Getting the right size glasses can be challenging. Generally, in order to get the right measurements for eyeglasses you will need to visit an eye doctor or an eyewear retailer.

The eye doctor or eyewear specialist will use a device called a pupillometer to measure the distance between the centers of your pupils.
They will ask you to look straight ahead and will hold the pupillometer up to your face, aligning it with the center of your pupils.
They will then take a measurement in millimeters (mm) and record the result.
Frame size measurement:
The eye doctor or eyewear specialist will measure the width of your eyeglass frames to determine the best size for you.
They will measure the distance across the lens (lens width) and the distance between the lenses (bridge width).
The lens width and bridge width are typically indicated on eyeglass frames using millimeters (mm).
Lens prescription:
Your eye doctor will conduct an eye exam to determine your eyeglass prescription.
During the exam, they will use a variety of tools and tests to assess your vision and determine the type and strength of the lenses that will help you see better.
They will also consider the size and shape of the lenses based on the frame you have chosen.