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$100 a month for 10 years

What $100 a month becomes over 10 years, and the point where the interest starts outgrowing what you put in.

This is arithmetic on a rate held constant for the whole period. Real returns vary, and taxes and fees are not included. Contributions are added at the end of each month.

Final balance

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Contributed—
Interest—

How the money grows

Over 10 years you deposit $12,000 of your own money. At 7% a year, compounded monthly, the balance ends at $17,105 — so interest added $5,105, and the total is 1.4× what you paid in.

Over 10 years the interest never overtakes the deposits: at the end the balance is still mostly your own money. That tipping point depends on the rate, not on the amount, and at 7% it lands well beyond this horizon — which is the honest answer for a short term. Compounding rewards time more than it rewards the deposit.

Compound interest means the interest itself earns interest. A deposit made in month one keeps working for every month that follows, which is why the curve bends upward rather than climbing in a straight line — and why the last few years of a long plan add more than the first ten.

The deposits here land at the end of each month, which is the conservative convention: the month-one deposit starts earning in month two. Tools that assume deposits at the start of the month produce a slightly larger number, and most of them do not say which they use.

The rate is a stated reference, not a promise. Real returns vary year to year, inflation eats part of the result, and tax treatment depends on where you are. The field above is editable for exactly that reason — the shape of the curve is the lesson, not the last digit.

Frequently asked questions

Is $100 a month enough?

Enough for what is the part the calculator cannot answer. What it can say is that $100 a month for 10 years puts $12,000 in and ends at $17,105 at 7% a year. Whether that meets a goal depends on the goal.

Does the calculation account for inflation?

No. The figures are nominal, so a balance of 100,000 in thirty years buys less than 100,000 buys today. To reason in today’s money, subtract expected inflation from the rate — a 7% return with 3% inflation behaves like 4%.

Why does the total not simply scale with the deposit?

It does, actually — doubling the monthly deposit doubles both the money in and the balance out, because the maths is linear in the deposit. What does not scale is time: doubling the years does far more than doubling the result, and that asymmetry is the whole point of compounding.

What happens if I stop depositing halfway through?

The balance keeps compounding on what is already there; it just stops growing from new money. A plan abandoned at the halfway point ends well below half the full result, because the deposits you skipped were the ones with the least time left to work.

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