Free Compound Interest Calculator — See How Your Money Grows

Compound interest is the single most powerful force in personal finance — and most people don’t use it because they can’t see it working. This calculator shows you exactly what happens when your money earns returns on top of returns. Enter your starting amount, what you can add each month, and your expected rate. Then watch what time actually does to the numbers.

What This Calculator Shows

The three numbers that matter most are your Final Balance, Total Contributed, and Interest Earned. The gap between what you put in and what you end up with is compound interest doing its job. The longer the time period, the bigger that gap gets — that’s why starting early matters more than starting with a lot.

A Worked Example

Start with $1,000, add $200 a month for 20 years at 7% annual return (roughly the long-term average of a broad index fund, inflation-adjusted):

• Total contributed: $49,000
• Final balance: ~$106,000
• Interest earned: ~$57,000 — more than you put in, and it cost you nothing extra

That extra $57,000 is the math of compounding — your returns earning their own returns, month after month. The earlier you start, the longer that snowball rolls.

What Rate Should You Use?

• High-yield savings account: 4–5% (as of 2026 — changes with Fed rate decisions)
• Broad stock market index fund (S&P 500 or total market): 7–10% long-term average, with year-to-year swings
• Conservative portfolio (stocks + bonds mix): 5–7%
• Target-date retirement fund: Check the fund’s prospectus for expected returns

For long-term retirement projections, 7% is the standard conservative estimate for a diversified stock index fund. For a HYSA, use your bank’s current rate.

How to Use This for Real Decisions

The most useful thing this calculator does is show you the cost of waiting. Run it twice — once starting now, and once starting five years from now with the same contribution. That difference in final balance is what delaying costs you. It tends to make people act.

You can also reverse-engineer a goal: if you want $500,000 in 30 years at 7%, adjust the monthly contribution until the final balance hits your target.

Next Steps

If this convinced you to start, here’s the order that works for most people:

1. Pay off any high-interest debt first — 20% APR credit card debt beats any investment return
2. Capture your employer’s 401(k) match — that’s an immediate 50–100% return
3. Open a Roth IRA — tax-free growth makes compounding work even harder
4. Invest in low-cost index funds — see the VTI vs VOO vs SCHD guide for a plain-English comparison
5. Automate contributions — systems beat willpower every time

If you don’t have a 3–6 month emergency fund yet, build that before investing beyond your employer match. It’s what keeps you from selling investments at the worst possible moment.

This calculator provides estimates for educational purposes only. It assumes a constant rate of return and regular contributions. Actual investment returns vary and are not guaranteed. This is not personalized financial advice.

Frequently Asked Questions

What is compound interest?

Compound interest means you earn interest on your interest, not just on your original deposit. If you invest $1,000 and earn 7% in year one, you end year one with $1,070. In year two, you earn 7% on $1,070 — not just $1,000. Over decades, this “interest on interest” effect becomes the dominant force in your investment growth.

How often does interest compound?

It depends on the account or investment. High-yield savings accounts typically compound daily. CDs may compound daily, monthly, or quarterly. Investment returns from index funds don’t compound in the traditional sense — gains are reinvested, which produces a similar effect. More frequent compounding slightly increases your total return, but the difference is small compared to the rate and time variables.

What rate of return should I use for investing projections?

A common planning assumption for diversified stock index funds is 7% annually (adjusted for inflation) or 10% nominally, based on long-term historical averages for the U.S. market. Neither number is guaranteed. For savings accounts and CDs, use the current rate your bank is offering — that’s a known rate, not a projection.

What’s the difference between compound interest and simple interest?

Simple interest is calculated only on your original principal. If you invest $1,000 at 7% simple interest for 10 years, you earn $700 total. Compound interest calculates on the growing balance — the same scenario with compound interest yields about $967 in interest. The gap grows dramatically over longer time horizons.

How long does it take to double your money?

Use the Rule of 72: divide 72 by your annual interest rate. At 6%, your money doubles in about 12 years. At 9%, about 8 years. At 4% (closer to current high-yield savings rates), about 18 years. The rule is a quick estimate — this calculator gives you exact projections with monthly contributions factored in.

Does this calculator account for taxes on investment gains?

No. The results assume tax-deferred growth (like a traditional 401(k) or IRA) or a taxable account where taxes aren’t explicitly modeled. If you’re investing in a taxable brokerage account, your actual after-tax returns will be lower. Investing inside a Roth IRA means qualified withdrawals are tax-free, which matches the calculator’s assumptions more closely.

Sources

• SEC Investor.gov — Compound Interest Calculator and Explainer
• Consumer Financial Protection Bureau — What Is Compound Interest?
• IRS — Retirement Plan Contribution Limits

About the Author

Nolan Briggs — Founder, Up From Zero HQ

Nolan Briggs spent years working a regular job while carrying more debt than he knew how to handle. No finance degree. No safety net. Just a lot of bad decisions and a determination to dig out of them the hard way. After paying off tens of thousands in debt and rebuilding his finances from scratch, he started Up From Zero to give other working people the plain-English money education he wished he had. Everything on this site is built for beginners — no jargon, no get-rich promises, and no shame. Just real systems that actually work for people working real jobs. Not a licensed financial advisor — everything here is plain-English education based on publicly available information, personal experience, and primary sources like the IRS, CFPB, and HUD.