Rule of 72: How Long Until Your Money Doubles?

By HSP · Published October 14, 2026 · 4 min read · Money & Finance

The rule of 72 is one of the quickest ways to understand compound growth without a spreadsheet. You take one number, your yearly rate of return or interest, and get a rough answer to a big question: how many years until this amount doubles? The same trick works in reverse for inflation, debt and population growth.

The rule of 72 formula

Divide 72 by the annual rate written as a whole number (8 for 8%, not 0.08):

Years to double ≈ 72 ÷ r

You can also flip it. If you want money to double in a set number of years, the rate you need is roughly r ≈ 72 ÷ years. To double in 6 years you need about 72 ÷ 6 = 12% a year.

The rule assumes the growth compounds, meaning each year's gain is added to the balance and earns returns itself. That is how savings accounts, reinvested investment returns and most loans work. If you want the background first, read compound interest explained.

Worked example: 10,000 at 8% a year

1. Rate = 8, so 72 ÷ 8 = 9 years.
2. Check it the long way: 10,000 × 1.08⁹ = about 19,990. That is almost exactly double.
3. Confirm in the compound interest calculator: enter 10,000, 8% a year, compounded yearly, for 9 years, with no extra deposits.

So the shortcut lands within a few cents of the precise answer at this rate, which is why it is popular.

How accurate is it? A quick table

The exact doubling time with yearly compounding is ln(2) ÷ ln(1 + r/100). Here is how the rule compares:

The rule is closest around 6% to 10%. At very low or very high rates it drifts by a few months or more, which is fine for a rough plan but not for a contract.

Rule of 72 vs rule of 70 and 69.3

You may see two cousins of the rule:

• Rule of 70: easier with rates like 2%, 5% or 7%, and slightly better at low rates. Economists often use it for GDP and population growth.
• Rule of 69.3: the most accurate when interest compounds continuously, because ln(2) is about 0.693. It is awkward to do in your head.

72 wins for everyday use because it divides evenly by 2, 3, 4, 6, 8, 9 and 12.

Using it for inflation and debt

The rule also tells you how fast things shrink or grow against you:

• Inflation: at 3.7% a year, prices double in roughly 72 ÷ 3.7 ≈ 19.5 years (the exact answer is about 19.1). Put another way, cash under the mattress loses about half its buying power in that time. Try different rates in our inflation calculator.
• Credit card debt: an unpaid balance at 24% a year can double in about 72 ÷ 24 = 3 years if interest keeps compounding and you make no payments.
• Fees: a 2% yearly fee "halves" the growth you keep over about 36 years, which is why small percentages matter for long-term savings.

When the shortcut does not fit

• Simple interest: if interest is not added back to the balance, money doubles in exactly 100 ÷ r years. See how to calculate simple interest.
• Regular deposits: monthly contributions make a balance grow faster than the rule suggests. Use the savings calculator instead, and read how much should I save each month.
• Returns that change: stock market returns vary year to year. The rule only works with an average, and past returns do not guarantee future ones.
• Monthly compounding: it shortens the doubling time slightly. Our guide on calculating compound interest monthly shows the exact formula.

Step by step: use the rule in 3 moves

1. Write down the yearly rate as a whole number (for example 6).
2. Divide 72 by it (72 ÷ 6 = 12 years to double).
3. Check the precise answer in the compound interest calculator before you make a real decision.

For long-range goals, pair the result with our retirement planning basics guide.

Last updated: 14 Oct 2026. General educational information, not financial advice. ViralScopeHub is not responsible for investment or borrowing decisions; check rates and terms with your bank or a licensed adviser.