Measure the opening from the inside
Interior frames fit at a specific mounting plane inside the existing window. Record actual measurements to 1/16 inch. Do not average the sides or make your own installation deductions.
Choose the mounting position
Decide exactly where the storm frame will sit. Compression windows normally need 3/4 inch of usable depth; a 5/8-inch recess requires review. Double hung and sliding systems need 1 1/8 inches. Surface-mount panels overlap the glass area, so record the intended finished panel size and available mounting surface.
Measure all four sides
Use a locking tape measure, a framing square, and a small ruler. Record the top width, bottom width, left height, and right height separately. Avoid bending the tape into a corner; include the tape-case length when measuring that way. Label every window by room and opening.
Check the corners and sill
Hold a framing square against each side and lower it to the sill. Record the gaps at both bottom corners rather than assuming the opening is square. Check whether the sill bows up or down; if it does, add a center-height measurement and a sketch. Photos make these conditions easier to review.
Check access and obstructions
Look for locks, handles, blinds, projecting screws, and changes in the opening between the mounting plane and the room. A panel that fits its final position still needs clearance to pass into that position. Report obstructions or narrowed access before ordering.
Record rails and panel divisions
For a double hung system, measure from the sill to the top of the original meeting rail. For a triple slider, record the existing center-panel width. A double hung opening taller than 72 inches generally needs review for two separate units with a horizontal mullion. Record the whole opening and proposed division without subtracting mullion dimensions yourself.
Flag special glazing locations
Tell us about locations within 12 inches of a door or 18 inches of the floor. The manufacturer requires acrylic or tempered glazing there. Standard glass panels are limited to 25 square feet and 125 united inches; larger panels need a glazing review and a confirmed price.
Review before manufacturing
Online pricing uses dimensions rounded to the nearest whole inch for the frame charge and glazing area rounded up to the next square foot. Your fabrication worksheet still needs exact measurements to 1/16 inch. Keep pricing measurements and final manufacturing dimensions separate, and confirm the complete worksheet before production.