How to measure your pupillary distance (PD) at home

Pupillary distance, or PD, is the distance between the centers of your two pupils, measured in millimeters. To measure it at home, stand close to a mirror with a millimeter ruler held flat against your brow, line up the zero mark with the center of one pupil, then read the mark at the center of the other, using one eye at a time. Do it three times and take the average. Most adults land somewhere between about 54 and 74 mm.

When I worked in an optical shop, PD was the number people asked about most once they started ordering glasses online, usually because it wasn't on their prescription. A rough home measurement is easy. Whether rough is good enough depends on the glasses you're ordering, and that's worth knowing before you place the order.

What the PD numbers mean

PD tells the lab where to put the optical center of each lens, so it sits right in front of your pupil. You'll see it written in a few ways.

• Single PD (also called binocular PD) is one number, such as 63, measured from pupil center to pupil center.
• Dual PD (also called monocular PD) is two numbers, such as 31.5 and 32, each measured from the center of the bridge of your nose to the center of one pupil. Few faces are perfectly symmetrical, so the two halves often differ by a millimeter or so.
• Distance PD and near PD: when you read, your eyes turn inward slightly to meet on the page, so the near PD is smaller, typically about 3 mm less than the distance PD. A prescription that says 63/60 means 63 for distance and 60 for reading.

Children's PD is smaller than adults' and widens as the face grows. In adults it stays much the same.

Why the number matters

Every lens has an optical center: the point you see through with no sideways shift. Away from that point the lens bends light a little to one side, an effect opticians call prism. Your eyes can work around a small amount, but too much can cause eye strain, headaches, a pulling feeling, or occasionally double vision.

How much a misplaced center matters depends on the strength of the lens. Opticians use a simple rule of thumb: the prism effect equals how far the center is off (in centimeters) multiplied by the lens power (in diopters). So a center 5 mm out in a 2 diopter lens gives about 1 unit of prism, while the same 5 mm in a 6 diopter lens gives about 3. With a mild prescription, a millimeter or two hardly matters. With a strong one, it does. If you're not sure how strong your lenses are, the glasses prescription reader explains the numbers on your slip, including the PD line.

How to measure PD in a mirror

You need a ruler with millimeter marks (a clear plastic one is easiest), a mirror and good light. Take your glasses off.

1. Stand about 8 inches (20 cm) from the mirror and keep your head straight.
2. Hold the ruler flat and level against your brow, just above your eyes.
3. Close your right eye. Looking with your left eye, line up the zero mark with the center of your left pupil.
4. Keep the ruler still. Close your left eye and open your right. Read the millimeter mark that lines up with the center of your right pupil. That number is your distance PD.
5. Repeat the whole thing three times and average the results. If your readings differ by more than about 2 mm, slow down and start again.

Switching eyes matters. Each eye then looks straight at its own reflection, which gives a much truer reading than staring at both pupils at once.

If your eyes are dark and the pupil is hard to pick out, use the edge of the iris, the colored ring, instead. Line up zero with the left edge of one iris and read the mark at the left edge of the other. Using the same edge on both eyes gives the same distance as center to center.

Measuring with a helper

A friend can usually measure more steadily than you can on your own. Sit face to face about 16 inches (40 cm) apart, with your eyes at the same height. Your helper holds the ruler flat against your brow.

1. Your helper closes their right eye. You look straight into their open left eye. They line up the zero mark with the center of your right pupil.
2. Your helper closes their left eye and opens their right. You switch to looking into their open right eye. They read the mark at the center of your left pupil.
3. Repeat three times and average.

For a dual PD, your helper also notes the mark at the center of the bridge of your nose and measures from there to each pupil. Treat a home dual PD as rough, since the middle of a nose is hard to pin down.

Using an old pair of glasses

If you have a pair you see well through and find comfortable, it can serve as a cross-check. Put them on and look at something across the room. Your helper, sitting directly in front of you, puts a small dot on each lens over the center of each pupil, using a washable marker or a tiny piece of tape. Take the glasses off and measure between the dots. Clean the lenses afterward with water and a drop of dish soap, as in the guide to cleaning glasses without scratching, never with a solvent. An optician can also read where the lens centers sit in your old pair with a lens-measuring instrument.

