Here's a party trick that will make you sound smarter than everyone in the room. Someone mentions their savings account earns 5% interest. You nod and say, "So your money doubles in about 14 years." They ask how you calculated that so fast. You tell them: divide 72 by the interest rate.
72 / 5 = 14.4 years
That's the Rule of 72, and once you learn it, you'll use it constantly.
Why does dividing by 72 work?
The actual formula for compound interest is A = P(1 + r)^t, where P is your principal, r is the annual rate, and t is time in years. To find the doubling time, you set A = 2P and solve for t:
2 = (1 + r)^t
ln(2) = t * ln(1 + r)
t = ln(2) / ln(1 + r)
For small values of r, ln(1 + r) approximates to r. And ln(2) is 0.693. So t is approximately 0.693 / r, or 69.3 / (r as a percentage).
Mathematically, 69.3 is more accurate. But 72 is divisible by 2, 3, 4, 6, 8, 9, and 12, which makes the mental math trivial. The slight overestimation also compensates for the approximation error at higher rates. It's one of those rare cases where the "wrong" number gives better practical results.
The doubling time table
Here's what the Rule of 72 gives you across common rates of return:
- 2% (savings account): 36 years
- 4% (bonds): 18 years
- 6% (balanced portfolio): 12 years
- 8% (stock market average): 9 years
- 10% (aggressive growth): 7.2 years
- 12% (very aggressive): 6 years
- 24% (credit card debt): 3 years
That last one is the gut punch. The Rule of 72 works in both directions. If you carry a $5,000 credit card balance at 24% APR and make no payments, you owe $10,000 in three years. The same math that builds wealth destroys it.
Historical context: what does the stock market actually return?
The S&P 500 has returned roughly 10.5% annually since 1957 when you include dividends. That sounds great until you account for inflation, which has averaged about 3.2% over the same period. Your real return — the number that actually matters for purchasing power — is closer to 7%.
At 7% real returns, your money doubles in purchasing power every 10.3 years.
This is the distinction between nominal and real returns, and it's where most financial projections mislead people. A retirement calculator that shows your portfolio growing to $2 million in 30 years at 10% nominal returns is technically correct but practically dishonest if it doesn't account for the fact that $2 million in 2054 won't buy what $2 million buys today.
A more honest projection uses 7% and tells you what your money will be worth in today's dollars. That's a harder sell for financial products, which is why you rarely see it.
The three variables you actually control
Compound interest has three inputs: principal, rate, and time. Most people fixate on rate — chasing higher returns through stock picks or crypto. But rate is the variable you have the least control over. The market does what it does.
Time is the most powerful variable, and it's the one you waste by waiting. Here's an example I think about a lot.
Investor A starts putting $500/month into an index fund at age 22 and stops at age 32. Total invested: $60,000 over 10 years. Then she never invests another dollar.
Investor B starts at 32 and invests $500/month until age 62. Total invested: $180,000 over 30 years.
At 8% average annual returns, Investor A ends up with roughly $1.15 million at 62. Investor B ends up with about $745,000. Investor A put in a third of the money and ended up with 54% more. That's what a 10-year head start does.
The math is almost offensive in how clearly it punishes procrastination.
Applying this to real decisions
The Rule of 72 reframes everyday financial choices. That $50,000 car you're considering at age 30 — at 8% returns, it represents $200,000 you won't have at 60. Two doublings in 30 years.
A $5/day coffee habit is $1,825/year. Over 30 years at 8%, the invested alternative grows to about $206,000. I'm not saying don't buy the coffee. I'm saying understand the actual trade-off, not the sticker price.
This framing also works for debt decisions. If you're paying 18% on a credit card while earning 8% in the market, every dollar of debt is costing you the difference. Pay the debt first. The math isn't close.
Running the actual numbers
The Rule of 72 is great for quick estimation, but when you're making real decisions about retirement contributions or comparing investment options, you want precise numbers. I built a compound interest calculator that lets you adjust principal, monthly contributions, interest rate, and time horizon — it shows you year-by-year growth with a breakdown of contributions versus earnings at zovo.one/free-tools/compound-interest-calculator.
The single most important thing I've learned about money is that the math is simple. Earning more, spending less, investing the difference, starting early. None of it is complicated. The hard part is behavior, not arithmetic. But understanding the arithmetic at least removes the excuse of ignorance.
Divide 72 by your rate. Know your doubling time. Make decisions accordingly.
I'm Michael Lip. I build free developer tools at zovo.one. 350+ tools, all private, all free.
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