How Much Will $100 a Month Be Worth in 30 Years?

Put $100 a month into an S&P 500 index fund for 30 years and you end up with roughly $217,000. You contributed $36,000 of your own money. The other $181,000 is compound interest doing the work for you. That is the single most important number to understand about personal finance, and most people never see it laid out plainly.

But the $217,000 figure is just one scenario. Where you put that $100 a month changes the outcome by hundreds of thousands of dollars. This is not a rounding error. The gap between a savings account and a stock market index fund, over 30 years, is larger than the average American's entire retirement savings.

What Different Return Rates Actually Produce

The math below assumes $100 invested at the start of each month for 360 months (30 years). Total money you put in: $36,000.

The HYSA rate in this table assumes current rates, which will not stay at 4-5% forever. Historically, HYSA rates track the federal funds rate and have spent long periods near 0.5%. The stock market figures use long-term historical averages, which do smooth over brutal years like 2000-2002 and 2008.

How Starting Age Reshapes Everything

The $100 a month amount matters less than most people think. When you start is the dominant variable. The table below shows the same $100 per month contribution, stopping contributions at age 60, assuming a 10% average annual return.

Starting at 22 versus 35 produces a $365,000 difference at age 60, even though the 22-year-old only invested $15,600 more. Those 13 extra years of compounding are worth more than a quarter million dollars. This is not a metaphor. It is arithmetic.

The Milestone View: What You Have at Each Decade

One reason people underestimate compound interest is that it looks slow at first. The early years feel like nothing is happening. Here is a year-by-year milestone breakdown for $100 a month at 10% annual return, starting at age 30:

Notice what happens in the final five years. Between year 25 and year 30, the account grows by $94,000. That is more than it grew in the entire first 20 years. The account is essentially doubling in its final stretch because the base has become so large that 10% of it is a significant sum on its own. In year 30, you earn roughly $20,000 in a single year just from interest and growth, while putting in only $1,200.

Where to Actually Put $100 a Month

A savings account, even a high-yield one, is the wrong vehicle for 30-year money. HYSA rates are excellent for emergency funds and short-term savings. They are not built for retirement wealth. The better answer for a 30-year horizon is a Roth IRA invested in a broad index fund.

A Roth IRA offers two advantages that a taxable brokerage account cannot match:

• All growth is completely tax-free. The $217,000 you accumulate is yours, not the government's.
• You can withdraw your original contributions (not earnings) at any time without penalty. This makes it a more flexible vehicle than most people realize.

The 2025 Roth IRA contribution limit is $7,000 per year (approximately $583 per month). A $100-per-month contribution uses only 17% of that limit, which means there is significant room to increase contributions as your income grows.

Income limits apply: single filers earning more than $161,000 in 2024 face phase-out restrictions. For most people just starting out, eligibility is not a concern.

The Fund Question: VTI or FXAIX

Once you have a Roth IRA open at Fidelity, Vanguard, or Schwab, the question is what to buy with your $100. The answer, for most people, is simple.

Two excellent choices:

• VTI (Vanguard Total Stock Market ETF): Expense ratio of 0.03%. Holds roughly 3,700 US stocks. Available at any broker. The broadest US market exposure you can buy.
• FXAIX (Fidelity 500 Index Fund): Expense ratio of 0.015%. Tracks the S&P 500. Available only at Fidelity but with fractional shares and no minimums.

Both are correct. The difference in long-term performance between total market and S&P 500 is negligible for most investors. VTI includes small and mid-cap companies that the S&P 500 excludes, which adds a small amount of diversification. Neither choice will meaningfully change your outcome over 30 years compared to the much larger decision of which account you use and how consistently you contribute.

At Fidelity, you can also consider FZROX, which has a 0% expense ratio. Zero fees, forever. The catch is that it is only available at Fidelity, so if you ever transfer your account, you would need to sell and rebuy in a similar fund.

