What is the Rule of 72?

The Rule of 72 is a mental shortcut for compound growth: divide 72 by your annual percentage return to estimate how many years it takes money to double. At 8%, money doubles in about 9 years; at 6%, about 12 years.

How accurate is the Rule of 72?

Very accurate for typical investment returns. Between 6% and 10% the estimate lands within about a tenth of a year of the true doubling time. At very low rates it slightly overestimates (36 vs a true 35 years at 2%) and at high rates it slightly underestimates, but it's close enough for planning either way.

Does the Rule of 72 work for inflation and debt?

Yes, it works on anything that compounds. At 3% inflation, prices double (and cash loses half its buying power) in about 24 years. A credit card charging 24% doubles what you owe in about 3 years if unpaid.

Why 72 and not another number?

The mathematically exact constant is about 69.3 (100 × ln 2), but 72 is used because it's almost as accurate at ordinary rates and divides cleanly by 2, 3, 4, 6, 8, 9, and 12, which makes the mental math trivial.

Compound Interest Calculator

The Rule of 72: How Fast Will Your Money Double?

You don't need a spreadsheet to know roughly what compounding will do; you need one division. The Rule of 72 turns any growth rate into a doubling time in your head, and it's accurate enough that professionals still use it. Check any answer against the real math with the free compound interest calculator.

The rule: years to double ≈ 72 ÷ annual return. At 8%, money doubles in ~9 years. At 6%, ~12 years. At 12%, ~6 years.

How to use it

Take 72, divide by the annual percentage return, and you have the approximate number of years for a sum to double with compounding. It works in reverse too: to double your money in 10 years, you need about 72 ÷ 10 = 7.2% a year.

Stack doublings to think in decades: at 8%, $25,000 becomes ~$50,000 in 9 years, ~$100,000 in 18, and ~$200,000 in 27. Three doublings is 8× your money. That's the compounding curve in three division problems.

How accurate is it, really?

Annual return	Rule of 72 says	True doubling time
2%	36 years	35.0 years
4%	18 years	17.7 years
6%	12 years	11.9 years
8%	9 years	9.0 years
10%	7.2 years	7.3 years
12%	6 years	6.1 years

In the range where real portfolios live (roughly 6% to 10%), the rule is nearly exact. The mathematically pure constant is 69.3 (from the natural log of 2), but 72 wins in practice because it divides cleanly by 2, 3, 4, 6, 8, 9, and 12.

It works on things you'd rather not double

Inflation. At 3% inflation, prices double (and cash under the mattress loses half its buying power) in about 24 years. At 6% inflation, just 12.
Debt. A credit card at 24% APR doubles an unpaid balance in about 3 years. The same math that builds wealth compounds against you on the other side of the ledger.
Fees. A 1% annual fee doesn't sound like much, but compounded over a 36-year career it quietly eats about 30% of your final balance.

Where the shortcut ends

The rule assumes a steady rate, no contributions, and no taxes. Real markets zig-zag, and regular contributions change the picture entirely (for the better). Use the rule to frame a decision in seconds, then run the real numbers, with your monthly contributions included, in the calculator.

Frequently asked questions

What return doubles money in 5 years?

About 72 ÷ 5 = 14.4% a year, well above what diversified markets have historically delivered, which is why "double your money in 5 years" pitches deserve skepticism.

Does the rule account for monthly contributions?

No, it describes a lump sum only. Contributions accelerate growth beyond what the rule predicts; use the calculator to model them.

Is there a rule for tripling?

Yes: the Rule of 114. At 8%, money triples in about 114 ÷ 8 ≈ 14 years. (And 144 for quadrupling, which is two doublings.)

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