Glasses are one of the few things people buy at a markup of several hundred percent without ever being shown a price list. Buying online cuts that down a lot, and what stands between most people and a cheaper pair is a piece of paper they already own plus two measurements nobody ever explained to them.
Your prescription is yours by law
In the US, the Federal Trade Commission’s Eyeglass Rule requires optometrists and ophthalmologists to give you a copy of your prescription after an eye exam at no extra cost, and bars them from making the exam conditional on you buying glasses from them. The rule has been in place since 1978, and the FTC tightened it in 2024 after years of complaints that prescribers were not handing prescriptions over automatically.
In practice this means you should not have to ask, you cannot be charged a fee for the copy, and “we only release that if you buy here” is not a policy, it is a violation. Get the paper or the emailed file before you leave the office, every time, even if you plan to buy from that optician anyway.
What the numbers mean
OD is your right eye and OS is your left, from the Latin. Sphere, often written SPH, is the main correction, with a minus for nearsightedness and a plus for farsightedness, measured in units called diopters. Cylinder and axis appear only if you have astigmatism: cylinder is how much correction you need and axis is the orientation of it in degrees. Add is the extra close-up power in multifocals. The American Academy of Ophthalmology has a clear breakdown of how to read the prescription if yours has fields you do not recognize.
You do not need to interpret any of it. You need to copy it across without a single error, since a transposed minus sign or a decimal in the wrong place is the most common reason an online order arrives unwearable. Type it in, then check it against the original twice before paying.
The measurement they usually leave off
Pupillary distance, or PD, is the gap between the centers of your pupils. The lab uses it to place the optical center of each lens directly in front of each eye. Get it wrong by two or three millimeters on a strong prescription and you will get eye strain and headaches from glasses that are technically the right power.
PD is often missing from the prescription, because it is a fitting measurement rather than a medical one, so the rule above does not force anyone to include it. Ask for it anyway. If you get nowhere, measure it against a millimeter ruler in a mirror or have someone else measure it, do it three times, and take the average. Weak prescriptions forgive a small error here. Strong ones do not.
Your size is printed inside the arm
If you already own glasses that fit, pick them up and look at the inside of one temple arm. You will find three numbers, something like 52-18-145. That is lens width, bridge width, and temple length in millimeters, and it is the most reliable way to shop online. Filter by those numbers, allow a millimeter or two either side, and you skip most of the guesswork that comes from judging scale in a product photo. Style advice about face shapes is worth reading, but fit is measurable and style is not.
Lenses are where the money goes
The frame price is the headline and the lens options are the actual bill. A higher index lens is thinner and lighter, which matters at strong prescriptions and is wasted money at mild ones, so let your sphere number decide rather than the upsell. Anti-reflective coating is the one add-on almost everyone should take, since it kills the glare that makes night driving unpleasant. Blue light filtering is sold much harder than the evidence for it supports, so buy it only if you like how it looks. Progressives are the one purchase where paying more, and buying somewhere with a good remake policy, is usually the right call.
Try the frames before you commit
The obvious weakness of buying online is that you cannot put the frames on your face, which is exactly the problem home try-on programs exist to solve. Warby Parker, a New York company that sells online and runs stores across the US, will ship you a set of frames to try at home for free before you order any lenses, and its stores will take your measurements and adjust the finished glasses for you. Several other online sellers run similar programs. Whoever you use, check the return window and the remake policy before you pay, since those are what actually protect you.
Check them when they arrive
Put the new glasses on and read something at normal distance. Text should be sharp straight away, the frames should sit level without pinching behind your ears, and lines in the room should stay straight when you turn your head. Progressives take a week or two of adjustment, but single vision lenses either work on day one or they are wrong. If something feels off, say so inside the return window and ask for a remake rather than living with it.
Once you have your prescription, your PD, and your frame numbers written down somewhere, a second pair costs a fraction of what the first one did. That is the real reason the paperwork is worth chasing.
