Monthly compound interest calculator

Key terms

Monthly compounding

Interest added every month; next month earns on the new balance.

FV of annuity

Sum of regular contributions + interest on each. The math behind monthly DCA.

Dollar-cost averaging

Fixed monthly amount invested regardless of price. The "monthly contribution" field models this.

Why monthly compounding is the default for investing

Monthly compounding is the convention for long-term financial planning because most people invest monthly (paycheck contributions to 401(k), automated IRA deposits, DCA into index funds) and most mortgages amortize monthly. The standard formula A = P × (1 + r/12)12×t matches how real cash flows actually move in and out of accounts.

The 30-year picture: $500/month with monthly compounding

At 8% annual return (a conservative long-term S&P 500 nominal default):

• 10 years → $91,473 (you contributed $60,000)
• 20 years → $294,510 (you contributed $120,000)
• 30 years → $745,180 (you contributed $180,000)
• 40 years → $1,747,477 (you contributed $240,000)

Notice the gap between contributions and final balance widens dramatically. By year 30, compound interest has produced 4× your total contributions. By year 40, 6×. This is the mathematics behind why starting in your 20s makes a million-dollar difference vs starting in your 30s — see the cost-of-waiting index.

Monthly vs daily vs annual compounding — actual numbers

$10,000 lump sum at 7% APR over 20 years:

• Annual compounding: $38,697
• Monthly compounding: $40,387
• Daily compounding: $40,545
• Continuous compounding: $40,552

Monthly captures 99.6% of the benefit of continuous compounding. The bigger lever is the rate, time, and contribution size — not compounding frequency. See our APY converter to convert any APR to its true APY.

When to use monthly compounding vs other frequencies

• Monthly: retirement accounts (401k, IRA, Roth IRA), index funds, mortgages, long-term savings goals — the standard for financial planning
• Daily: high-yield savings accounts, CDs, money market accounts. Use our daily compound interest calculator for short-term cash projections
• Annual: bonds with annual coupons, some certificates, simple academic problems
• Continuous: theoretical maximum — rare in real-world finance

The compound interest formula explained

For a lump sum: A = P × (1 + r/n)n×t

• A = future value
• P = principal (initial amount)
• r = annual rate (as decimal — 8% = 0.08)
• n = compounding periods per year (12 for monthly)
• t = time in years

For monthly contributions added on top: FV = PMT × [(1 + r/n)n×t − 1] / (r/n)

This calculator computes both simultaneously and visualizes year-by-year growth.

The single biggest compound-interest mistake

Treating "compound interest" as a passive trick that runs by itself. Compound interest only works if you keep contributing during market drops. The investor who panic-sold in March 2020 saw $0 of the next-5-year compound effect. The one who kept contributing saw their balance triple. Behavior beats math.

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FAQ

Methodology & sources

Formula: A = P(1 + r/12)^(12t) + PMT × [((1 + r/12)^(12t) − 1) / (r/12)]. S&P 500 historical returns verified against Shiller CAPE dataset; long-run nominal ≈ 10%, real ≈ 7%. Updated 2026-06-15. See editorial standards.