What $217,000 Actually Buys You in Retirement

$217,000 is not "retire comfortably" money on its own. It is important to be honest about this. Using the 4% rule as a withdrawal guideline, a $217,000 portfolio supports roughly $8,680 per year in withdrawals, or about $723 per month. That is meaningful supplemental income but not a full retirement.

The $100 a month example is a starting point, not a destination. Here is how the 4% rule scales across different portfolio sizes:

The path from $100 a month to $461 a month does not require a dramatic event. A promotion, a raise, or simply increasing contributions by $10-20 every year will close the gap. Many financial planners suggest a simple rule: increase your investment contribution by 1% of your salary every year. At a $50,000 salary, that is $500 a year, or about $42 more per month, automatically.

The Inflation Factor

Every projection in this article uses nominal returns, not inflation-adjusted ones. Over 30 years, inflation erodes purchasing power significantly. The Federal Reserve targets 2% inflation annually. At that rate, $217,000 in 2055 dollars is worth approximately $120,000 in today's purchasing power.

This changes the calculus on what "enough" looks like but does not change the underlying logic: the stock market has historically outpaced inflation by a significant margin. At 10% nominal returns and 2-3% inflation, the real return is roughly 7-8%. That still trounces cash savings, which typically produce negative real returns when inflation is running at 3-4%.

An $100-a-month investor who wants to think in real purchasing power should use 7% as their expected return in projections rather than 10%. At 7%, the same $100 a month over 30 years produces roughly $121,000 in real (inflation-adjusted) dollars. Still a remarkable outcome from $36,000 contributed.

When $100 a Month Becomes More

The most powerful lever most investors have is not picking better funds or timing the market better. It is increasing the contribution amount over time. If you start at $100 per month at 25 and increase by $10 each year, by 35 you are contributing $220 a month, and by 55 you are contributing $420 a month. The cumulative effect at 10% return through 55: approximately $680,000.

Many employer payroll systems allow you to set contributions as a percentage of salary rather than a fixed dollar amount. A 6% contribution rate on a $40,000 salary is $200 per month. If you get a 3% raise next year, your contribution automatically increases to $206 per month. This set-and-forget approach to escalation is underused but highly effective.

The Tax-Free Advantage: One More Calculation

If your $217,000 sits in a Roth IRA, you owe no taxes on withdrawals. If it sits in a traditional 401(k) or IRA, withdrawals are taxed as ordinary income. At a 22% tax bracket, a $217,000 traditional account effectively becomes a $169,000 after-tax account.

The Roth IRA advantage is clearest for younger investors who expect their income, and therefore their tax bracket, to rise over time. Paying taxes now at a 12% or 22% rate rather than later at a potentially higher rate is the calculus. For most people starting with $100 a month, Roth wins.

FAQ

Is 10% a realistic long-term return assumption?

The S&P 500 has returned approximately 10.7% per year on average since 1957, which is the index's inception. That figure includes multiple recessions, market crashes, wars, and interest rate cycles. It does not guarantee future performance, but it is the most defensible long-run assumption available. Many financial planners use 7% for projections to build in conservatism after adjusting for inflation.

Can I start a Roth IRA with just $100?

Yes. Fidelity and Schwab both have $0 account minimums and offer fractional shares, meaning you can buy partial shares of funds like VTI or FXAIX with whatever dollar amount you have. You do not need to have $100 at once: you can set up automatic monthly transfers and the broker invests as the money arrives.

What if I miss a month or two of contributions?

Missing one or two months is not the end of the world, but it does matter more than most people expect. If you miss 12 months of $100 contributions in year 5 of a 30-year run, the compounding loss on that $1,200 at 10% over 25 remaining years is approximately $13,000. Automating contributions from your paycheck eliminates this problem without requiring willpower.

Should I invest $100 a month or pay off debt first?

The answer depends on the interest rate. If your debt carries an interest rate above 7%, paying it off first produces a guaranteed "return" that matches or beats what the market offers. If you have student loans at 4% or a mortgage at 3%, investing while making minimum debt payments is mathematically sensible. High-interest credit card debt, typically 20-29%, should almost always be eliminated before investing.