The Glasses Fit Calculator compares the prescription your doctor prescribed with the prescription you are actually experiencing based on how your glasses fit. By entering both the written prescription and real-world fitting parameters, it determines whether your optics remain within accepted standards or drift out of spec. Poorly fit glasses can effectively alter a prescription to the same extent as an incorrect prescription being written, which is why lens alignment is just as critical as lens design and material.
Glasses Fit Reality Check
Move the sliders to see how small fitting errors can change what your eyes actually look through.
Choose a standard
Educational tool. Labs may target tighter values, but these are widely used minimum tolerances.
How the frame sits
Typical range is about 10 to 18 mm.
Often assumed during lens measurement or refraction setup.
Your prescription
Right eye
Enter the numbers from your prescription
PD error, right lens center
0.0 mm
Negative values: lens center shifts toward your nose (glasses PD too narrow). Positive values: lens center shifts toward your temple (glasses PD too wide).
Height error, right lens center
0.0 mm
Negative values: lens center sits lower than your pupil. Positive values: lens center sits higher than your pupil.
Left eye
Same idea, other eye
PD error, left lens center
0.0 mm
Negative values: lens center shifts toward your nose (glasses PD too narrow). Positive values: lens center shifts toward your temple (glasses PD too wide).
Height error, left lens center
0.0 mm
Negative values: lens center sits lower than your pupil. Positive values: lens center sits higher than your pupil.
Results