Compound interest formula
Without contributions: A = P (1 + r/n)nt, where P is principal, r annual rate (decimal), n compounds per year, t years.
With a fixed contribution at the end of each period, the contribution future value is PMT × [((1 + r/n)nt − 1) ÷ (r/n)].
Example
$5,000 at 7% for 10 years, compounded monthly, with $0 extra deposits grows to about $10,048 (rounded). Adding $100 per month increases the final balance substantially through both deposits and interest on deposits.
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