How to measure your PD (pupillary distance) at home
Your pupillary distance — PD — is the distance in millimetres between the centers of your pupils. Online glasses retailers ask for it because lenses only work properly when the optical center sits directly in front of each pupil. Here are five ways to get it, ranked by accuracy.
| Method | Accuracy | Cost |
|---|---|---|
| With a phone app (fastest, most consistent) | ±1–3 mm | Free |
| With a mirror and a millimetre ruler | ±3–5 mm | Free |
| With a friend | ±2–4 mm | Free |
| With a credit card as a known reference | ±1–2 mm (in an app); ±3 mm (manual) | Free |
| From your optician | ±0.5 mm | Often free with an eye exam |
1. With a phone app (fastest, most consistent)
The fastest method is to use a phone app that finds your pupils through the front camera. IPD Measure runs face tracking on-device — no upload, no account — and uses the horizontal iris diameter (biologically stable at 11.7 ± 0.5 mm) as a built-in ruler.
- Install IPD Measure on Google Play and grant camera access.
- Hold the phone at arm's length, face level, looking straight at the camera.
- Wait three seconds. The app averages three readings and shows your PD in millimetres.
The free reading uses an average iris size as its scale. Calibrating with a credit-sized card (Premium, one-time $4.99) measures your own iris for a more precise, personal result — well suited to ordering glasses online.
2. With a mirror and a millimetre ruler
The classic method. You need a flat mirror, a millimetre ruler (most rulers have a mm scale on one edge), and steady hands.
- Stand 20 cm (about 8 inches) from the mirror.
- Hold the ruler horizontally across your brow, with the 0 mm mark aligned over the center of your right pupil.
- Close your right eye, keep your head still, then read the millimetres directly under the center of your left pupil.
- That number is your PD. Repeat three times and average.
The hard part is keeping the ruler level and your head perfectly still. Parallax errors of 3–5 mm are common, which is why most opticians use a digital pupillometer.
3. With a friend
More accurate than the solo mirror method because your friend can align the ruler from in front of you instead of through a reflection.
- Stand facing your friend at arm's length. Look at a point past their shoulder so your eyes don't converge.
- They rest the ruler across your brow with the 0 mm mark over your right pupil.
- They read the millimetres over your left pupil while you keep your gaze locked.
The fixation point is the trick. If you look directly at your friend's face, your eyes converge inward and you'll under-measure by 2–3 mm.
4. With a credit card as a known reference
Every ID-1 card — credit card, debit card, driver's licence — is a precise 85.60 × 53.98 mm. That makes it a stable ruler if you can get one into the photo.
The manual version: hold the card flat against your forehead while a friend photographs you head-on, then measure pupil-to-pupil in the photo using the card width as a scale reference. Slow, but works on any device.
The IPD Measure Premium mode does the same thing automatically — hold up a card, the app detects it and calibrates to the known width for a more precise, personal reading.
5. From your optician
The gold standard. An optician uses a digital pupillometer (or mounted phoropter) that locks onto your pupils from a fixed distance. Accuracy is typically ±0.5 mm.
The catch: many opticians won't print the PD on your prescription unless you ask — and some charge a small fee for it, because the number makes it easy to buy glasses elsewhere. Always ask for it explicitly during the exam.
The fastest way: use the app
IPD Measure gives you a reading in about three seconds, runs the face tracker on-device so your camera frames never leave your phone, and is free.
FAQ
What's the difference between PD and IPD?
They're the same measurement. PD (pupillary distance) is the term opticians and online glasses retailers use. IPD (interpupillary distance) is the term VR and AR headset manuals use. Both refer to the distance in millimetres between the centers of your pupils.
What's a normal PD?
Adult PD typically falls between 54 mm and 74 mm, with an average around 63 mm. Children's PD is usually 43–58 mm. If your reading falls way outside that range, take a second measurement — it's almost always alignment error rather than a genuine outlier.
How accurate does my PD need to be for online glasses?
Within ±2 mm is fine for most single-vision prescriptions. Higher prescriptions (over ±4.00 D) and progressive lenses are more sensitive, so aim for a tight, careful measurement. The IPD Measure app uses an average iris size for its free reading; Premium card calibration measures your own iris for a more precise, personal result.
Single PD vs dual PD — what should I enter?
Single PD is the total pupil-to-pupil distance (e.g. 63 mm). Dual PD is two numbers — the distance from each pupil to the bridge of your nose (e.g. 31/32 mm) — and it accounts for facial asymmetry. Most online retailers accept single PD. If a retailer asks for dual PD, you'll usually need an optician measurement or a more advanced app.
Can I measure PD with my iPhone?
IPD Measure is Android-only at the moment; iOS is in the pipeline. In the meantime, the mirror-and-ruler method works on any phone, and several iOS apps use the iPhone's TrueDepth camera if your model has Face ID.