Whatever you can leave alone for decades, starting now. The math is simple enough to show in one table. At an assumed 7% annual return, $250 a month for 30 years is about $305,000, of which you put in $90,000. The other $215,000 is growth. No specific amount is right for everyone, but the table tells you what each one buys.
Most answers to this question are a percentage of income and a shrug. Here are the actual numbers instead, with the assumption stated plainly: a steady 7% a year, compounded monthly, no inflation adjustment, no taxes. Real markets do not deliver steady anything, so treat these as a map, not a promise.
What each monthly amount becomes
| You invest | After 10 years | After 20 years | After 30 years |
|---|---|---|---|
| $100/month | $17,300 | $52,100 | $122,000 |
| $250/month | $43,300 | $130,200 | $305,000 |
| $500/month | $86,500 | $260,500 | $610,000 |
| $1000/month | $173,100 | $520,900 | $1,220,000 |
These are hypothetical figures at an assumed 7% return. Past performance does not guarantee future results. To run your own amount, rate, and timeline, use the ETF investment calculator.
Years beat dollars
Look across a row and then down a column. Doubling the monthly amount doubles the result. Tripling the years multiplies it about sevenfold. $250 a month for 30 years ($305,000) ends up ahead of $500 a month for 20 years ($260,500), even though the second person contributed $30,000 more of their own money.
That is compounding, and it is the reason "how much" is the second question. The first is "how soon." Someone who waits five years to be able to invest $500 instead of $200 is usually worse off than someone who started with $200.
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How people actually pick a number
- Capture any employer match first. A 401(k) match is an immediate return that no ETF can compete with.
- A commonly cited target is 15% of gross income toward retirement, including any match. Plenty of people start at 5% and raise it one point a year.
- Keep short-term money out. Cash needed within a few years belongs somewhere steadier. See ETFs for short-term goals.
- Start below what feels comfortable, not above. An amount you never have to pause beats a bigger one you stop after a bad month.
There is no minimum worth waiting for. Most brokers sell fractional shares, so $25 buys a slice of a fund whose shares cost hundreds. See the minimum to invest in an ETF.
Making it automatic
The plan that works is the one that does not depend on remembering. Set a recurring transfer from your bank for the day after payday, and a recurring purchase of a broad fund such as VTI or the funds in a 3-fund portfolio. Buying the same dollar amount every month means you automatically buy more shares when prices are low and fewer when they are high, with no decision required. The step-by-step is in how to buy ETFs.
People spend months researching which ETF and about four minutes on how much and how long, which is backwards. Over 30 years the difference between two cheap broad index funds is a rounding error. The difference between $200 and $400 a month is about $244,000. The difference between starting now and starting in five years is bigger than both. Pick a boring fund, automate an amount that survives a bad month, and raise it when your income rises.
How much to invest in ETFs per month
- At an assumed 7% return, $250 a month becomes about $305,000 in 30 years; $500 becomes about $610,000. Hypothetical, not a forecast.
- Time outweighs amount: $250 for 30 years ($305,000) beats $500 for 20 years ($260,500).
- An employer 401(k) match comes first. A commonly cited target is 15% of gross income toward retirement.
- Fractional shares mean there is no meaningful minimum to start.
- Automate the transfer and the purchase so the plan does not depend on willpower.
Common questions
There is no single correct amount. A commonly cited guideline is to put about 15% of gross income toward retirement, including any employer match, and many people start lower and increase over time. As an illustration, $250 a month at an assumed 7% annual return grows to roughly $305,000 over 30 years. That figure is hypothetical; past performance does not guarantee future results. This is educational information, not advice.
Yes, it is enough to start. Most brokers offer fractional shares, so $100 can be invested in full each month. At an assumed 7% annual return, $100 a month grows to roughly $122,000 over 30 years, of which $36,000 is your own contributions. The result is hypothetical and actual returns will vary.
At an assumed steady 7% annual return compounded monthly, $500 a month grows to roughly $610,000 over 30 years, with $180,000 coming from contributions and the rest from growth. Real returns are uneven and the figure is not adjusted for inflation or taxes. Past performance does not guarantee future results.
If you already hold a lump sum, investing it at once has historically come out ahead more often than spreading it out, because markets have risen in most periods. For money arriving with each paycheck the question does not apply: investing a fixed amount monthly is simply investing as the money becomes available.