Compound Interest Calculator

About the Compound Interest Calculator

Compound interest is interest earned on interest: each period, the account grows by its rate, and the next period's growth is calculated on that larger balance rather than just the original principal. With no contributions, this follows the closed-form formula A = P × (1 + r/n)^(n×t), where P is principal, r is the annual rate as a decimal, n is compounds per year, and t is years. This calculator adds a monthly contribution on top, so instead of the closed form it simulates month by month: each month it grows the current balance by the equivalent monthly rate, then adds your contribution, and repeats for the full term.

To use it: enter a starting principal, an optional monthly contribution, an annual rate, how many years to project, and how often interest compounds (more frequent compounding — daily versus annually — produces slightly more growth at the same nominal rate). For example, $10,000 invested at 7% annually for 20 years with monthly compounding and no contributions grows to roughly $40,400 — over $30,000 in interest on $10,000 of principal, purely from compounding. Add a $200 monthly contribution and the same 20 years instead ends around $144,500, since each contribution also gets years to compound before the term ends. Use the year-by-year table and CSV export to see exactly how the balance, contributions, and interest build up over time.