Calculator
Compound Interest Calculator
🧮 Calculate Your Compound Growth
How It Works
The Math Behind Compound Interest
The compound interest formula: A = P(1 + r/n)nt
Where: A = final amount | P = principal (starting amount) | r = annual interest rate | n = compounding periods per year | t = time in years
For monthly compounding (the standard for most investment accounts): n = 12. Most retirement calculators and investment accounts compound monthly.
💡 The Rule of 72: Divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 6% → 12 years. At 8% → 9 years. At 10% → 7.2 years.
| Annual Return | 10 Years | 20 Years | 30 Years | Years to Double |
|---|---|---|---|---|
| 5% | $113,700 | $222,700 | $429,700 | 14.4 years |
| 7% | $124,400 | $275,400 | $601,900 | 10.3 years |
| 8% | $129,900 | $307,200 | $729,600 | 9.0 years |
| 10% | $142,500 | $385,900 | $1,082,000 | 7.2 years |
Assumes $10,000 starting amount + $500/month contributions. Values rounded. Returns are illustrative; actual investment returns vary.
Frequently Asked Questions
What is compound interest?+
What is the Rule of 72?+
How does the employer 401(k) match affect compound interest?+
What return rate should I use for retirement planning?+
📚 Sources
- Standard financial mathematics: Time Value of Money (TVM) formulas
- Vanguard Research, The Case for Low-Cost Index Funds
- Charles Schwab, Understanding Investment Returns and Compound Growth
- IRS, Retirement Plan Contribution Limits 2026 (irs.gov)
- S&P 500 historical return data: Bloomberg (nominal ~10%/yr; real ~7%/yr)
- Plootus Research 2026
