Measure the opening three times
Width is taken at the bottom, middle, and top of the opening. In an older house these three numbers commonly differ by a quarter inch or more, and the glass has to be cut to the smallest of them or to the actual taper.
Check plumb on both jambs
A level held against each side wall tells you how far the opening leans and in which direction. That reading determines whether the door can hang square, whether a wall channel can absorb the variance, or whether the panel edge needs to be cut to the lean.
Do not forget the curb
Tiled curbs are usually sloped inward on purpose. The slope changes where the sweep sits and how the door closes, so the curb gets measured for both width and pitch.
Common mistakes
Most fit problems trace back to one of these:
- Measuring before the tile is finished, then ordering to those numbers.
- Taking one width instead of three.
- Ignoring the plumb reading because the wall looks straight.
- Forgetting to check how the panel will physically reach the bathroom.
Why we template after tile
Tempered glass cannot be trimmed once it is made. Templating the finished surfaces is the only way to guarantee the panel fits the room you ended up with rather than the room on the plan.
The short version
If a supplier quotes fabrication from numbers you read off a tape measure, expect to own the fit risk yourself.