Use these steps for preliminary quoting. Final manufacturing dimensions should be verified by a qualified person before the order is released.
Measure at the top, middle and bottom of the opening. Record the smallest dimension and note whether you are measuring the rough opening, frame or existing unit.
Measure left, centre and right. Record the smallest dimension and identify sill/threshold conditions.
Record wall/jamb depth, interior trim, exterior cladding, brickmould and any obstruction affecting installation.
For quote and installation review, photos of the interior, exterior and surrounding finishes are useful.