The rule of 72: how fast your money (or debt) doubles

The short answer: the rule of 72 estimates how long money takes to double: divide 72 by the yearly interest rate. At 6%, savings double in about 12 years.

• Debt works the same way: a balance charging 24% could double in about 3 years
• Rate needed: 72 ÷ years; to double in 10 years, you need 7.2% a year
• Rank debts by doubling time
• Chart of 17 rates and how accurate it is below
• Next step: Plan it with the Debt Payoff Planner ($6.99).

What is the rule of 72? It's a quick way to estimate how long it takes money to double. Divide 72 by the yearly interest rate, and the answer is roughly the number of years. At 6%, 72 ÷ 6 = 12, so savings earning 6% a year double in about 12 years. The same math works in reverse on debt: a credit card balance charging 24% a year could double in about 3 years if it just sat there growing.

That second part is why I think every household budget needs this rule, not only investors. Below you'll find the formula, a chart comparing the rule's estimate to the exact answer at 17 rates, real numbers for today's savings and credit card rates, and a simple six-step way to put it to work in your own budget. All the dollar examples are illustrations, so swap in your own balances and rates.

What is the rule of 72?

The rule of 72 is a mental-math shortcut for compound growth. Compound growth means you earn (or owe) interest on the interest that's already been added, not just on the original amount. The SEC's Investor.gov explains it with a simple example: at a 9% expected return, an investment doubles about every 8 years, because 72 divided by 9 equals 8.

Why 72? It's close to the true math (more on that below), and it divides evenly by 2, 3, 4, 6, 8, 9 and 12. That makes it easy to use in your head at the kitchen table, in a bank lobby or while reading a credit card offer.

The rule only works for compound interest. If an account pays simple interest, meaning interest only on the original amount, it won't double on the rule of 72's schedule.

The rule of 72 formula

There's one formula, and you can flip it to answer two different questions.

Here's how to use it:

1. Write the rate as a plain number. Use 8 for 8%, not 0.08.
2. Divide 72 by that number. The answer is the approximate years to double.
3. Count the doublings. Divide the years you have by the doubling time. Each doubling multiplies the balance by 2.
4. Sanity-check big decisions. For anything important, run the exact numbers with our free compound interest calculator, which shows the exact time to double as you type.

Counting doublings is where the rule gets powerful. Say $10,000 earns 8% a year for 36 years. It doubles every 9 years, so it doubles four times: $20,000, $40,000, $80,000, then about $160,000. You didn't need a spreadsheet to see that leaving money alone for decades matters more than almost anything else.

Rule of 72 chart: estimate vs. exact doubling time

This chart compares the rule of 72 to the exact doubling time. The exact figures use the standard compound-interest formula (the natural log of 2 divided by the log of 1 plus the rate). I worked them out for interest added once a year and for interest added monthly, which is closer to how savings accounts and credit cards actually work.

Two things jump out. Between about 6% and 10%, the rule is almost spot-on. At very low rates, it's a few years off, but the big picture (money barely grows) stays the same. And at credit card rates, monthly compounding makes debt grow even a bit faster than the rule suggests.

How accurate is the rule of 72? Rule of 70, 69.3, 114 and 144

The rule of 72 is an estimate, not a promise. It assumes one steady rate the whole time, which real investments rarely deliver. Still, it's close enough to guide decisions. Here are the cousins you'll see online and when each one fits:

• Rule of 69.3: Most accurate when interest compounds daily or continuously, and at low rates. 69.3 ÷ 1 = 69.3 years, which matches the monthly column above almost exactly.
• Rule of 70: A rounder version of 69.3. Economists often use it for inflation and population growth, which tend to be low rates.
• Rule of 72: Best for everyday rates from about 6% to 10%, and the easiest to divide in your head.
• Rule of 114: Estimates years to triple. At 8%, 114 ÷ 8 = about 14.3 years. The exact answer is 14.3.
• Rule of 144: Estimates years to quadruple. At 8%, 144 ÷ 8 = 18 years, which is simply two doublings.

My take: use 72 for quick checks, use 70 if the rate is under about 4%, and use a calculator for anything you're signing.

The rule of 72 for savings: why your rate matters so much

Here's where the rule gets personal. The FDIC's national rates for October 2026 put the average savings account at 0.38% a year. Run the rule: 72 ÷ 0.38 = about 189 years to double. Nobody is waiting that long.

Now compare a few rates on the same $5,000 emergency fund:

Your emergency fund isn't meant to get rich. It's meant to be there on a bad day. But the rule shows why it's worth moving cash out of an account paying almost nothing. Our guide to what a high-yield savings account is walks through how to compare them, and the emergency fund guide covers how much to keep there.

For money you won't touch for decades, like retirement savings, the doubling count is what matters. Thirty years at 7% is about three doublings. The same 30 years at 4% is not quite two. That gap is why starting early matters more than picking the perfect fund.

