La Hucha deBOB STICKMAN

Compound interest calculator

What if you had started 10 years ago?

If inyou had put $ in at the start, and then $ every month into the US stock market, by the end of 2025 you'd have $30,321.
What your portfolio would be worthWhat you would have put inWhat the market did (right-hand scale)Years the market fell
$10,000$20,000$30,000×1×2×3

In 2022 the market fell 18.0%. Your portfolio went from $15,445 to $13,756 even though you kept putting money in.

The dashed line is the S&P 500 with dividends: how many times what you had at the start multiplied. Your portfolio rises more smoothly because new money goes in every month, buying cheaper just when the market drops. Below, what each year did; in your portfolio, each year's return is spread over twelve equal months and each month's deposit goes in at the end of the month.

You would have put in

$13,000

What the market added

$17,321

Average return over those years

14.7% a year

Your worst year

2022: −18.0%

What if you had started in a different year?

The same plan, 10 years putting in the same amount, starting in each year since 1928. That's 89 real stories, and in 87 of them you ended up with more than you put in.

The worst start: 1999

$10,791

The typical one (half ended with more)

$23,795

The best start: 1946

$42,209

What you put in, in all of them

$13,000

Starting in 1928In red, your year: 2016Starting in 2016

Each bar is what you would have had at the end, depending on the year you started. The yellow line is what you put in. What happened doesn't tell you what will happen, but it does show how much the result can vary.

Real S&P 500 data with dividends, in dollars, from Aswath Damodaran (NYU Stern), updated January 2026. No fees, taxes or currency conversion. What happened doesn't tell you what will happen.

And now, looking ahead

$
$
0%
Like your salary, or like prices. At 0%, you always set aside the same amount.
30 years
7%
The S&P 500 with dividends grew 10% a year, compounded, from 1928 to 2025 (the average of each year's percentages comes out higher, 11.9%, but it's no good for long-term math). A savings account or a deposit pays a lot less.
1.5%
An index fund usually charges less than 0.3%; many actively managed funds, more than 1.5%.
3%

In 30 years, the app would show

$87,442

What you put in

$36,000

What time puts in

$51,442

Fees take $29,503 along the way: without them, the app would show $116,945.

With prices rising 3% a year, a $3 coffee will cost $7.28 in 30 years. With what the app shows you can buy 12,008 coffees at those prices.

Year by yearWhat you put inWhat time puts in
Year 1Year 30

Compare two paths

The same numbers as above, with two different fees. What looks like a small-print detail, over 30 years.

0.2%

$112,433

1.8%

$82,608

The difference

$29,825

Path B ends up with $29,825 less: 27% of what you would have on the other path.

Bob's Anti-Panic Plan, free

What to have sorted before you invest, your contract with yourself for the day your money drops, and a reminder to review it every year. To print and fill in with a pen. And then, an email with each new Bob experiment; unsubscribe in one click.

Bob's piggy bank with a coin in the slot

How compound interest works

Compound interest means earning on what you have already earned. What your money earns in one year is added to what you had, and next year it earns too. At first you barely notice; over the years, the part that grows on its own ends up bigger than the part you put in.

this month's balance = last month's balance × (1 + this month's interest) + what you set aside

The monthly interest is not the yearly return divided by twelve, but the rate that, repeated twelve times, gives the yearly one: with 10% a year, 0.80% a month.

Why look back with real years

Almost every calculator uses a fixed average, as if the market rose the same amount every year. It doesn't: there are years up more than 30% and years down more than 30%. With real years you see what it truly costs to hold on, and that the order of the bad years changes the result.

Frequently asked questions

What is the difference between simple and compound interest?

With simple interest you only earn on what you put in at the start, and you earn the same every year. With compound interest what you earned is added in and earns too, so each year you earn a bit more than the last.

Why do fees matter so much?

Because they are charged every year on everything you have built up, not just on what you put in. Compare two paths above and see it with your own numbers.

Assumptions and sources

Annual S&P 500 returns with dividends: Aswath Damodaran (NYU Stern), Historical Returns on Stocks, Bonds and Bills: 1928-2025, updated January 2026. It is the history of a single market, in dollars, before fees and taxes.

Educational content. The calculations are examples with simplified assumptions, not a promise of returns or financial advice: we don't tell you what to buy. Before investing, get properly informed or talk to a professional.