Guide
Door and window seal calculator
Weather stripping is sold by the roll. Measure the opening, and this works out the run you need and whether one roll covers it.
How much stripping does the opening need?
A tenth is added for corners, offcuts and the piece that gets mismeasured. Roll lengths offered in this range: 26 ft, 40 ft, 60 ft, 90 ft.
Measuring beats estimating here, because stripping that lands a few inches short across the top of a door leaves exactly the gap the roll was bought to close. Measure each opening rather than one and multiplying, since frames in the same house are rarely identical.