Why Use This Calculator?
Compound interest is the reason small accounts become big ones: every dollar earned starts earning its own. This calculator combines a starting lump sum with optional monthly contributions and shows the future value, the total you actually put in, and the interest the compounding did on its own.
The year-by-year schedule underneath makes the exponential curve visible — the early years crawl, the later years sprint. Switch the compounding frequency to match your account, from annually to monthly.
How It Works
- Enter your starting principal, annual rate, and time horizon.
- Add a monthly contribution if you're investing regularly — this is where growth compounds fastest.
- Read the future value and scan the year-by-year schedule to see when growth accelerates.
Frequently Asked Questions
How does a compound interest calculator with monthly contributions work?
Each month your balance grows by the periodic rate, then your contribution is added on top. Contributions themselves start compounding immediately, which is why steady monthly investing beats waiting to invest a lump sum.
Is a lump sum investment better than monthly contributions?
Historically, investing a lump sum immediately wins slightly more often because the money spends longer in the market. Monthly contributions win on discipline and on smoothing out bad timing — the calculator shows both paths.
What rate should I use for a compound growth calculator?
Long-run US stock index returns averaged roughly 7–10% a year before inflation. Use a conservative 5–7% for planning, and remember past performance never guarantees future results.
Tips & Things to Watch
- Time in the market beats timing — the last decade of the schedule does most of the work.
- Increase the monthly contribution by even $50 and re-run it: the difference compounds shockingly.
- Match the compounding frequency to your actual account; savings accounts compound monthly, many bonds annually.