Compound Interest Calculator with Monthly Contributions
Compound interest means your interest earns interest — the engine behind every long-term fortune. Enter a starting amount, monthly contribution, and expected return to watch the curve bend upward year by year.
- Total contributed
- $70,000.00
- Total interest earned
- $100,619.05
Growth by year
| Year | Contributed | Interest earned | Balance |
|---|---|---|---|
| 1 | $3,000.00 | $821.05 | $13,821.05 |
| 2 | $3,000.00 | $1,097.27 | $17,918.32 |
| 3 | $3,000.00 | $1,393.46 | $22,311.78 |
| 4 | $3,000.00 | $1,711.07 | $27,022.85 |
| 5 | $3,000.00 | $2,051.63 | $32,074.48 |
| 6 | $3,000.00 | $2,416.81 | $37,491.29 |
| 7 | $3,000.00 | $2,808.39 | $43,299.69 |
| 8 | $3,000.00 | $3,228.28 | $49,527.97 |
| 9 | $3,000.00 | $3,678.53 | $56,206.50 |
| 10 | $3,000.00 | $4,161.32 | $63,367.82 |
| 11 | $3,000.00 | $4,679.01 | $71,046.83 |
| 12 | $3,000.00 | $5,234.13 | $79,280.95 |
| 13 | $3,000.00 | $5,829.37 | $88,110.33 |
| 14 | $3,000.00 | $6,467.65 | $97,577.98 |
| 15 | $3,000.00 | $7,152.07 | $107,730.04 |
| 16 | $3,000.00 | $7,885.96 | $118,616.00 |
| 17 | $3,000.00 | $8,672.91 | $130,288.91 |
| 18 | $3,000.00 | $9,516.74 | $142,805.65 |
| 19 | $3,000.00 | $10,421.58 | $156,227.23 |
| 20 | $3,000.00 | $11,391.83 | $170,619.05 |
How this calculator works
- Each month the balance grows by rate ÷ 12, then your contribution is added. Over decades the interest-on-interest term dominates: at 7% for 30 years, roughly two-thirds of the final balance is growth, not deposits.
- 7% is a common planning assumption for diversified stock portfolios after inflation; savings accounts and CDs run far lower.
FV = P(1 + r/12)¹²ʸ + PMT × [((1 + r/12)¹²ʸ − 1) ÷ (r/12)]
Frequently asked questions
How often should interest compound?
More frequent compounding helps slightly — $10,000 at 5% for 10 years yields $16,470 monthly-compounded vs $16,289 annually. Rate matters far more than frequency.
What return should I assume?
Historically US stocks returned ~10% nominal / ~7% after inflation. Conservative planners use 5–7%; using 10%+ risks painting an unrealistically rosy picture.
What's the rule of 72?
Divide 72 by your annual return to estimate doubling time: at 8%, money doubles roughly every 9 years.
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Disclaimer: Results are estimates for educational purposes only and are not tax, legal, or financial advice. Tax rules are simplified (credits, phase-outs, and local taxes may not be modeled). Verify important decisions with the IRS, your state tax authority, or a licensed professional.