You'll need numbers from both the outside and inside of the skylight. That means the first numbers will be found on the roof. Once you're up there, measure along the side of the skylight where the shingles meet the flashing, width first then length. Next, measure the height of your skylight or curb.

How are inside skylights measured?

For non-VELUX skylights, measure the drywall opening length and width on the inside of the house, the length and width of the exterior glass inside the cladding and the overall size of the skylight (inclusive of cladding).

How do I know what size skylight to buy?

As a rule of thumb, the skylight size should never be more than 5% of the floor area in rooms with many windows and no more than 15% of the room's total floor area for spaces with few windows. If you want to maximize daylighting and/or passive solar heating potential, consider a skylight's position.