Compound Interest Calculator
Start with a lump sum, add monthly if you like, and watch compounding do the heavy lifting — for stocks, funds, deposits, property or any asset with an annual return.
Your numbers
Year-by-year breakdown
| Year | Contributed | Interest | Balance |
|---|
How compound interest works
Compound interest means you earn returns on your returns. In year one, only your initial deposit grows. In year two, the deposit plus year one's interest grows — and so on. Over short periods the difference from simple interest is small; over 15–30 years it becomes the majority of your final balance. That curve you see in the chart bending upward is the compounding effect.
The classic formula is A = P(1 + r/n)nt, where P is your starting amount, r the annual rate, n the number of compounding periods per year and t the years invested. When you add monthly contributions, each deposit starts its own compounding clock — this calculator simulates every month so the contributions are treated exactly, not approximated.
What the results mean
- Future value — your projected balance at the end of the period.
- Total contributed — initial amount plus every monthly deposit: the money that came from you.
- Interest earned — the part compounding added. The longer the horizon, the bigger this slice.
- Growth multiple — future value divided by contributions. 2× means your money doubled.
Tips for realistic projections
- Use real (after-inflation) rates if you want the answer in today's purchasing power — subtract roughly 2–4% from your nominal estimate.
- Fees compound too: a 1% annual fee on an 8% return removes far more than 1% of your final balance.
- The rule of 72 is a handy check: money doubles in about 72 ÷ rate years (9 years at 8%).