No tape measure required. For the right size, find the window code reference located at the top of the open VELUX roof window Compatible with VELUX roof windows with reference codes: GGL U08, GGU U08, GPL U08, GPU U08, VFE UK38, VIU UK38 An inexpensive option from VELUX, ideal for replacing old and worn blinds Manually operated stepless positioning, almost complete blackout effect Easy installation – Tailored for VELUX roof windows for a perfect fit. Install in minutes with the pre-fitted mounting system. If mounting brackets aren't pre-fitted, you'll need to purchase the Z0Z 230 separately