How Does Compound Interest Actually Work for Minors? A Plain-English Guide for Parents

Put $1,000 into a child’s investment account at birth and leave it alone. At a hypothetical 7% average annual return, it grows to about $3,380 by the child’s 18th birthday and about $5,430 by age 25. By then, more than four of every five dollars in the account are money the investment earned on its own, not money anyone deposited.

That’s compounding, and it’s the biggest reason starting early matters so much. The math itself is simple. What surprises most people is the shape of the curve: slow for years, then steeper and steeper. For a child, that curve bends in your favor, because a newborn has decades ahead for money to grow before it’s needed.

This guide, part of our Ultimate Guide to Building a Generational Nest Egg for Your Child, explains how compound interest works in plain English, how it shows up in savings accounts and investment accounts, why the first dollars you save end up doing most of the work, and what quietly slows it down.

Compound interest, in plain English

Interest is what a bank or borrower pays you for the use of your money. With simple interest, you earn it only on the amount you first deposited, called your principal. With compound interest, each period’s interest is added to the balance, so the next period’s interest is figured on a bigger number. The Securities and Exchange Commission’s guide for teachers and parents puts it in a phrase: compound interest is “the interest you earn on interest.”

Here’s how the two compare for $1,000 earning 7% a year.

Simple interest pays 7% of the original $1,000, or $70, every year. Compound interest pays 7% of the growing balance. Hypothetical example with no fees or taxes.

In the first year, the two are identical: $70 of interest either way. After that, compounding pulls ahead, because each year’s interest starts earning interest of its own. By 18, the extra interest on interest comes to about $1,120, almost as much as the $1,260 the simple-interest account earned in total. By 25, it’s about $2,680, well past the simple account’s $1,750.

Did Einstein really call it the eighth wonder of the world?

You may have seen compound interest called “the eighth wonder of the world,” usually credited to Albert Einstein. There’s no good evidence he said it. The line appears in a section titled “Probably Not By Einstein” in Princeton University Press’s The Ultimate Quotable Einstein, and the earliest close match Quote Investigator could find was a 1925 newspaper ad for a savings and loan in Cleveland. Whoever wrote the ad, the math lives up to the billing.

Where compounding happens in a child’s accounts

Compounding works the same way everywhere, but it looks different depending on where your child’s money sits.

Savings accounts and CDs: true compound interest

Savings accounts, money market accounts, and certificates of deposit (CDs) pay interest, and banks typically compound it daily or monthly. To compare accounts, look at the annual percentage yield, or APY. Under Regulation DD, the federal Truth in Savings rule, the APY reflects both the interest rate and how often interest compounds over a year, while the stated interest rate doesn’t reflect compounding at all.

The catch is the rate. The Federal Deposit Insurance Corporation’s national rate for savings accounts, an average of what banks and credit unions pay weighted by deposits, was 0.37% in September 2026, according to FDIC data published on FRED. At that pace, $1,000 deposited at birth would be worth about $1,069 at 18. If money grows more slowly than prices rise, it loses buying power, as the SEC’s guide points out. Savings accounts are a good home for short-term money and emergency funds, where safety and quick access matter most. For a goal 18 years away, they barely keep the snowball rolling.

Investment accounts: compounding without interest

Stocks and stock funds don’t pay interest. They make money in two ways: the price can rise, and companies can pay out part of their profits as dividends, as the same SEC guide explains. Compounding happens when those gains stay invested. Reinvested dividends buy more shares, and those shares can grow and pay dividends of their own. Strictly speaking, that’s compound growth rather than compound interest, but the snowball works the same way.

Reinvesting matters more than most people expect. Going back to 1960, 85% of the S&P 500’s cumulative total return can be traced to reinvested dividends and the compounding they made possible, according to Hartford Funds’ analysis of Morningstar data.

In most 529 plan portfolios, earnings are reinvested for you. In a brokerage account, including most custodial UTMA and UGMA accounts, look for a dividend reinvestment setting on each fund you own. If dividends are paid out as cash instead, they sit on the sidelines rather than compounding.

The Rule of 72: how fast money doubles

You don’t need a spreadsheet to see compounding at work. The Rule of 72, a rule of thumb the SEC’s investor educators recommend, estimates how long money takes to double: divide 72 by the annual rate of return. At 7%, that’s about 10 years. So a dollar invested at birth roughly doubles by age 10 and has nearly doubled again by 20.

Doubling times use the Rule of 72. Values assume the same return every year, with no fees, taxes, or added deposits. The 0.37% rate is the FDIC’s national savings rate for September 2026. The 10% rate is close to the S&P 500’s compound annual return from 1928 through 2025, a stretch that included many losing years. Not a prediction or guarantee.