Some phone apps and retailer websites measure PD with the camera, often with a card held up for scale. Results vary, so compare them with a ruler measurement before you rely on one.

Why your prescription may not show PD

People are often surprised that the prescription from their eye exam leaves PD off. In many places it is treated as a fitting measurement taken when glasses are made, rather than as part of the prescription. Optometrists and opticians usually measure it with a ruler or a pupillometer, a small device you look into that measures from the reflection of light on each eye.

You can simply ask for it. When you book an exam, or at the end of one, ask whether your PD can be measured and written on your copy; some offices will do it, and some measure it only when they make your glasses. The guide to your first eye exam explains what else to ask for and what happens during the visit.

Progressives, strong prescriptions and children

A home ruler measurement is fine for many simple single-vision glasses. For some lenses, I always recommended having it measured in a shop.

• Progressive lenses and bifocals need a dual PD and a fitting height: where your pupil sits from top to bottom in the frame you've chosen. That can only be measured with that frame on your face, adjusted to sit properly. Get it wrong and the narrow reading channel ends up off to the side or too low. The column on reading glasses and progressives explains how those zones work.
• Strong prescriptions and lenses with prism written on the prescription are less forgiving, for the reasons above.
• Children should be fitted by an optician. Their faces are small, their PD changes as they grow, and most young children find it hard to keep still for a ruler.

Ready-made reading glasses are a different case. They are made with the lens centers at one fixed spacing, which suits most people for short spells of reading. If your PD is a long way from average, you may find them tiring, and glasses made to measure will feel better. The reading glasses strength tool gives a starting strength to try, and it also reminds you that an eye exam comes first.

When to see an eye doctor

Knowing your PD tells you nothing about your eyes. It doesn't show whether your prescription is current or whether your eyes are healthy. The American Academy of Ophthalmology recommends a baseline comprehensive eye exam at age 40 for adults without symptoms or risk factors, and earlier for people with diabetes, high blood pressure or a family history of eye disease. An optometrist can examine your eyes and measure your prescription; an ophthalmologist is a medical doctor who treats eye disease. The guide to finding an eye doctor and planning a visit is a good place to start.

If new glasses give you headaches, eye strain or double vision, take them back to whoever made them first and ask them to check the centers and the fit. If the problem continues, or your sight isn't clear even with the new glasses, see your eye doctor.

Get same-day eye care for new flashes of light, a sudden shower of floaters, or a shadow or curtain across part of your vision.

Go to an emergency department, or call your local emergency number (such as 911, 999, 112, or 119 in Japan), for:

• Double vision that comes on suddenly, especially with headache, weakness, a drooping eyelid or trouble speaking
• Sudden loss of vision in one eye, even if it returns within minutes, which can be a warning sign of stroke
• Severe eye pain with blurred vision, especially with haloes around lights, headache or nausea

Eye emergencies and first aid explains what to do while you get help.

Questions people ask

What is a normal pupillary distance?

There's no single normal number. Most adults fall between about 54 and 74 mm, and children's PD is smaller. What matters is your own measurement, not how it compares with anyone else's.

Is my PD on my contact lens prescription?

No. A contact lens sits on the eye and moves with it, so it doesn't need a PD. A contact lens prescription carries other measurements from a lens fitting instead, and it can't be swapped for a glasses prescription in either direction.

How accurate does a home PD measurement need to be?

For a mild single-vision prescription, a careful average of three readings is usually close enough. For strong lenses, prism or progressives, small errors matter more, so have an optician take the measurement.

Can I reuse the PD from an earlier pair?

For an adult, usually yes: a PD that an optician measured for you earlier is generally still usable for single-vision glasses, because adult PD changes little. A fitting height for progressives belongs to one particular frame, though, and has to be measured again with each new frame.