Fitting a fresh sheet of glass to an existing window frame depends mainly on cutting the glass to just the right size: too large and it wo not fit, too small and it will not fill the hole. The distance where the glass must fit is bigger than the hole you see, so simply quantifying across that hole will not get the job done. You also must determine the dimensions of the recess to which the glass fits, a measurement known as the “tight fit.”
Locate the outer borders of the glazing beading all over the frame. Glazing beading is that the strips of wood or plastic which overlap the edges of the glass and hold it from the window frame.
Determine the horizontal “tight fit” measurements by measuring the horizontal distance between the two outer borders of the beading on both sides of the window frame at the top and bottom of the frame. Record the dimensions as the bottom and top “tight-fit” values.
Determine the vertical “tight fit” measurements by measuring the vertical spaces between the bottom and top outer borders of the beading on the right and left borders of the frame. Again record the dimensions.
Deduct 1/4 inch from each of the recorded dimensions. For example, if the width throughout the top of the frame is 30 inches, bend 1/4 inch to create a new width of 29 3/4 inches. The adjusted measurements are the values to use for cutting the new sheet of glass. A 1/8 inch is deducted from each side of every dimension so that the cut glass is slightly smaller than the frame. This allows the frame to contract and expand without squeezing and fracturing the glass. Because 1/8 of an inch is removed from each side, the complete removed is 1/4 inch.