Free Tool — Show Them the Snowball · Created August 3, 2026

Compound Interest Calculator for Kids

Ten dollars a week, a growth rate, and a lot of patience — watch what time does to small money.

The money your child saves this week can multiply itself for fifty years. This calculator makes that visible — run it with your kid.

Starting amount ($)

Added every week ($)

Child’s age now

Yearly growth rate (%)

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At age 18

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At age 30

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At age 65 (no new money after 18)

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Total actually saved by 18

Weekly additions stop at age 18; after that the balance just keeps compounding at the same rate. 8% reflects a long-run stock-market-like return; real returns vary year to year and are not guaranteed.

How to Explain It to Your Kid

Try this script:

“Your money earns money while you sleep. Then the money it earned starts earning money too. That is the whole trick — and the earlier you start, the more time you have for your money to work for you.”
Then point at the different age boxes. The huge number at age 65 doesn’t come from saving harder — it comes from how long the money was left alone to work.

Ten dollars a week starting at ten years old beats far larger amounts started at forty, because compounding rewards time more than effort — the lesson behind our whole age-by-age roadmap.

Where does a kid get an 8% engine? Not a piggy bank — a savings account is the first jar, but the long-run growth lives in index funds, explained in Investing Basics. Around age 8 is a good moment to open a custodial brokerage (UTMA) account for your child — old enough to follow the balance, young enough for the runway to matter. In our own family it changed the way our kids saw saving: money stopped being something you spend and started being something you own.

For children with real earnings, the custodial Roth IRA runs this exact math with the taxes removed. And the “total actually saved” box is where the magic shows up: the gap between that number and the age-65 number is money nobody had to earn. That’s the money your money made for you.

The Trick

Compound interest pays you for waiting. Kids have more waiting ahead of them than anyone — which makes them the richest investors in the house.

References & Resources

SEC Investor.gov: Compound Interest Calculator — The government’s version of this math.
CFPB: Money as You Grow — Age-appropriate money activities for kids.
Long-run return context: the S&P 500’s historical average annual return is roughly 10% before inflation; 8% is a common planning assumption. Past performance does not guarantee future results.