely. At 18% credit card APR, your balance doubles in 72 ÷ 18 = 4 years if you make no payments. It illustrates how quickly high-interest debt compounds — a powerful motivation to pay it down aggressively.

Rule of 72 Calculator

Use the Rule of 72 to quickly estimate how long it takes to double your money at a given interest rate — or find the rate needed to double in a target number of years. Includes a reference table of common rates.

Rule of 72 Results

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The Rule of 72 Explained

Divide 72 by the annual interest rate to estimate how many years it takes to double your money. It's a useful mental math shortcut that works well for rates between 5% and 15%.

Formula: Years ≈ 72 ÷ Rate

Example: At 8% per year, 72 ÷ 8 = 9 years to double.

The Precise Formula

For exact doubling time: Years = ln(2) ÷ ln(1 + r) = 0.6931 ÷ ln(1 + r). The Rule of 72 is accurate within 1% for rates in the typical investment range.

Application to Debt

At 24% APR (some credit cards), your balance doubles in just 3 years. At 18%, it doubles in 4 years. Understanding this makes it clear why high-interest debt should be eliminated as quickly as possible.

Rule of 72 for Inflation

You can apply the same formula to inflation. At 3% inflation, prices double in about 24 years. At 7% (2022 peak), prices double in roughly 10 years. This makes it easy to visualize how inflation erodes the real value of fixed incomes, savings accounts, and any asset that doesn't grow to keep pace.

Frequently Asked Questions

Why 72 and not another number?

72 is a mathematical approximation derived from the natural logarithm of 2 (≈ 0.693). It works because ln(2) ÷ ln(1 + r) ≈ 0.693 ÷ r for small values of r, and 72 is close enough to 69.3 while being far more divisible — you can divide it evenly by 1, 2, 3, 4, 6, 8, 9, 12, and 18. For very low rates, the Rule of 69.3 is more accurate; for higher rates, some analysts use 74.

How accurate is the Rule of 72?

At rates between 5% and 15%, the Rule of 72 is accurate within about 1% of the exact answer. At lower rates (1–2%) it slightly overestimates doubling time; at very high rates (25%+) it underestimates. For most personal finance and investing purposes, the approximation is more than close enough for quick mental math.

Can the Rule of 72 be used for halving time?

Yes — the Rule of 72 also tells you how long before something halves in value. At 6% inflation, the purchasing power of a fixed sum halves in about 12 years. This is useful for understanding how quickly the real value of a pension, annuity, or fixed income stream declines over time without cost-of-living adjustments.