Compound Interest Calculator for Kids

Compound interest by the numbers

• $100 single deposit at 7% annual return: grows to $196.72 in 10 years, $761.23 in 30 years, and $1,497.45 in 40 years. No additional deposits.
• $5/week at 7% (compounded weekly): reaches about $3,770 after 10 years, $11,200 after 20 years, and $26,700 after 30 years. The longer the runway, the more of the total comes from interest, not deposits.
• S&P 500 average return, 1928 to 2024: roughly 10% nominal, near 7% after inflation (long-run historical average, not a forecast). Source: NYU Stern annual returns dataset.
• Rule of 72: money roughly doubles every 72 / rate years. At 7%, that is about 10 years and 3 months. At 10%, about 7 years and 2 months.

How do you explain compound interest to a 10-year-old?

Use the snowball image. A small snowball rolled down a long hill picks up more snow each turn, and what it picks up itself picks up more. Then show the math: $1 today that doubles every day for 30 days reaches over $5.3 million on day 30. The shock does the teaching.

How much do I need to invest monthly to reach $100,000?

At a 7% annual return, you would need about $585 per month for 10 years, $190 per month for 20 years, or $80 per month for 30 years. The longer the timeline, the smaller the monthly amount, because compounding does more of the work the more years it has.

How much is $1,000 at birth worth at 18?

A $1,000 deposit at birth with nothing added grows to about $3,400 by age 18 at a 7% average yearly return. Monthly additions do most of the work: $50 a month reaches about $23,800, $100 a month about $44,200, and $250 a month about $105,400. Returns vary year to year and are not guaranteed.

These figures assume yearly compounding, each year's contributions added at the end of that year, and no fees, taxes, or withdrawals, rounded to the nearest $100. A $1,000 start at birth is also the federal seed for a Trump Account opened for a child born 2025 through 2028. Our Trump Account calculator runs these numbers for any amount you add.

Why Starting Early Changes Everything

Compound interest is the one financial concept that rewards patience more than income. A child saving $5 per week from age 10 with a 7% average return reaches $168,760 by age 65. Wait until 25 to start, and that same $5 per week only reaches about $56,800. The child who starts at 10 wins by about $112,000. Not because they saved more, but because they started sooner.

That 15-year head start is the entire point of this calculator. When your child can see the number - their specific number, with their savings rate and their timeline - the concept stops being abstract.

How Compound Interest Works

Simple interest pays you based on your original deposit only. Compound interest pays you on your deposit plus all the interest already earned. Year one, you earn interest on $100. Year two, you earn interest on $107. Year three, on $114. The base keeps growing, so the interest payment grows too, even if you never add another dollar.

Add regular contributions and the effect becomes dramatic. Every $5 your child saves today earns interest for the full remaining timeline. A $5 deposit at age 10 is worth about $207 at age 65 at 7% - a 41x multiplier from a single week of saving.

The Right Growth Rate to Use

This calculator offers three options. The 2% savings account rate is a simple round number for teaching. Real rates vary widely: the national average savings account pays under 1% (FDIC national rates), while high-yield online accounts pay more. The 7% S&P 500 rate is the historical inflation-adjusted average - useful for teaching about investing, but it comes with real-world variability (some years up 30%, some down 20%). For teaching, start with 2% to show basic compounding, then switch to 7% to show the difference investing makes. The contrast is the lesson.

What to Do After the Calculator

Use this number as a conversation starter, not a financial plan. Open a savings account with your child and let them watch the balance grow. Many banks have youth accounts with no fees. For older teens, a custodial investment account can introduce real market investing with small amounts. If your child was born in 2025 or later, our Trump Account calculator shows how the $1,000 federal seed compounds to age 18. The goal is building the habit of consistent saving, not hitting an exact number.

Start with an allowance and a savings goal. Penny Time lets kids practice saving real money and watch it grow over time, building the habits that make compound interest work later.

This calculator is for educational purposes only. It shows how compound interest works using the growth rate you choose. It is not financial advice.

Compound interest chart by age: what $5 a week looks like

The numbers below use $5 per week at a 7% annual return (the S&P 500 historical average, adjusted for inflation). Same amount, same rate. The only variable is when you start.

Look at the last column. A child who starts at age 8 ends up with $194,597 by age 65. A child who starts at 14 ends up with $126,746. That six-year head start adds $67,851 - but the extra deposits over those six years only total $1,559. The rest is compound interest doing the work.

The middle columns matter too. By age 30, the child who started at 8 has $13,524 from just $5 a week. That is real money for a down payment, a business, or a head start on retirement investing. All from pocket-change contributions that started in elementary school.

Use the calculator above to plug in your child's actual age and see their specific numbers. The table uses $5 a week, but even $2 or $3 follows the same pattern. The curve does not care about the amount. It cares about time.

Compound Interest Explained for Kids

Start with the penny question. Ask your child: "Would you rather have $1,000 right now, or a single penny that doubles every day for 30 days?"

Most kids pick the $1,000. Here is what happens with the penny:

• Day 1: $0.01
• Day 10: $5.12
• Day 20: $5,243
• Day 25: $167,772
• Day 30: $5,368,709

That penny reaches $5.37 million by day 30. The first half of the month looks like nothing is happening. Then the growth explodes. This is exactly how compound interest works - slowly at first, then all at once.

Want them to see it instead of just hearing about it? Scroll up to the calculator and change the numbers together. Watching the total jump is what makes the idea stick, and it is the part a text explanation alone cannot do.