The rate you earn changes everything. Money that doubles about every 10 years at 7% would take roughly two centuries to double at 0.37%. The rule is less precise at very low rates, and the exact figure is about 188 years, but the point stands. Over a childhood, that’s the difference between a savings account that barely moves and an investment account that has more than tripled.

To test your own numbers, try the SEC’s free compound interest calculator on Investor.gov.

Why the first dollars do the most work

Compounding rewards time, so the dollars you invest earliest do the heaviest lifting. Picture a family that invests $100 a month from birth until their child turns 18. Every deposit is the same size, but the deposits don’t end up equally valuable.

• What you contribute
• Investment growth

Hypothetical example: $100 deposited at the end of each month from birth through age 18, or $3,600 in each three-year stretch, with a 7% average annual return compounded monthly and no fees or taxes. The age 25 column below assumes the money stays invested with no new deposits. Not a prediction or guarantee.

Show the numbers

Deposits at ages	You put in	At 18	At 25
0 to 2	$3,600	$10,981	$17,634
3 to 5	$3,600	$8,964	$14,394
6 to 8	$3,600	$7,317	$11,750
9 to 11	$3,600	$5,973	$9,592
12 to 14	$3,600	$4,876	$7,830
15 to 17	$3,600	$3,980	$6,391
All 18 years	$21,600	$42,092	$67,590

The $3,600 invested while the child is a baby and toddler, from birth to age 2, grows to about $11,000 by 18. The same $3,600 invested in the last three years, ages 15 to 17, grows to about $4,000. Deposits from the first six years make up only a third of the $21,600 contributed, but they produce about 62% of the $20,500 in investment growth. The first year’s deposits alone earn about 70 times as much growth as the final year’s.

That’s why early seed money is so valuable. A lump sum invested in the first year or two, such as a grandparent’s gift or the one-time $1,000 federal deposit for eligible children born from 2025 through 2028, described in this Wolters Kluwer overview, gets the longest possible runway. At 7%, that $1,000 could grow to about $3,380 by 18 and about $5,430 by 25 without another dollar added.

If your child is already older, don’t read this as a reason to give up. Read it as a reason to start now, since today is the earliest start date you still have. The Cost of Waiting compares starting at birth with starting at age 10 and shows what it takes to catch up.

From 18 to 25: when growth takes over

Growth starts slowly. In the $100-a-month example, investment growth is only about 16% of the balance at age 5. Around the 10th birthday, the account starts earning more in a typical year than the $1,200 a year the family adds. And a few months after the 18th birthday, total growth passes the $21,600 the family put in.

Hypothetical example: $100 deposited at the end of each month from birth through age 18, then no new deposits. Assumes a 7% average annual return compounded monthly, with no fees, taxes, or inflation adjustment. Not a prediction or guarantee.

The years after 18 are where compounding really shows. If the money isn’t needed for college and stays invested with no new deposits, it grows from about $42,100 at 18 to about $67,600 at 25. That’s about $25,500 of growth in seven years, more than every dollar the family contributed over the previous 18.

Not all of a child’s money should stay invested in stocks that long, of course. Money earmarked for tuition due at 18 should shift gradually toward safer investments as college approaches, as our guide to building a target-date glide path explains. But money meant for later goals, such as a first car, a wedding, or a down payment on a home, can keep compounding into your child’s 20s, as we cover in Using Custodial Funds for Life Starts. Retirement-style accounts can compound for decades longer. A Trump Account is a new type of traditional IRA, according to the IRS, and a custodial Roth IRA lets a teen with a paycheck invest part of their own earnings.

These figures aren’t adjusted for inflation. At 3% inflation, the $42,100 balance at 18 would buy about what $24,700 buys today, as The Cost of Waiting explains in more detail.

Steady averages vs. real markets

Every example in this guide assumes a steady 7% a year. Real markets don’t move that way. Over the 98 years from 1928 through 2025, the S&P 500 with dividends reinvested lost money in 26 of them, including drops of about 44% in 1931 and 37% in 2008, according to annual return data compiled by NYU Stern professor Aswath Damodaran.

Ups and downs also mean the average of yearly returns overstates what investors actually earn. Imagine $1,000 that gains 50% one year and loses 50% the next. The average return is 0%, but the account is down to $750. The rate that matters is the compound annual return, the steady rate that would have produced the same result. From 1928 through 2025, the S&P 500’s yearly returns averaged about 11.9%, but money invested for the whole period compounded at about 10.0% a year, by our calculation from Damodaran’s data.

