Let r be the annual rate divided by 12 and by 100, and n the number of months. The starting amount becomes start × (1+r)^n. The deposits become deposit × ((1+r)^n − 1) ÷ r. A zero rate is the start plus deposit × n. Deposits land at the end of each month, so the last deposit earns no interest in this formula.
$1,000 already saved, plus $200 a month, at 5% for 2 years, ends at $6,142.13. You deposited $4,800 along the way. A withdrawal, a skipped month, or a rate that changes is a different series than the one this page runs.