Compound Interest Calculator (with monthly contributions)

What compound interest does

Compound interest = your money earning interest on its previous interest.

A simple example: $1,000 at 10% APR for 3 years.

• Simple interest: $1,000 + (3 × $100) = $1,300
• Compound interest: $1,000 × 1.1³ = $1,331

The $31 difference is small over 3 years. Over 30 years, it explodes:

• Simple: $1,000 + (30 × $100) = $4,000
• Compound: $1,000 × 1.1³⁰ = $17,449

That’s the famous “compounding is the eighth wonder of the world” effect. The longer the timeframe, the more dramatic.

With monthly contributions

If you also contribute $100/month to that 10% account for 30 years, the math gets even more interesting:

• Total you contributed: $1,000 + ($100 × 12 × 30) = $37,000
• Total final value: ~$245,000
• Compounding earned you ~$208,000 — more than 5× what you put in.

That’s why retirement accounts (401k, IRA, Roth IRA, HSA) work so well — long time horizons + monthly contributions + compounding. Starting at 25 vs 35 with the same monthly contribution typically results in 2-3× more at retirement.

Realistic interest rates (US, 2026)

• High-yield savings (HYSA): 4-5% APR — short-term, FDIC-insured
• CDs (1-5 year): 4-5% — locked-up savings
• Treasury bonds (10y): 4-5% — government debt
• Corporate bonds: 5-7% — higher yield, default risk
• S&P 500 historical: ~10% nominal, ~7% real (after inflation), but with high volatility — short timeframes can see -30%+ years
• Real estate (REITs): 8-10% historical including dividends

For long-term retirement planning, 7% real (inflation-adjusted) is the standard conservative assumption.

Compounding frequency — does it matter?

Marginal at most rates and timeframes. The differences:

• Annual compounding: 1 + r once per year
• Monthly compounding: (1 + r/12)^12 ≈ 1 + r + tiny bit more
• Daily compounding: (1 + r/365)^365 ≈ 1 + r + slightly more

At 7% annual rate over 20 years: $1,000 grows to $3,870 (annual) vs $4,022 (monthly) vs $4,055 (daily). Real impact: a few percent over decades. Most savings accounts compound daily; investment accounts compound continuously through reinvestment of dividends/interest.

Frequently asked questions

faq: - q: “Does this account for inflation?” a: “No — this calculates nominal growth. To get inflation-adjusted (‘real’) returns, use a real rate of return: stock market 7% real (10% nominal − 3% inflation), bonds 1-2% real. The dollars at the end will be in today’s purchasing power.” - q: “What about taxes?” a: “Not modeled here. Tax-advantaged accounts (Roth IRA, 401k, HSA) shelter compounding from tax — these are dramatically more powerful than taxable accounts over long timeframes. In a taxable brokerage, you owe capital gains on dividends and at sale; this can reduce effective return by 0.5-1.5% annually.” - q: “Can I use this for mortgage payoff or loan calculations?” a: “This calculator is for INVESTMENT growth (positive contributions). For loan amortization (paying down debt), the math is similar but inverted. Look for a mortgage calculator instead.” - q: “How do I use this for retirement planning?” a: “Set principal = current retirement savings, monthly contribution = your monthly retirement savings amount (incl. any employer match), years = years until retirement, rate = 7% (conservative real return). The final value is your retirement balance in today’s dollars. Compare against expected expenses × 25-33 years (the 4% rule).”

Related calculators

• FIRE calculator — financial independence number
• Hourly to salary — annual income calculator
• Net profit margin — business profitability
• Pay raise — salary increase impact

Sources

• SEC investor.gov — Compound Interest Calculator
• Investopedia — Compound Interest
• Federal Reserve Economic Data (FRED)

The Rule of 72

A famous mental shortcut for compound growth:

Years to double = 72 ÷ interest rate

So at 7% APR: 72/7 ≈ 10.3 years to double your money. At 10% APR: 72/10 = 7.2 years. At 4% APR: 72/4 = 18 years.

Useful for back-of-napkin retirement projections.

Realistic US tax-advantaged accounts

For long-term compounding, tax-advantaged accounts compound dramatically faster than taxable accounts:

• Roth IRA: $7,000/year (2025) — tax-free growth and withdrawals
• Traditional 401(k): $23,500/year (2025), often with employer match — pre-tax contributions
• HSA: $4,300 self / $8,550 family — triple-tax-advantaged with HDHP
• Roth 401(k): $23,500/year — same limits as traditional but post-tax

A 30-year-old maxing Roth IRA + 401(k) at $30k/year for 30 years at 7% real returns ends with ~$3M in 2025 dollars.

Last verified: April 2026.