Compound interest calculator
Free compound interest calculator with monthly contributions and visual charts. Works for savings accounts, HYSAs, CDs, and index funds — daily, monthly, or annual compounding.
How to use this compound interest calculator
Enter four numbers: your initial amount, optional monthly contribution, an annual interest rate, and the number of years you'll let it grow. The calculator solves the standard compound interest formula and shows the result as a chart and a year-by-year table.
Use this calculator for any product that compounds: savings accounts, high-yield savings (HYSAs), CDs, money market accounts, Treasury bills rolled over, or long-term index fund investments.
The compound interest formula
With contributions added at the end of each period, future value equals:
FV = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)] P = principal (initial deposit) r = annual interest rate (decimal) n = compoundings per year (1, 12, or 365) t = number of years PMT = contribution per compounding period
The first term is your principal compounding. The second is the future value of an ordinary annuity — every contribution you make also compounds, from the day it's deposited.
What rate should I use?
- High-yield savings accounts: 4–5% APY in 2026. Use today's rate; HYSA rates float with the Fed funds rate.
- CDs (1–5 year): 4–5.25% depending on term. Use the locked APY.
- S&P 500 index funds: ~10% nominal long-term average, ~7% after inflation. Use 7% for conservative real-return projections.
- Balanced 60/40 portfolio: 6–8% nominal long term.
Compound interest examples
| Starting | Monthly | Rate | Years | Final value |
|---|---|---|---|---|
| $1,000 | $100 | 7% | 30 | ~$130,000 |
| $5,000 | $250 | 7% | 30 | ~$345,000 |
| $10,000 | $500 | 7% | 25 | ~$460,000 |
| $10,000 | $500 | 10% | 30 | ~$1.13M |
| $25,000 | $1,000 | 8% | 30 | ~$1.65M |
Daily vs monthly vs annual compounding
More frequent compounding earns slightly more at the same nominal rate, but the difference is tiny. At 5% APR over 30 years on $10,000:
- Annual compounding: $43,219
- Monthly compounding: $44,677
- Daily compounding: $44,812
Daily beats monthly by 0.3% over 30 years — meaningful, but rate and contribution size matter far more. Don't chase compounding frequency; chase higher APY.