The rule of 72 for debt: how fast a credit card balance can double

This is the part most rule of 72 explainers skip, and it's the part I'd want a friend to hear first. The rule works on anything that compounds, including what you owe.

The Federal Reserve's consumer credit report (G.19) shows the average credit card rate on accounts that paid interest was 22.36% in August 2026. Using the rule: 72 ÷ 22.36 = about 3.2 years. In plain words, if a card balance at that rate were left alone with interest piling on, it could double in a little over three years. With monthly compounding, it's closer to 3.1.

Minimum payments slow that down, but they don't stop interest from eating a big share of each payment. A few ways to use the rule on debt:

• Compare it to your savings rate. Savings at 4% doubles in about 18 years. Card debt at 22% doubles in about 3. Paying down high-rate debt is almost always the better "return."
• Rank debts by doubling time. The shortest doubling time is the most expensive debt. That's the logic behind the avalanche method in our debt snowball vs. avalanche guide.
• Check promo offers. A 0% balance transfer stops the doubling clock, but only until the promo ends. Note the date the regular APR starts. Our guide to what APR means explains the terms.

Once you see how fast debt doubles, the next question is how fast you can make it disappear. The Debt Payoff Planner ($6.99, Excel and Google Sheets) lists up to 10 debts with their balances, rates and minimums. It lays out your payoff order using snowball or avalanche and shows your debt-free date and the interest you save compared with paying only the minimums. Change the extra payment and the date updates right away. If you'd like a quick free estimate first, try the debt payoff calculator.

The rule of 72 for inflation and fees

The rule also works in reverse, for things that shrink your money.

Inflation. Divide 72 by the inflation rate to estimate how long until prices double, which means your cash buys half as much. These are examples, not forecasts:

That's why cash sitting at 0.38% loses buying power whenever inflation runs higher than that rate. It's also why long-term goals need some growth, not just a safe spot.

Fees. Investment fees compound too. By the rule, a 1% yearly fee on its own would cut a balance in half in about 72 years. That sounds harmless until you compare outcomes over a working lifetime: $10,000 earning 7% for 36 years grows to about $114,000, but at 6% (the same investment after a 1% fee) it grows to about $81,000. One percentage point cost about $33,000 in this example. Always ask what a fund or advisor charges each year.

How to use the rule of 72 in your budget: 6 steps

The rule is only useful if it changes what you do next. Here's a simple routine you can do in one sitting.

1. List every rate in your life. Write down each savings account, CD, retirement account (use a conservative expected return), credit card, car loan and personal loan, with its yearly rate. For more on this topic, see our budget and debt calculators.
2. Find each doubling time. Divide 72 by each rate. Write the answer next to it.
3. Circle anything that doubles in under 10 years against you. That's high-rate debt. It goes to the top of your plan.
4. Move idle cash. If your savings doubling time is over 100 years, compare a higher-yield account for your emergency fund.
5. Give each goal its own fund. For goals one to five years away, growth matters less than steady saving. Our guide to sinking funds explains how to set them up.
6. Recheck every six months. Rates change. Add the date to your calendar and run the numbers again.

To keep the savings side organized, the Sinking Funds Tracker ($4.99) takes each goal and its due date and shows the monthly amount needed to hit it on time, for up to 20 goals. If you want the full set, the Ultimate Budget Bundle ($19.99, regularly $46.42 if you buy the eight items separately) includes the Debt Payoff Planner, the Sinking Funds Tracker, four more budget spreadsheets and two printable planners. You can browse every option on our Spreadsheets page.

I grew up as the oldest of three, and I learned early that the person keeping things together needs simple tools, not complicated ones. The rule of 72 is one of those. It fits on a sticky note.

Common rule of 72 mistakes

• Using it for simple interest. The rule assumes interest on interest. Simple-interest loans and bonds don't double on this schedule.
• Treating returns as guaranteed. Stock returns swing year to year. The rule shows a long-run average, not what happens next year.
• Forgetting taxes and fees. Subtract fees from your rate before you divide. Taxes can slow growth in a regular, taxable account too.
• Ignoring extra deposits or payments. The rule tracks a single lump sum. Adding money each month (or paying extra on debt) changes the timeline, so use the compound interest calculator for that.
• Using it at very high or very low rates without a check. It's a few months to a few years off at the extremes, as the chart shows.

Your next step

Grab one bill or statement today, find the yearly rate and divide 72 by it. If it's a card balance, you'll see how fast it could grow, and you'll know where your next extra dollar should go. If it's savings, you'll know whether your money is working or just sitting. Slow progress is still progress, and one number on a sticky note is a fine place to start.

Put compound growth to work with our step-by-step guide on how to save for retirement.

Want the full math behind it? Read what is compound interest, with the formula, worked examples and how it works on debt.