- Purchasing eyewear online seems daunting at first. If you're like me, purchasing and fitting prescription glasses always seemed like a complex procedure best left to professionals. However, if you know your prescription and have your existing glasses handy, it is really quite simple to purchase glasses online. The Seven Critical Numbers First, find your current prescription. I keep mine in the health file in my filing cabinet. If you can't find your prescription, visit wherever you last had your eyes examined and request a copy. Your optician, optometrist, or ophthalmologist is required by law to release your eyewear prescription to you. You now need to make note of a few critical pieces of information from your prescription. Among other information, you should see three columns on your prescription: sphere, cylinder, and axis. In the example above, these are abbreviated SPH, CYL, and AXIS. These three columns will have two rows, namely Right and Left. In the prescription above, this is simply abbreviated R and L. Many doctors instead use the Latin abbreviations, which are OD and OS. OD means right eye and OS means left eye. Jot down a quick copy of the six numbers or simply keep your prescription handy. These six critical numbers define how your lens will be carved. Any online eyewear retailer will ask for the input of these six numbers. In addition to these six numbers, there is one more critical number, the Pupillary Distance, or PD. This is a measure, in millimeters, of the space between your eyes. In the example prescription above, you'll see the Pupillary Distance of the patient is 62. Jot down this number also. It is the seventh of your seven critical numbers. A Final Measurement A somewhat less critical number is your temple length. If you're lucky, you'll also see a temple length and bridge size in your prescription. If not, don't sweat it. You can figure it out yourself. The temple length describes the length of the two bars that connect the glasses to your ears. It is measured from where the 90-degree curve from your lens structure to the side bars (temples) begins to the very end of the temple, including the curve. It is not measured from the flex point of the temples, because the "stub" between the flex point and lens structure is not a standard size. Now, take out some measuring tape (preferably the type of tape used for fitting clothes and sewing, as it is very flexible) and measure an existing pair of glasses that fit comfortably. An average temple length for men is 135mm or 145mm. For women, 130mm to 140mm is standard. If your tape only has inches, measure to the nearest eighth or sixteenth and multiply that number by 25.4 to get the measurement in millimeters. Temple lengths are generally available in 5mm increments, such as 130, 135, 140, 145, etc. At most online eyeglasses stores, temple length is shown within a frame description and is not customizable. Therefore, you need to find a frame with an appropriate temple size. This sometimes varies if you are at a site specializing in designer glasses, where you have the option to input your own temple size. Honestly, temple size is not a huge deal. I have one pair of glasses with 135mm temples and another with 140mm temples. Both pairs fit comfortably. If you can't come up with a temple measurement, don't let that stop you from making an eyewear purchase online. Instead, simply choose 140mm temples if you are a man and 135mm temples if you are a woman. Chances are, this length will fit comfortably. What's Your Style? Armed with your seven critical numbers and an idea on your temple length, start browsing the myriad of online retailers. Pick out some favorite frames and open the individual pages in separate tabs or windows of your web browser. Similar to trying on glasses at a standard retailer, you need to figure out if the frames you like are appropriate for your face. If you have a nice oval face, you're in luck; most any pair will look good on you. If you have strong, angular features, a rounded or rimless pair will look good on you. If you have a very round face, choose a rectangular or hexagonal pair of glasses. If you just want a basic pair of professional glasses, go with titanium rimless. Using your trusty measuring tape, you can compare other measurements of your existing glasses with what you are looking at online. The bridge measurement describes the distance between lenses, the height describes the distance from the lowest point on the front of the frame to the highest point, and the lens describes the distance between the end of the bridge and the beginning of the temple. Some retailers also offer free virtual try-on. This service will allow you to upload a picture and place a variety of frame types on your face.