How it works
Your savings grow by the yearly return divided by 12 each month, with the contribution added at the end of the month, until college starts. Each year of college costs today's price grown by college inflation for every year until then, so a $28,000 year that starts in 15 years at 4% costs about $50,400. Your goal is the share of the total you want to cover. The monthly amount needed is the contribution that makes the projected savings equal the goal; the lump sum is what you could add today instead. The spreadsheet does this for up to four children, with a balance for every birthday.