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 | Rule's error |
|---|---|---|---|
| 2% | 36 years | 35.0 years | +1.00 years |
| 4% | 18 years | 17.7 years | +0.33 years |
| 6% | 12 years | 11.9 years | +0.10 years |
| 8% | 9 years | 9.0 years | -0.01 years |
| 10% | 7.2 years | 7.3 years | -0.07 years |
| 12% | 6 years | 6.1 years | -0.12 years |
The pattern is worth knowing. The rule runs slightly slow at low rates and slightly fast at high ones, crossing into exactness at about 7.85%. Across roughly 6% to 10%, the band most long-horizon planning sits in, the error never exceeds a tenth of a year, which is far tighter than any 30-year rate assumption you would feed into it. Down at 2% the rule is a full year off, and that is the one region where reaching for the calculator is worth the trouble.
Why 72, and not 69.3
The mathematically pure constant is 69.3, not 72. It comes from the natural logarithm of 2 (0.693), and it is exact for continuous compounding: at 8% compounded continuously, money doubles in 0.693 ÷ 0.08 = 8.66 years, and 69.3 ÷ 8 returns the same 8.66. Almost every account you can actually open compounds in discrete steps instead, daily or monthly or annually, and discrete compounding is a shade slower than continuous. That makes the truly exact constant drift upward as the rate rises:
| Annual return | Constant that would be exact |
|---|---|
| 2% | 70.01 |
| 4% | 70.69 |
| 6% | 71.37 |
| 8% | 72.05 |
| 10% | 72.73 |
| 12% | 73.40 |
No single number is right everywhere, so the only real question is which range to be right in. Seventy-two lands almost exactly on 8%, near the middle of the band people plan around, and it holds up on both sides of it.
Then there is the arithmetic, which matters more than it sounds. The rule has to survive being done in your head, and 72 divides cleanly by 2, 3, 4, 6, 8, 9, 12, 18, 24, and 36. Sixty-nine divides evenly by 3 and 23, and nothing else. A slightly less accurate constant you can actually divide beats a slightly more accurate one that sends you hunting for a calculator, which is the whole value of a rule of thumb.
Tripling and quadrupling: the Rules of 114 and 144
The same trick works for any multiple; only the constant changes. Tripling uses 114, quadrupling uses 144. They are built the same way: the natural logs of 3 and 4 are 1.099 and 1.386, giving continuous constants of 109.9 and 138.6, then nudged upward for discrete compounding. Note that 144 is exactly twice 72, which it has to be, because quadrupling is simply two doublings.
| Annual return | 114 ÷ r (triple) | Exact tripling | 144 ÷ r (quadruple) | Exact quadrupling |
|---|---|---|---|---|
| 4% | 28.5 years | 28.0 years | 36.0 years | 35.3 years |
| 6% | 19.0 years | 18.9 years | 24.0 years | 23.8 years |
| 8% | 14.2 years | 14.3 years | 18.0 years | 18.0 years |
| 10% | 11.4 years | 11.5 years | 14.4 years | 14.5 years |
| 12% | 9.5 years | 9.7 years | 12.0 years | 12.2 years |
Both rules inherit the Rule of 72's sweet spot near 8% and drift the same direction outside it. Running the compounding itself confirms them: $10,000 at 8% reaches $29,371.94 after 14 years, which is 2.94 times the starting amount against a 114 ÷ 8 = 14.2 year estimate, and $39,960.19 after 18 years, which is 4.00 times the starting amount against a 144 ÷ 8 = 18 year estimate.
Worked planning examples
Stacking doublings. $25,000 at an illustrative 8% doubles roughly every 9 years, so the mental ladder reads $50,000 at year 9, $100,000 at year 18, $200,000 at year 27. Running the actual compounding gives $49,975.12, $99,900.49, and $199,701.54. After 27 years the rule is off by 0.15%, which is closer than any 27-year rate assumption will ever be.
Working backwards to a required rate. To double in 10 years you need about 72 ÷ 10 = 7.2% a year. The exact requirement is 7.177%, and $25,000 at 7.2% for 10 years reaches $50,105.78, just past the target. Framed this way the rule converts "can I double this in a decade?" into one concrete question: is 7.2% a year plausible for the account in front of me?
Pointed at inflation. At an illustrative 3% inflation, prices double in about 72 ÷ 3 = 24 years, with an exact figure of 23.4. A basket costing $1,000 today runs $2,032.79 after 24 years at that rate. The arithmetic that grows a balance also shrinks what the balance buys, worked through on inflation and compound interest.
Pointed at debt. A card at 24% doubles an unpaid balance in about 72 ÷ 24 = 3 years, or exactly 3.22 years with annual compounding. $5,000 left alone becomes $9,533.12 after 3 years. Cards typically compound daily rather than annually, which makes the real figure worse still; that effective-rate math is on APY vs APR.
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 the gap between a 7% gross return and a 6% net one compounds all the same. Over a 36-year career it quietly eats 28.7% of the final balance: on an illustrative $10,000 start, $114,239.42 becomes $81,472.52.
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.)
Does the rule change if my account compounds monthly?
Only slightly. The rule is calibrated for annual compounding. At 8%, annual compounding doubles money in 9.01 years, monthly in 8.69, and daily in 8.67, so the whole span from annual to daily is about a third of a year. Compounding frequency moves less than most people expect, which is the subject of daily vs monthly compounding.
Related reading: How compound interest works · Daily vs. monthly compounding · APY vs APR · Inflation and compound interest · Compound interest FAQ
Examples assume constant returns for illustration. Real markets vary and can lose money. Educational only, not investment, tax, or financial advice.
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