Step 1. Measure your windows width at three locations (see figure 3) between the inside edge of the exterior side casings, subtract 1/8" from the longest dimension.

Step 2. Measure your windows height at two locations (see figure 4) between the bottom of the exterior head casing and the sill, subtract 1/8" from the longest dimension.

Figure 3

Figure 4

NOTE: Use the EDGE of the Exterior Casing as your measuring point (DO NOT USE THE BLIND STOP). Spencerworks All-Season Hanging Windows™ mount flush against the outer edge of the blind stop. The height of the window is equal to the longest dimension of the exterior casings outside edge minus 1/8”.

NOTE: When ordering windows oversized for trimming added 1/4" to the width found in step one. If you feel that your situation requires more than 1/4" of extra material in the storm window it may be necessary to repair your homes exterior casing to achieve a perfect fit.