Finance
Compound interest calculator
See how a starting deposit grows over time when interest compounds and you optionally add monthly contributions. Pick the compounding frequency (annual to daily) and the rate to match your savings account, CD, or investment.
Formula
Compound interest is interest earned on the original principal AND on the accumulated interest from previous periods. The more often interest compounds, the more total return you get for the same nominal rate — though the difference between monthly and daily compounding is small at typical rates.
The formula treats monthly contributions as an ordinary annuity (payments at the end of each period). If your account credits contributions at the beginning of the period instead, your real future value will be slightly higher than this calculator shows. The difference is one period's interest on each contribution.
This calculator ignores fees, taxes, inflation, and variable rates. For US tax-advantaged accounts (401k, IRA, Roth), reduce the rate by 0.5–1.5%/yr for expense ratios. For inflation-adjusted purchasing power, subtract expected inflation (~2–3%) from the rate.
Examples
- 01$10,000 at 7% for 10 years, monthly compounding, no contributions→ FV ≈ $20,096.61 (interest $10,096.61 — money roughly doubles in 10 years)
- 02Same start + $100/month contribution→ Contributions $22,000 · FV ≈ $37,400 · Interest $5,304 above just contributions
- 03$5,000 at 5% APR for 30 years, annual compounding→ FV ≈ $21,609.71 (4.32× growth from compounding alone)
FAQ
- Divide 72 by the annual rate to get a rough doubling time: 6% → ~12 years to double, 9% → 8 years. It overestimates slightly for high rates and underestimates for low ones, but it's accurate within a year for 4–12% rates.