The Rule of 72: Estimate How Fast Your Money Doubles
Here's a question that trips people up: if your investments return 7% per year, how long until your money doubles?
Most people grab a calculator. Or a spreadsheet. Or give up.
But there's a shortcut that's been around since at least 1494 (Luca Pacioli mentioned it, the same guy who basically invented accounting). It takes about three seconds. No calculator needed.
Divide 72 by your interest rate. That's it.
At 7%? Your money doubles in roughly 10.3 years. At 10%? About 7.2 years. At a measly savings account rate of 2%? You're waiting 36 years.
See It in Action
Pick a rate. Watch how the doubling point shifts. The gold bars show growth up to the doubling year — green is everything after.
At 7%, your money doubles in roughly 10 years
The Math Behind It (30 Seconds, Promise)
The formula is dead simple:
Years to double = 72 ÷ annual return
That's the whole thing. Some examples:
- S&P 500 average (10%): 72 ÷ 10 = 7.2 years
- Bond fund (4%): 72 ÷ 4 = 18 years
- High-yield savings (4.5% as of March 2026): 72 ÷ 4.5 = 16 years
- Credit card debt (22%): 72 ÷ 22 = 3.3 years — your debt doubles that fast too
That last one is worth reading twice. The Rule of 72 works both ways. Your investments double, but so does unpaid debt. A $5,000 credit card balance at 22% becomes $10,000 in just over three years if you're only making minimum payments.
I remember working this out on a napkin at a Chipotle in 2019. I had around eight grand sitting in a Chase savings account earning something like 1.5%. Plugged it in: 72 divided by 1.5 is 48 years. I'd be 74. I don't think I even finished my burrito — I went home and opened a Fidelity account that night. Probably overthought the fund selection for three hours, but the point is: that napkin math is what got me to actually move.
How Accurate Is It, Really?
The Rule of 72 isn't exact. It's a shortcut. But how far off is it? Here's a side-by-side comparison:
| Annual Return | Rule of 72 Says | Actual Years | Accuracy |
|---|---|---|---|
| 1% | 72 years | 69.7 years | ≈ Rough |
| 2% | 36 years | 35 years | ≈ Rough |
| 3% | 24 years | 23.4 years | ✓ Close |
| 4% | 18 years | 17.7 years | 🎯 Spot on |
| 5% | 14.4 years | 14.2 years | 🎯 Spot on |
| 6% | 12 years | 11.9 years | 🎯 Spot on |
| 7% | 10.3 years | 10.2 years | 🎯 Spot on |
| 8% | 9 years | 9 years | 🎯 Spot on |
| 10% | 7.2 years | 7.3 years | 🎯 Spot on |
| 12% | 6 years | 6.1 years | 🎯 Spot on |
Between 4% and 10%, the Rule of 72 is almost perfect — within a few months of the actual number. That covers most investment scenarios people care about. It gets rougher at the extremes, but honestly, if you need decimal-level precision, you should be using a compound interest calculator anyway.
Three Real Scenarios
1. Maria, age 25, invests $10,000 in index funds
Average S&P 500 return: ~10% per year (before inflation). Rule of 72 says her money doubles every 7.2 years.
She didn't add a single dollar after 25. That's $10,000 turning into $160,000 — purely from compounding. And by 65? North of $400,000. One investment. Zero extra contributions. That's why people won't shut up about starting early.
2. Jake's credit card nightmare
Jake carries a $8,000 balance at 24% APR. He makes minimum payments and tells himself he'll "deal with it later."
Rule of 72: 72 ÷ 24 = 3 years to double.
In 3 years, he effectively owes $16,000 in total cost. In 6 years, $32,000. This is the same math working against him. Same rule, opposite result. If there's one takeaway from this entire article, it's this: get compound interest on your side, not working against you.
3. Inflation eating your cash
US inflation averaged 3.4% in 2023 and 2.9% in 2024, according to the BLS. At 3% inflation, the purchasing power of your cash halves every 24 years (72 ÷ 3).
That means $100,000 sitting in a checking account (0% interest) buys the equivalent of $50,000 worth of stuff in 2050. It's not losing value on paper. It's losing value in reality. This is why holding too much cash is its own kind of risk.
How to Actually Use This
- Quick gut-check on investments. Someone pitches you a "guaranteed 4% return." You now know that means doubling in 18 years. Exciting? Not really.
- Negotiate debt payoff priority. Compare your credit card rate (72 ÷ 22 = 3.3 years) vs. student loans (72 ÷ 5 = 14.4 years). Attack the one that doubles fastest.
- Explain compounding to your kids. "If you save $1,000 at age 15 and earn 8%, it doubles to $2,000 by 24, $4,000 by 33, $8,000 by 42..." Watch their eyes widen.
- Sanity-check retirement projections. Advisor says your 401k will grow to $2M by 65? Work backward with the Rule of 72 to see if the math even makes sense.
- Understand inflation's bite. Next time someone says "just keep it in savings," do the math. At 4.5% APY vs. 3% inflation, your real return is 1.5%. Doubling time? 48 years. Not great.
When the Rule of 72 Doesn't Work
Nothing works everywhere. Here's where this shortcut falls apart:
- Very low rates (under 2%). Use the Rule of 70 instead — more accurate at these levels.
- Very high rates (over 15%). The estimate gets increasingly optimistic. At 20%, Rule of 72 says 3.6 years, actual is 3.8. Not a huge deal, but it compounds (pun intended).
- Variable returns. The stock market doesn't return a steady 10% every year. It might do +25% one year and -15% the next. The Rule of 72 uses average returns, which smooths over the ugly years.
- Taxes and fees. A 10% gross return isn't 10% in your pocket. Fund fees (0.5-1%), capital gains tax (15-20%), and inflation (2-3%) chew into it. Your real after-everything return might be 5-6%, not 10%.
Want the exact number instead of an estimate?
The Rule of 72 is great for mental math. For real planning, use our calculator with your actual numbers.
Try the Compound Interest Calculator →Frequently Asked Questions
Does the Rule of 72 work for any interest rate?
It works best between 2% and 12%. Below 2%, use the Rule of 70 instead. Above 12%, the estimate gets less accurate — off by a year or more.
Can I use the Rule of 72 for inflation?
Yes. If inflation runs at 3%, prices roughly double every 24 years (72 / 3 = 24). A $5 coffee becomes $10 in about two and a half decades.
What about the Rule of 70 or Rule of 69.3?
The Rule of 70 is more accurate for low rates under 4%. The Rule of 69.3 is mathematically exact for continuous compounding. 72 is just easier to divide in your head.
Does this account for taxes and fees?
No. The Rule of 72 uses the nominal return. After taxes and fund fees, your effective rate is lower. A 10% return with 2% in taxes/fees is really 8% — doubling in 9 years instead of 7.2.
How accurate is the Rule of 72?
At 6-8% it is nearly perfect — within a few months of the real answer. At extreme rates (1% or 20%+) it can be off by a year or more. Good enough for napkin math, not for a financial plan.