The answer provided below assumes that the curtain will be face fitted.
The number of strips will create a curtain slightly wider than the actual door opening to stop draughts creeping around the edges.
If your curtain is to be soffit fitted between jambs (DEAD WIDTH). Order the same number of strips but, when hanging, adjust the spacing of the two outer strips to make them fit.
Strip width
200mm
300mm
400mm Strip overlap
Door width
metres (to 3 decimal places)
300mm
400mm Strip overlap