Compound Interest Visualizer
See what consistent saving looks like over time.
What compound interest actually is
Compound interest means earning returns on your returns. If you invest $1,000 and it grows 7% in a year, you have $1,070. The next year, you earn 7% on $1,070 — not just on the original $1,000. That extra $4.90 does not sound like much, but over decades it transforms small contributions into large sums.
The single most important variable is time. Starting early matters more than saving more. A person who saves $100 per month from age 20 to 65 at 7% annual returns accumulates roughly $380,000. Someone who starts at 30 and saves $200 per month — twice as much — accumulates roughly $340,000 over the same period. The earlier saver contributed less money and ended up with more, because ten extra years of compounding outweighed doubling the contribution.
How to read this chart
The darker blue area at the bottom represents your total contributions — the money you actually put in. The lighter blue area on top represents investment growth — the returns your money earned. The line at the top is the total balance.
Notice that early on, contributions dominate: the darker area is most of the chart. But after 15–20 years, the growth area starts exceeding contributions. By the end of a 40–50 year period, growth is typically 2–4 times larger than what you put in. That crossover point is where compounding becomes visually obvious and emotionally motivating.
Why we use 7% as the default return
The S&P 500 has returned roughly 10% annually in nominal terms since 1926. After subtracting inflation (historically about 3%), the real return is approximately 7%. Real returns are what matter for planning because they represent actual purchasing power growth — what your money can buy, not just how many dollars you have.
This is an average over very long periods. Individual years range from -37% (2008) to +52% (1954). Entire decades have underperformed. The 7% figure is a useful planning assumption, not a promise. If you want a conservative projection, use 5%. If you want to see nominal growth, use 10%. The slider lets you explore the range.
Why this matters if you are a teenager
You have something that no amount of money can buy later: time. Starting at 16 instead of 25 gives you nine extra years of compounding. With identical savings — same dollar amount, same frequency — those extra years typically produce a final balance 50–80% larger. That is not a rounding error. It is the difference between a comfortable retirement and a spectacular one.
Even small amounts matter. Five dollars a week from age 16, invested at 7%, grows to roughly $100,000 by age 65. Twenty-five dollars a week grows to roughly $500,000. These are not fantasy numbers. They are the mathematical consequence of consistency and time. Try it with the sliders above.
Where this calculator fits in your planning
This tool visualizes growth but does not model tax treatment. For tax-advantaged accounts, consider: a custodial Roth IRA if your teen has earned income (contributions grow tax-free), a 529 plan for college savings (tax-free growth for qualified education expenses), or a UTMA/UGMA custodial account for general long-term gifting (gains are taxable, with some kiddie tax considerations). Each has tradeoffs worth understanding before opening an account.
What this calculator does not model
Market volatility. Real returns are not smooth. Your balance will go up and down year to year, sometimes dramatically. This chart shows the long-term average path, not the turbulent reality.
Inflation. The default 7% rate is already inflation-adjusted. If you use a higher rate, the result is in nominal (future) dollars that will buy less than today’s dollars.
Taxes. Gains in taxable accounts are subject to capital gains tax. Roth IRAs and 529 plans avoid this, but have contribution limits and rules. This calculator does not deduct taxes from returns.
Fees. Mutual fund expense ratios and trading costs reduce actual returns. Low-cost index funds typically charge 0.03–0.20% annually. Higher-fee funds can reduce returns by 1% or more per year, which compounds significantly over decades. See our methodology page for full disclosure.
Frequently asked questions
What’s a realistic return rate to assume?
The S&P 500 has returned roughly 10% annually before inflation since 1926, or about 7% after inflation. We default to 7% (the real return) because that reflects actual purchasing power growth. Conservative planners use 5–6%. Aggressive projections use 8–10%. No rate is guaranteed; markets have had full decades of underperformance.
Is it better to start earlier or save more?
Starting earlier almost always wins. Saving $100/month from age 20 to 65 at 7% yields roughly $380,000. Saving $200/month from age 30 to 65 yields roughly $340,000 — despite contributing twice as much per month for 10 fewer years. The extra decade of compounding matters more than doubling the contribution.
Should a teen even worry about investing?
Not worry, but understand. The biggest advantage a teenager has is time. Even $5 or $10 per week invested consistently from age 16 becomes a meaningful sum by 40. Understanding compound growth early changes how you think about saving versus spending for the rest of your life.
What kind of account lets this actually work?
For teens with earned income: a custodial Roth IRA grows tax-free and is one of the most powerful accounts available. For general savings: a custodial brokerage account (UTMA/UGMA) has no contribution limits but gains are taxable. For college: a 529 plan offers tax-free growth for education expenses. Each has tradeoffs.
What happens if the market crashes?
Markets crash regularly — about once per decade historically. Over 20–30 year periods, they have always recovered and grown. This calculator shows the long-term average, not the year-to-year reality. The key is staying invested through downturns, which is hard emotionally but historically the right move.
Is this showing after-inflation or before-inflation dollars?
The default 7% rate is a real (after-inflation) return. The dollar amounts shown approximate future purchasing power in today’s terms. If you switch to 10%, you’re seeing nominal dollars that will buy less in the future due to inflation. We default to 7% because it’s more honest for planning.
Can I share these projections with my family?
Yes. Every input is saved in the page URL. Copy the URL and send it to anyone — they’ll see the exact same chart and numbers. This is useful for showing a teen what their savings could become, or for sending a college savings projection to a partner.