Compound interest calculator

How compounding builds wealth over time

Compound interest means your interest starts earning its own interest. Each period, the bank (or the market) pays interest not just on what you originally put in, but on every dollar of interest that has already accumulated. In the early years the effect looks small, but the curve gets steeper the longer money is left untouched — which is why starting early usually matters more than the exact rate you earn.

The formula behind this calculator

• A = P × (1 + r/n)^(n×t) — where P is your starting amount, r is the annual rate as a decimal, n is how many times per year interest compounds, and t is time in years.
• Monthly contributions are added at the start of each month and grown for whatever time remains until the end of the period, then summed together with the principal's own growth.

Reading the yearly table

The breakdown table shows exactly how much of each year's growth comes from your own deposits versus interest. Early on, deposits usually make up most of the balance growth. Later, interest can end up contributing more per year than your contributions do — that crossover point is often called the moment compounding "takes over."

Things this calculator doesn't account for

This tool assumes a constant interest rate for the entire period and doesn't factor in taxes on interest income, investment fees, or inflation eating into real purchasing power. Treat the result as a useful estimate for planning, not a guaranteed outcome.