Can I use my contact lens prescription for glasses?

No. Contact lenses sit on the eye, so the powers are adjusted and the prescription includes a base curve and diameter that glasses do not use. Ask for a separate glasses prescription.

What if PD is not on my prescription?

It often is not, because the optician who sells the glasses usually measures it. Ask the office that did your exam, or measure it yourself with a ruler and a mirror as described above.

How long is a glasses prescription valid?

Commonly one or two years, set by your state and your eye doctor. The expiry date is printed on the prescription. Online retailers will not accept an expired one.

What does a negative number mean?

A minus sphere means you are nearsighted and the lens corrects distance vision. A plus sphere means you are farsighted, or the prescription is for reading.

How to read your prescription (and find your PD)

The rows: OD and OS

A glasses prescription has two rows. OD is the right eye, OS is the left eye. Some forms write OU for both eyes when the values are the same. Keep them straight when you type them into a retailer’s form; swapping the eyes is the most common ordering mistake.

The columns

SPH (sphere) is the main power of the lens, in diopters. A minus number corrects nearsightedness, a plus number corrects farsightedness. The further from zero, the stronger the lens. Mild prescriptions sit within about two diopters either side; strong ones go past four, which is where lens index starts to matter.

CYL (cylinder) and AXIS together correct astigmatism. Cylinder is a power, axis is an angle from 1 to 180 that says where that power sits. If both are blank or marked DS or SPH, you have no astigmatism correction. If one is filled in, both must be.

ADD is the extra near power for progressives or bifocals, always a plus number, usually the same for both eyes. If you only have a distance prescription, there is no ADD. Over-the-counter readers are, in effect, a prescription that is all ADD.

PRISM and BASE appear rarely. They correct eye alignment. If they are on your prescription, order from a retailer that supports prism and expect it to cost more.

PD: the one that is usually missing

PD (pupillary distance) is the distance between the centers of your pupils in millimeters. The lens has to be centered on each pupil, so the retailer needs it. Many prescriptions leave it out, because the optician who sells the glasses normally measures it.

Adult PDs mostly fall between the high fifties and the high sixties. You will see it written one of three ways:

a single number, such as 63, for distance;
two numbers, such as 31.5/31.5, one per eye, which add up to the single number;
two numbers such as 63/60, where the second, smaller one is the near PD used for reading glasses.

If you have two numbers that add up to roughly 60-something, they are per-eye values. If the second number is about three smaller than the first, it is the near PD.

Three ways to measure PD

Ask the office. The place that did your exam measured it or can measure it in a minute.
Mirror and ruler. Stand about arm’s length from a mirror with a millimeter ruler across your brow. Close your right eye, line up the zero with the center of your left pupil. Close your left eye and open your right; read the number at the center of your right pupil. Repeat three times and take the middle value.
A friend and a ruler. They hold the ruler across your brow while you look at a point far behind them, and read the distance between pupil centers. Looking at the distance matters: looking at your friend narrows the PD.

Retailers also offer phone-based measurement during checkout. It works, but check the result against a manual reading if it is far from the middle sixties.

Before you order

Check the expiry date.
Enter each eye on the correct row.
If CYL is filled in, make sure AXIS is too.
Enter PD as the retailer asks for it: single or per eye.
Save a photo of the paper. Every retailer lets you upload it instead of typing.