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MoneyMath

Compound Interest Calculator — With Every Step Shown

See your investment grow with transparent compound interest math. Unlike the SEC's black-box calculator, this one shows the formula and your actual numbers substituted — so you can verify.

🟢 Updated April 2026👤 Reviewed by MoneyMath Editorial⚡ Runs in your browser · inputs never leave your device
$
$
After 30 Years
$691,150
$501,150 growth · 264% total return
Starting amount$10,000
Total contributions$190,000
Interest earned$501,150
From initial deposit$81,165
From monthly contributions$609,985
Final balance$691,150
Show the formula
A = P(1 + r/n)^(nt) + PMT × [(1+r/n)^(nt) − 1] / (r/n)
where: P = principal, r = rate, n = compounding periods/yr
t = years, PMT = monthly contribution × 12 / n

Compound interest: the 8th wonder of the world

Albert Einstein (probably apocryphally) called compound interest "the most powerful force in the universe." The math is boring; the result is astonishing. $500/month at 7% for 30 years = $612,000. Of that, only $180,000 is your contributions. The remaining $432,000 is interest on interest on interest.

The rule of 72 (quick mental math)

To estimate doubling time, divide 72 by your rate. At 7%, money doubles every ~10.3 years. At 10%, every 7.2 years. At 4% HYSA, every 18 years. Useful for napkin math.

Why "start early" matters more than "invest more"

A 25-year-old who invests $5,000/year for 10 years then stops ends up with MORE at age 65 than a 35-year-old who invests $5,000/year for 30 years straight. Time in the market beats timing the market.

Typical return assumptions

  • S&P 500 long-term: ~10% nominal, ~7% after inflation (real return)
  • Diversified stock/bond portfolio: 6-8% nominal
  • High-yield savings (HYSA): 4-5% in 2026, but taxable
  • Treasury bonds (10-year): 4-5% tax-advantaged at state level
  • TIPS (inflation-protected Treasuries): ~2% real above inflation
  • Crypto: Highly variable. Don't use this for retirement planning.

Real returns vs nominal returns

When you read "S&P 500 returns 10% historically," that's nominal (before inflation). Real returns (inflation-adjusted purchasing power) average 6-7%. Use nominal for retirement projections but adjust your target retirement number for inflation too — $1M in 2055 has the purchasing power of about $500k today at 2.5% inflation.

Compounding frequency doesn't matter much

Monthly vs daily compounding on a 7% rate for 30 years differs by less than 1%. The marketing hype around "daily compounding" on savings accounts is mostly marketing. What matters: the rate itself, contribution amount, and time.

Frequently Asked Questions

Why show the formula?

Because calculators that hide the math are indistinguishable from magic. We believe showing the formula with your actual numbers substituted builds trust AND teaches people how interest actually works. Other major calculators (SEC, Bankrate, NerdWallet) hide the formula — we show it.

Should I use 7%, 8%, or 10% as my return assumption?

Conservative: 6-7%. Standard retirement planning: 7-8%. Aggressive: 9-10%. The S&P 500 has averaged ~10% nominal since 1950, but using the historical mean ignores sequence-of-returns risk. Most financial planners recommend 6-7% for retirement projections.

How do taxes affect compound interest?

In a taxable account, dividends and interest are taxed annually, dragging returns. In a 401(k) or IRA, all growth is tax-deferred (or tax-free in Roth). The difference compounds: a taxable account growing 7% might net 5.5% after tax drag — losing 30% of final value over 30 years.

What about bear markets and crashes?

This calculator assumes smooth returns. Reality has bumps. The 2008 crash wiped 50% of portfolios in 18 months. Sequence of returns matters near retirement. For serious planning, use Monte Carlo tools (Engaging Data, PortfolioVisualizer) that simulate thousands of market paths.

Is this safe for retirement planning?

As a starting point, yes. As the sole basis for a retirement decision, no. Use this to get intuition, then use dedicated retirement tools (Fidelity, Schwab, Empower) for personalized projections, and consult a fiduciary financial planner for life-affecting decisions.