Time has narrowed the range of outcomes. In our analysis of the same data, a lump sum held in the S&P 500 for any 18 calendar years from 1928 through 2025 never lost money, but the results ranged from about 2% a year (1929 to 1946) to about 18% a year (1982 to 1999). Over 25-year stretches, the worst result was about 5.2% a year, and 72 of the 74 periods beat 7%. That history is part of why GoodNest’s examples use a more conservative 7%, and a reminder that it’s an illustration, not a forecast. Past returns don’t guarantee future ones, and these figures leave out fees and taxes.

The compounding in those long stretches went to investors who stayed invested through the bad years. The Psychology of Generational Wealth covers how to stay the course when markets drop, and Index Funds vs. Stock Picking for Minors explains why a simple, diversified fund is a sensible way to capture market returns.

What slows compounding down

Compounding needs two things: money left invested, and as much of each year’s return as possible staying in the account. Five things chip away at it.

• Fees. Fund expenses and account fees come out of your balance every year, and money paid in fees stops compounding for you. In an SEC investor bulletin, $100,000 growing 4% a year for 20 years ends at about $208,000 with a 0.25% annual fee, but only about $179,000 with a 1% fee. In our $100-a-month example, a 1% annual fee would leave about $38,100 at 18 instead of about $42,100. Trump Accounts are designed to keep this drag small: under the IRS’s proposed rules, the funds they hold until the year a child turns 17 can charge no more than 0.1% a year, which would trim the same example by only about $420.
• Taxes. Growth compounds fastest when it isn’t taxed along the way. In a 529 plan, earnings aren’t subject to federal tax when used for qualified education expenses, according to the IRS. Qualified distributions from a Roth IRA are tax-free, and Trump Accounts grow tax-deferred. In a custodial UTMA or UGMA account, investment income is taxable each year, and a child’s unearned income above $2,700 can be taxed at the parents’ rate under the kiddie tax. For 2025 and 2026, the first $1,350 is generally tax-free and the next $1,350 is taxed at the child’s own rate, as U.S. Bank summarizes.
• Uninvested cash. A contribution that lands in a brokerage account as cash earns little until it buys something. Automating the Nest Egg explains how to make sure every deposit gets invested. Trump Accounts handle this for you: under the proposed rules, money is invested in an eligible fund chosen by the trustee if you don’t pick one.
• Early withdrawals. Money taken out stops compounding for good, along with everything it would have earned. Long pauses in contributions have a similar effect.
• Inflation. Rising prices shrink what the final balance can buy, which is why the rate you earn needs to outpace inflation over time.

How to explain compounding to your child

The SEC’s guide for teachers and parents calls youth the best time to learn about money, and compounding is one of the best lessons to start with. It helps your child understand the account you’re building for them and, eventually, how to manage it. Here’s one way to teach it as they grow.

• Ages 3 to 5: roll a snowball. A snowball rolled across the yard picks up more snow the bigger it gets. Money that stays in an account grows the same way. The Early Years covers more first money lessons.
• Ages 6 to 10: pay interest at home. If your child saves in a jar, add a small “interest” payment each month based on what’s inside. As the jar fills, the payments grow, and your child can see why leaving money alone pays off. Grade School explains how this fits with allowances.
• Ages 11 to 14: show them the real account. During a yearly portfolio review, point out how much of the balance came from deposits and how much from growth. Try the Rule of 72 or the Investor.gov calculator together. Middle School walks through the review.
• Ages 15 to 18: show the flip side. Compounding works against borrowers, too. In the second quarter of 2026, credit card accounts that were charged interest paid an average rate of 22.15%, according to the Federal Reserve. By the Rule of 72, an unpaid balance at that rate doubles in just over three years. A teen with a paycheck can also put compounding to work by investing part of it. High School covers first paychecks and avoiding debt.

Where GoodNest fits

Compounding rewards early money, and some of the earliest money a child receives comes from family. Grandparents, aunts, uncles, and friends want to celebrate birthdays and milestones, and GoodNest lets them mark those moments with a contribution toward your child’s future, often paired with a small keepsake like a book, instead of another toy. GoodNest doesn’t hold or invest the money. Family send gifts straight to you by Zelle, Venmo, or PayPal, and you move them into the 529 plan, custodial account, or Trump Account you’ve chosen.

Timing is where compounding comes in. At a hypothetical 7% return, a $100 gift invested at a first birthday could grow to about $316 by 18 and about $507 by 25. A first birthday’s worth of gifts totaling $500 could grow to about $1,580 by 18 and about $2,540 by 25. So move gift money into your child’s account soon after it arrives, and consider showing the givers how their gifts are growing.

