Why these calculators are different
Most material calculators hand you a number and leave you to trust it. These show the formula, a worked example and a reference table on the same page, so a contractor can sanity-check the result and a homeowner can learn what the number means.
Nothing here quotes a price. Material costs move constantly and vary by region and supplier, so a calculator that bakes them in is wrong the week after it is published. Where cost matters, you enter your own figure from your own quote.
Every result is an estimate for planning. Measure twice on site, allow for waste, and check your local building code before you order.