The snowball version (ages 8-10)

For younger kids, skip the math and use a snowball. "Imagine you roll a tiny snowball down a long hill. It picks up more snow as it goes. By the bottom, it is huge. Your money works the same way - it picks up more money as it rolls." A short hill (saving for 5 years) makes a small snowball. A long hill (saving for 40 years) makes a giant one. The only thing that matters is how long the hill is.

The money-making-money version (ages 11-13)

Older kids can handle the actual mechanic. "You put $100 in a savings account. The bank pays you $7 for keeping it there. Now you have $107. Next year, the bank pays you 7% on $107 - that is $7.49 instead of $7. The year after, you earn on $114.49. Every year, your earnings get a little bigger because the base keeps growing." Pull up the calculator and let them type in their own numbers. Seeing $5 a week turn into $26,412 over 30 years makes the concept click faster than any explanation.

The real-account version (ages 14+)

Teens are ready for specifics. "The stock market has averaged about 7% per year after inflation over the last century. If you invest $5 a week starting now and never stop, you could have over $126,000 by age 65. You would deposit about $13,250 total. The rest - over $113,000 - is compound interest." For teens with jobs or allowance income, a custodial brokerage account lets them invest real money with a parent's oversight. Our investing for teens guide shows what starting at 15 instead of 25 is worth.

A penny doubled for 30 days: the day-by-day table

A penny that doubles every day is worth $5,368,709.12 on day 30, starting from a single cent on day 1. Here is every day, so you can show a child exactly where the jump happens.

Day 28 (highlighted) is the first day the penny passes $1 million. Day 20 is still only $5,242.88, so the first two-thirds of the month look like almost nothing and then the total explodes. That shape is the whole lesson.

Three figures people mix up

• Value on day 30: $5,368,709.12 (counting the first penny as day 1).
• Value on day 31 (or day 30 if you call the first penny "day 0"): $10,737,418.24.
• Running total handed over across all 30 days: $10,737,418.23.

Different websites quote different numbers because they answer different questions. All three above are correct for the question each one asks.

Would you take $1,000,000 today, or the penny?

Most kids (and most adults) take the million, because on day 20 the penny is worth only $5,242.88. But real money does not double every day. At a 7% yearly return, the Rule of 72 says money doubles about every 10 years, so that same penny would need roughly a decade for one doubling, not a day. That is why starting early matters more than chasing a high rate: you are buying more doublings, and each doubling is worth more than the one before it. To let your child choose before they see the answer, play our penny doubled for 30 days game.

When Should Kids Start Learning About Investing?

Ages 8-10: The concept of money growing

Kids this age can understand that money in a savings account grows over time. They do not need percentages or formulas. They need to see it. Open a savings account together, deposit their birthday money, and check the balance a few months later. When they see a number higher than what they put in, the concept lands. Use this calculator to show them what their balance could look like in 5 or 10 years.

Ages 11-13: Compound interest math

Middle schoolers can handle multiplication and percentages. Show them the difference between simple interest (earning on the original amount only) and compound interest (earning on the growing total). The penny-doubling question works well here. Let them experiment with the calculator - change the years slider and watch the interest portion of the bar chart grow faster than the deposit portion. That visual gap between blue and green is compound interest at work.

Ages 14 and up: Real accounts and real investing

Teens with earned income can open a custodial Roth IRA. Teens with allowance or gift money can use a custodial brokerage account. Both let them invest in index funds with a parent as the account holder. The amounts do not matter much at this stage. What matters is building the habit of consistent contributions and seeing real market returns (including the down years). A teenager who watches their own portfolio through a 10% drop learns more about investing than any textbook.

The calculator above works at any age. Plug in your child's current age and their realistic weekly savings to see what their head start is worth.

Compound interest practice problems (with answers)

Homework-style questions with the working shown. For a single deposit, the balance is: starting amount × (1 + rate)^years, with the rate written as a decimal (4% = 0.04). The last problem adds money every month, which builds even faster. Try each one on paper, then check your answer.

Problem 1 (ages 10-12): a birthday deposit

Your parents put $100 in your bank account when you turn 13. You do not spend any of it for 5 years, and the account earns 4% per year. What is the balance when you turn 18?

Answer: 100 × (1.04)⁵ = $121.67. The $100 earned $21.67 in interest, and about $1.67 of that came from interest earning its own interest.

Problem 2 (ages 11-13): three years of saving

You save $500 in an account that pays 5% per year and leave it alone for 3 years. How much is in the account at the end?

Answer: 500 × (1.05)³ = $578.81, so $78.81 in interest.

Problem 3 (ages 13-15): simple interest vs compound interest

You invest $1,000 at 6% per year for 10 years. How much do you have if the interest is simple (paid only on the original $1,000), and how much if it compounds?

Answer: Simple interest pays $60 a year, so 1,000 + (60 × 10) = $1,600. Compound interest is 1,000 × (1.06)¹⁰ = $1,790.85. Compounding is worth $190.85 more over the same 10 years, without adding a single extra dollar.

Problem 4 (ages 14-16): adding money every month

You put in $25 every month for 4 years, and the account earns 6% per year compounded monthly (0.5% each month). How much have you built?

Answer: about $1,352. You contributed $1,200 of your own money ($25 × 48 months) and earned roughly $152 in interest. Steady monthly saving plus compounding is how small amounts turn into real money.

Want to check your own numbers? Put the starting amount, rate, and years into the calculator at the top of this page and compare.