Using a size guide for glasses seemed unnecessary until I kept buying frames that didn't fit. Now I never shop without consulting one.
A good size guide explains:
Step 1: Measure My Face
I measure from cheekbone to cheekbone: 142mm
Step 2: Find My Size Category
142mm face width = Medium sizing (130-140mm frames)
Step 3: Determine Specific Measurements
Step 4: Shop Within My Range
I only consider frames that match these measurements.
Following a size guide eliminated:
Take five minutes to use a size guide properly. It's the difference between glasses that fit perfectly and glasses that constantly frustrate you.