The bottom line

Compound interest is simple: your money earns money, and then that money earns money, too. What makes it powerful for children is time. At a hypothetical 7% return, $100 a month from birth grows to about $42,100 by 18, with nearly half of it coming from growth, and to about $67,600 by 25 if it’s left alone. The earliest deposits do most of that work. Start as early as you can, keep fees and taxes low, reinvest what the account earns, and give the snowball time to roll. For the full plan, from choosing accounts to eventually handing the money over, head back to The Ultimate Guide to Building a Generational Nest Egg for Your Child.

Frequently asked questions

How does compound interest work for kids?

Compound interest means your child’s money earns interest or investment returns, and then those earnings earn returns, too. Because children have many years before they need the money, early deposits have time to double and double again. At a hypothetical 7% return, $1,000 invested at birth grows to about $3,380 by age 18 and about $5,430 by 25, and most of that final balance is growth rather than deposits.

How much will $100 a month grow by age 18?

At a hypothetical 7% average annual return, $100 invested at the end of every month from birth grows to about $42,100 by age 18. You would contribute $21,600, so nearly half the balance is investment growth. Left invested with no new deposits, it could reach about $67,600 by age 25. Real returns vary from year to year, and fees and taxes reduce them.

Do stocks and index funds earn compound interest?

Not interest, strictly speaking. Stocks and stock funds grow when prices rise and when companies pay dividends. When those dividends are reinvested, they buy more shares that can grow and pay dividends of their own. That’s compound growth, and it snowballs the same way compound interest does. Hartford Funds attributes 85% of the S&P 500’s cumulative total return since 1960 to reinvested dividends and compounding.

What is the Rule of 72?

The Rule of 72 is a shortcut for estimating how long money takes to double: divide 72 by the annual rate of return. At 7%, money doubles in about 10 years. At 0.37%, the national average savings rate in September 2026, it would take nearly two centuries. The rule works on debt, too: at a 22% credit card rate, an unpaid balance doubles in just over three years.

Does daily or monthly compounding make a big difference?

Not much. On $1,000 earning 5% for 18 years, compounding daily instead of monthly adds only about $4, and daily instead of once a year adds about $53. The rate you earn, how long you stay invested, and what you pay in fees and taxes matter far more. When you compare savings accounts, use the annual percentage yield (APY), which already accounts for how often interest compounds.

Sources

• U.S. Securities and Exchange Commission, Office of Investor Education and Advocacy, “Tips for Teaching Students About Saving and Investing” (last modified September 14, 2005).
• Quote Investigator, “Quote Origin: The Eighth Wonder of the World Is Compound Interest” (September 9, 2019; updated June 27, 2025).
• Consumer Financial Protection Bureau, Regulation DD (Truth in Savings), 12 CFR Part 1030, Electronic Code of Federal Regulations.
• Federal Deposit Insurance Corporation, “National Rate: Savings” (SNDR), retrieved from FRED, Federal Reserve Bank of St. Louis (September 2026 observation, updated September 21, 2026).
• Hartford Funds, “The Power of Dividends: Past, Present, and Future” (2026; data through December 31, 2025).
• U.S. Securities and Exchange Commission, Investor.gov, Compound Interest Calculator.
• Wolters Kluwer, “Trump Accounts: Highlights and Operational Considerations” (September 8, 2026).
• Internal Revenue Service, “Treasury, IRS Issue Proposed Regulations on Eligible Investments for Trump Accounts Under the Working Families Tax Cuts”, IR-2026-96 (August 20, 2026).
• Aswath Damodaran, NYU Stern School of Business, “Historical Returns on Stocks, Bonds and Bills” (annual data through 2025, updated January 5, 2026). Averages and rolling-period results are GoodNest calculations from this data.
• U.S. Securities and Exchange Commission, Investor.gov, “How Fees and Expenses Affect Your Investment Portfolio”, Investor Bulletin.
• Internal Revenue Service, “529 Plans: Questions and Answers”.
• Internal Revenue Service, “Roth IRAs”.
• Internal Revenue Service, “Topic No. 553, Tax on a Child’s Investment and Other Unearned Income (Kiddie Tax)”.
• U.S. Bank, “What Is the Kiddie Tax?” (updated August 4, 2026).
• Board of Governors of the Federal Reserve System, “Consumer Credit – G.19”, July 2026 release (September 8, 2026).

This article is for general educational purposes only and is not investment, tax, or legal advice. Examples are hypothetical, assume a steady 7% average annual return compounded monthly unless noted, and don’t reflect fees, taxes, or any specific investment unless noted. Historical market figures are past results and don’t predict future returns. Tax figures reflect 2025 and 2026 federal rules, which can change, and Trump Account investment rules described here come from proposed regulations. Investing involves risk, including possible loss of principal. GoodNest does not provide investment advice and does not hold or invest funds. Consider consulting a qualified professional about your family’s situation.