Calculate final amount, total contributions and interest earned with optional monthly contributions.
Compound interest (no contributions): A = P × (1 + r/n)^(n×t), where P = principal, r = annual rate (decimal), n = compounds per year, t = years.
With monthly contributions: Future value of initial amount plus future value of the contribution series.
Monthly contributions are assumed to be deposited at the end of each period (ordinary annuity model).
Interest earned on both the initial principal and on interest already earned.
Use A = P(1 + r/n)^(nt). With regular contributions, add the future value of each contribution.
Each period you earn interest on the previous balance, so growth accelerates over time.