How Compound Interest Works When You Invest Every Month

Compound interest is often explained with a single deposit left alone for decades. Most people don't invest that way: they put in a bit every month. Here's what happens then, with real numbers.

The mechanism in one sentence

Each month your balance earns a return, and next month's return is calculated on the bigger balance — so you earn returns on your earlier returns. Early on, growth comes almost entirely from your deposits. Later, the returns themselves become the main engine.

A worked example: $300 a month

In the compound interest calculator, start from zero, invest $300 a month and assume 7% a year, compounded monthly:

The last ten years add $36,000 of your money but more than $200,000 to the balance. That's the whole argument for starting early: the final decade is where compounding does the heavy lifting, and you only get a final decade if you started early enough.

Choosing a realistic return

The result is only as good as the rate you type in. Savings accounts and bonds pay much less than a diversified stock index fund over the long run — and stocks swing a lot from year to year. A sensible habit is to run three cases (say 3%, 5% and 7%) and plan around the middle one.

Fees and taxes come out of your return, so subtract them first. A fund charging 1% a year turns a 7% market return into 6% — over 30 years that difference is tens of thousands on the example above.

Don't forget inflation

$366,000 in 30 years won't buy what $366,000 buys today. Enter an inflation rate and the calculator also shows the balance in today's money. Alternatively, use a real return (return minus inflation) from the start — that's the approach the FIRE calculator uses for retirement planning.

Three takeaways

• Start now, even with a small amount; you can raise it later.
• Automate the monthly deposit so it happens before you can spend it.
• Keep costs low — fees compound too, against you.