Updated October 30, 2026
Money is tight in a lot of households right now. Consumer prices rose 3.7 percent over the 12 months ending in September, according to the Bureau of Labor Statistics [4]. Meanwhile, the Federal Reserve reports that the average interest rate on credit card accounts carrying a balance was 22.32 percent across 2025 [5]. When every paycheck already feels spoken for, saving for retirement can look like a problem for some other year.
That is exactly why one old piece of math deserves a second look. The Rule of 72 tells you how long it takes money to double. It works whether that money is growing for you in a retirement account or growing against you on a credit card statement. It needs no spreadsheet and no advisor, just one division problem you can do in your head.
Used honestly, the rule makes two things clear. Patient, steady saving can build real wealth on an ordinary income. And inflation and high interest debt run on the very same math, which is why ignoring them costs so much.
Quick Links
- What Is the Rule of 72?
- How Much Can $300 a Month Grow Over a Working Life?
- What Will a Million Dollars Actually Be Worth?
- What If You Are Starting Later Than 20?
- How Much Can You Contribute in 2026 and 2027?
- How Does the Rule of 72 Work Against You?
What Is the Rule of 72?
The rule is simple. Divide 72 by an annual rate of return, and the answer is roughly the number of years it takes money to double at that rate. At 6 percent, money doubles in about 12 years. At 8 percent, about 9 years. At 3 percent, about 24 years.
The shortcut is an estimate, but a good one in the range most savers care about. The precise doubling time at 6 percent works out to about 11.9 years, so the rule lands within a few weeks. It also works on anything that compounds, which is what makes it so useful. Savings compound. Debt compounds. Inflation compounds, too, only in the wrong direction for anyone holding cash.
Two cautions keep the rule honest. First, it assumes a steady rate of return, and real markets do not deliver steady returns. A 6 percent average can include years that are sharply up and years that are sharply down, so any projection built on the rule is an illustration, not a promise. Second, the rule gets less accurate as rates climb well above 10 percent, which matters a great deal when the subject is credit card interest rather than investment growth. For anyone who would rather see the math worked out on screen, MindCraft’s explainer, The Rule of 72 Explained (Most People Get This Wrong), walks through the rule visually [3].
The rule also explains why small, consistent amounts matter so much. Doubling is not something that happens once. Every dollar saved early gets to double again and again, and the doublings that happen late in a working life are the largest ones. Finding room for regular contributions usually starts with knowing where each dollar goes, and learning how to create a budget is the practical first step.
How Much Can $300 a Month Grow Over a Working Life?
Consider a worker who starts at age 20 and puts away $300 a month, earns an average 6 percent annual return compounded monthly, and pays no fees or taxes along the way. The table below shows what that steady habit can build, along with what each balance would be worth in today’s dollars if prices rise 3 percent a year.
| Age | Years Saving | Total Contributed | Approximate Balance at 6% | In Today’s Dollars (3% Inflation) |
|---|---|---|---|---|
| 30 | 10 | $36,000 | $49,200 | $36,600 |
| 40 | 20 | $72,000 | $138,600 | $76,700 |
| 50 | 30 | $108,000 | $301,400 | $124,200 |
| 60 | 40 | $144,000 | $597,400 | $183,200 |
| 65 | 45 | $162,000 | $826,800 | $218,600 |
| 70 | 50 | $180,000 | $1,136,200 | $259,200 |
By age 65, $162,000 in total contributions grows to roughly $826,800. The account crosses the $1 million mark at about age 68, after 48 years of saving. In other words, an average earner really can become a millionaire on $300 a month, but only by starting early, staying consistent, and working a few years past the traditional retirement age.
Notice where the growth happens. In the first 10 years, the account gains about $13,200 beyond what was put in. Between ages 60 and 70 alone, it gains more than $538,000, while the saver adds only $36,000 of new money. That is the Rule of 72 at work. Each doubling builds on every doubling before it, so the reward for patience arrives late and arrives large.
What Will a Million Dollars Actually Be Worth?
Here is the part many retirement projections leave out. The Rule of 72 applies to prices as well as savings. If prices rise 3 percent a year, they double in about 24 years. At the 3.7 percent pace the Bureau of Labor Statistics recorded for the year ending in September [4], prices would double in under 20 years.
That changes what a million dollars means. Assuming 3 percent annual inflation, $1 million received 45 years from now would buy roughly what $264,000 buys today. The last column of the table above makes the same point. The $826,800 balance at age 65 has the purchasing power of about $218,600 in today’s money.
None of this makes saving pointless. It makes saving more urgent. Cash left sitting idle loses ground every year, and money that earns less than the inflation rate is quietly shrinking even as the balance looks stable. The practical response is to treat inflation as a reason to raise contributions over time. When a raise or a cost of living adjustment arrives, sending part of it to savings before it gets absorbed into everyday spending keeps a retirement plan growing in real terms, not just on paper.
What If You Are Starting Later Than 20?
Most people do not start saving at 20. Student loans, rent, children, and job changes all get in the way, and many households are only now finding room in the budget. The Rule of 72 explains why a late start costs so much. Every year of delay removes a year of compounding from the end of the timeline, which is exactly where the biggest gains happen.
| Starting Age | Years Until 65 | Monthly Savings Needed to Reach $1 Million by 65 |
|---|---|---|
| 25 | 40 | $502 |
| 30 | 35 | $702 |
| 35 | 30 | $996 |
| 40 | 25 | $1,443 |
| 45 | 20 | $2,164 |
| 50 | 15 | $3,439 |
Each five year delay raises the required monthly amount by roughly 40 to 60 percent. A 40 year old would need to save $1,443 a month to reach $1 million by 65, which is out of reach for many families. But a million dollars is a milestone, not a pass or fail test. The same $300 a month started at age 40 still grows to roughly $207,900 by 65 under the same assumptions, and that is a meaningful cushion that did not exist before.
