The formula
For a single deposit, the balance after t years is A = P(1 + r/n)^(nt), where P is the starting amount, r is the annual rate as a decimal and n is the number of compounding periods per year. Regular contributions add PMT × ((1 + r/n)^(nt) − 1) ÷ (r/n), which assumes each contribution is made at the end of its period. With a 0% rate the balance is simply the starting amount plus all contributions.
How compounding frequency matters
Compounding more often means interest starts earning interest sooner. 1,000 at 5% for 10 years grows to 1,628.89 when compounded annually and 1,647.01 when compounded monthly. Adding 100 at the end of every month brings the monthly-compounded balance to 17,175.24, of which 13,000.00 is money you put in.
Reading the results
The final balance, total contributions and total interest are rounded to the cent once, and interest is calculated as the difference, so the three figures always add up exactly. The year-by-year table uses the same formula for each year; its last row is the final balance.
Limitations
The calculator assumes a constant rate for the whole period and ignores taxes, fees and inflation. Real investment returns vary from year to year, so use the result as an illustration of how growth compounds, not as a forecast.