Fixed-rate growth model
Set a starting sum, a rate, a monthly deposit and a horizon. The engine runs the balance forward month by month and separates what you added from what growth added.
Compounding is the process by which the returns on an investment themselves begin to earn returns. In the early years the balance grows mostly because of what you put in; over time, the growth on prior growth quietly takes over and becomes the dominant force. This calculator makes that crossover visible.
The engine assumes interest compounds monthly and that each deposit lands at the end of the month. It is deliberately simple: no taxes, no fees, no inflation adjustment. The point is not a precise forecast of any real account but a clear feel for how the shape of compounding behaves as you change the inputs.
Enter four numbers: the amount you start with, the annual return you expect, the amount you add each month, and the number of years you will leave it alone. Every keystroke re-runs the model instantly — nothing is sent anywhere, and all the arithmetic happens in your browser.
Read the three output registers together. ‘Final balance’ is what you end with. ‘You contributed’ is the sum of your starting amount plus every deposit. ‘Growth on growth’ is the difference — the part compounding produced for you rather than you saving it.
Watch the plot line rather than just the final number. A straight line would mean simple, linear saving; the upward curve is compounding at work, and the steeper it bends near the end, the more of your result is coming from growth rather than deposits. The readout line beneath the console shows the multiple on your deposits — how many times your contributions your final balance represents.
Use it to build intuition before a long-term decision — whether to start investing now versus later, how much a small monthly increase changes a decades-out result, or why time in the market tends to matter more than the size of any single deposit.
Start with $1,000, add $200 a month at an 8% annual return for 30 years. You contribute $73,000 in total, but end with roughly $309,000 — meaning about $236,000, over three-quarters of the result, is growth the deposits never touched. Cut the horizon to 20 years and the growth portion collapses far more than the balance, showing how much of compounding's power lives in the final stretch.
Underlying idea: Compounding ›