Later starters also have tools younger savers do not. Federal rules allow extra catch-up contributions after age 50, covered in detail below [1]. A worker who reaches 50 with little saved and contributes $32,500 a year for 15 years would have roughly $787,600 by 65 at a 6 percent return. That is a demanding target, but it shows that the door does not close at midlife. If an employer offers a matching contribution, putting in at least enough to collect the full match is one of the strongest returns available anywhere.
How Much Can You Contribute in 2026 and 2027?
For 2026, the Internal Revenue Service raised the employee contribution limit for 401(k), 403(b), most 457 plans, and the federal Thrift Savings Plan to $24,500, up from $23,500 in 2025 [1]. Workers 50 and older can add a catch-up contribution of $8,000, up from $7,500. Under the SECURE 2.0 Act, workers aged 60 to 63 get a higher catch-up limit of $11,250 instead [1].
“Generally can contribute up to $32,500 each year, starting in 2026.”
Internal Revenue Service, on workers 50 and older [1]
The annual limit for traditional and Roth IRAs rose to $7,500 for 2026, up from $7,000, and savers 50 and older can add a $1,100 catch-up contribution [1]. The IRA catch-up is now adjusted for inflation each year under SECURE 2.0.
The 2027 limits have not been announced yet. Benefits consulting firm Milliman projects that the 401(k) employee limit will rise to $25,500 in 2027, with the standard catch-up rising to $8,500 and the catch-up for workers aged 60 to 63 rising to $11,750 [2]. Milliman based its estimates on the September Consumer Price Index and describes them as projections rather than official figures [2]. Milliman did not project a 2027 IRA limit, so the official IRS announcement will settle that number.
“The IRS is expected to publish the final 2027 limits in the coming weeks.”
Milliman [2]
For households on tighter budgets, the most valuable item in the IRS announcement may be the Saver’s Credit, a tax credit for low and moderate income workers who contribute to a retirement account. For 2026, the credit’s income limit is $80,500 for married couples filing jointly, $60,375 for heads of household, and $40,250 for single filers and married individuals filing separately [1]. A credit that reduces the tax bill for money already being saved effectively raises the return on those first contributions, which is exactly where a modest saver needs the most help.
How Does the Rule of 72 Work Against You?
The same math that grows savings also grows debt, and it grows debt much faster. The Federal Reserve reports that commercial bank credit card accounts assessed interest carried an average rate of 22.32 percent in 2025 [5]. Divide 72 by 22.32, and the answer is about 3.2 years. A $5,000 balance left untouched at that rate would grow to roughly $10,000 in a little over three years.
This is also where the shortcut starts to slip. Because card interest compounds monthly, the precise doubling time at 22.32 percent is closer to 3.1 years, slightly faster than the rule suggests. At high rates, the Rule of 72 understates the danger rather than overstating it.
The lesson for a household with both savings goals and card balances is straightforward. Paying off a card charging 22 percent is the equivalent of earning a guaranteed 22 percent return, far above the 6 percent assumption used throughout the tables here. For most families, clearing high interest debt comes before investing beyond any employer match, and there are proven strategies for paying off debt and keeping it paid off.
Fees work against savers in a quieter way. A fund charging 1 percent a year turns a 6 percent return into roughly 5 percent, which stretches the doubling time from 12 years to about 14.4. On the $300 a month plan started at age 20, that single percentage point cuts the balance at 65 from about $826,800 to about $607,900, a difference of roughly $218,900. Low cost index funds and close attention to expense ratios protect more of every doubling for the saver instead of the fund manager.
Final Thoughts
The Rule of 72 does not promise anyone a fortune. What it offers is clarity. It shows that steady saving on an ordinary income can build serious wealth over a working life, that inflation cuts the real value of that wealth more than most projections admit, and that high interest debt runs on the same math in reverse.
The households that come out ahead are rarely the ones with the highest incomes. They are the ones that start where they are, keep contributing through good years and bad, raise their savings as their paychecks grow, and refuse to let a card balance double behind their backs. The simplest way to make that habit stick is to remove the decision entirely, and learning how to automate your savings turns good intentions into a monthly routine.
The IRS is expected to confirm the official 2027 contribution limits soon [2]. Whatever the final numbers turn out to be, the most important figure in any retirement plan is the date of the first contribution, and the best time to start is the next paycheck.
Editor’s note: An earlier version of this article, published in November 2024, included a growth table whose figures did not follow a consistent compounding method. All figures have been recalculated, and the article has been updated with 2026 contribution limits and projected 2027 limits.
Works Cited
[1] “401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500.” Internal Revenue Service, 13 Nov. 2025, www.irs.gov/newsroom/401k-limit-increases-to-24500-for-2026-ira-limit-increases-to-7500.
[2] Lantz, Nina, et al. “2027 IRS Limits: Final Estimates.” Milliman, 15 Oct. 2026, www.milliman.com/en/insight/2027-irs-limits-forecast-final-estimates.
[3] “The Rule of 72 Explained (Most People Get This Wrong).” YouTube, uploaded by MindCraft, [UPLOAD DATE], [VIDEO URL WITHOUT PROTOCOL].
[4] United States, Bureau of Labor Statistics. “Consumer Price Index: September 2026.” News release USDL-26-1574, 14 Oct. 2026, www.bls.gov/news.release/archives/cpi_10142026.htm.
[5] United States, Board of Governors of the Federal Reserve System. “G.19 Consumer Credit: August 2026.” 7 Oct. 2026, www.federalreserve.gov/releases/g19/20261007/g19.pdf.