Glasses frames are measured in millimeters. The best place to pick up a new perfect fit is to start with the measurements printed on your current glasses. Once you get the detailed measurements, you can go with the style you like. 
*Click to know how to select the frame style based on the face shape.

1. Read the Measurements Printed on the Frame and Measure the Frame

First of all, search for the three little numbers stamped on the inside of the temple of your glasses. It's usually printed as "XX XX XXX" or "XX-XX-XXX".
The measurements in picture "49☐24 140" are lens width, bridge width, and temple length. Frame size also refers to other detailed measurements like lens height and frame width.

Getting the following accurate measurements of your current glasses can better help you locate the perfect fit from our hundreds of stylish frames. Keep in mind that all these are in millimeters.

1. Lens Width      49-24-140
It is the horizontal width of each lens at its widest point. It typically ranges from 40 to 60 mm.
This measurement plus lens height determines whether the lenses look big or small on your face.

2. Bridge Width    49-24-140
It is the distance between your two lenses which is the space where your frames fit against your nose. The size of this piece ranges from 14 to 24 mm.
This measurement plus temple length indicates if the glasses will sit stably and comfortably on your face.

3. Temple Length   49-24-140
It is the distance from each screw to its temple tip, including the bend that sits on your ear. The temple is typically 120 to 150 mm long.
Temple length and bridge width indicate if the glasses will sit stably and comfortably.

4. Lens Height  (measure it yourself)

It is the vertical height of each lens at the highest point of the lens within the frame. It is vital for bifocals and progressives production.
This and lens width determines whether the lenses look big or small on your face.

5. Frame Width  (measure it yourself)
It is the horizontal width across the front of the frame from one screw to the other. It is necessary to find out if the frame is wider or narrower than your face.
No more than one finger width between the temple and your face is better.

Frame Width Standard Chart
Zinff has four frame width standards (XS, S, M, L). You may need to pay attention to the lens size (lens width and height) and temple length to make sure your new glasses correctly fit your face.

2. Search the Perfect Fit due to the Measurements You Got

A perfect pair of eyeglasses are always sitting stably and comfortably on your face to give you clear vision. Your eyes should be directly in the center of the lens, from left to right, and fill the top half of the lens. The bottom of the eye should roughly touch the vertical midpoint of the lens.
Opticians usually suggest that you keep the lens size within 2 mm of your current frame (unless you are intentionally choosing an over-sized style), within 2 mm of your current bridge size, and within 5 millimeters of your current frame's temple length.

Step 1: Find the Perfect Frame Width
A glasses frame that is too narrow will make your face look wider, while a wide glasses frame will make your face look too narrow. Your perfect fit should be slightly wider than your face - no more than a finger's breadth (3/4 of an inch or 19 mm) between the temple and your face.
If your current frame doesn't fit, stick with the "one finger-breadth" rule when selecting frames. If your glasses are a little wider or narrower, you can slightly bend the temples outward.
*Click here to find out more information about adjusting the frame to fit.

Step 2: Get Cozy with the Temple Arm
The temple should fit horizontally and evenly along your face and fit comfortably along the outside of your ear.
If you select a frame with longer temples than your current ones, you may have a problem with glasses sliding down your nose. However, you can still have the temples bent to keep them in place. 
*Click here to find out how to adjust the frame to fit.

Step 3: Pick the Right Lens Height and Width
Lens height and width indicate the size of lenses. Your eyes should be directly in the center of the lens, from left to right, and fill the top half of the lens. The bottom of the eye should roughly touch the vertical midpoint of the lens.
If the lens size is too big, it may cause you vision issues (blurry vision, headache) with the glasses. The bottom frame rim may touch your cheeks as well. The most important is that the lens would be thicker than the smaller frame, especially with a strong prescription.

Step 4: Select the Ideal Bridge Width
The bridge of your glasses should sit stably and comfortably against your nose. If your PD measurement is small, the ideal bridge width is a lower number. Otherwise, a higher number is better. 
*Click to find out what PD is and how to measure it.
You can calculate the frame PD comparing to your PD to check if the frame will sit properly on your face: Frame PD = Lens Width+Bridge Width.