How to Measure Your PD With a Credit Card (and Why It Works)

Last updated: October 2026 | Reading time: 8 minutes

Short answer: hold a bank card flat against your forehead, look straight at your camera, and take a photo. Every bank, debit, credit and ID card follows the same standard size, 85.60 mm wide, so the card gives the software a ruler it can trust. The software compares that ruler with the gap between your pupils and converts pixels into millimetres.

You can try it with our free PD measurement tool. This guide explains the method so you can get a cleaner result and understand what the number means.

Why a credit card works as a ruler

Cards that follow the ISO/IEC 7810 "ID-1" format measure 85.60 mm × 53.98 mm. That makes the width-to-height ratio about 1.586, a shape that is easy for software to recognise even in a mediocre photo.

A photo has no built-in scale. A face could be large in the frame because it is close or because the camera is zoomed. Putting an object of known size in the same picture solves that: if the card is 400 pixels wide, each pixel is 0.214 mm.

What you need

• A card with clearly visible edges. A dark card on a light wall works better than a white card on a white wall.
• A phone or webcam, at about arm's length from your face.
• Even, soft light on your face, from the front. Avoid a window directly behind you.
• Your glasses off, if you can. See measuring PD while wearing glasses.

Step-by-step

1. Choose distance PD or near PD

Pick distance PD for normal single-vision glasses. Pick near PD for reading glasses used at about 40 cm. If you're unsure, read near PD vs distance PD.

2. Hold the card flat on your forehead

Place the long edge of the card across your forehead, just above your eyebrows, with the face of the card toward the camera. Keep it level and flat against your skin: if the top edge tilts away from you, the card looks narrower than it really is and the result is skewed.

Make sure all four corners are in view. A hidden corner is the most common reason a card is not detected.

3. Look at the camera lens, not the screen

Your eyes turn inward when you look at something close. If you look at your own face on the screen instead of the lens, the apparent gap between your pupils changes slightly. Look at the small camera lens, keep your head level, and don't tilt your chin up or down.

4. Take three photos

One photo can be a bad frame: you blinked, shifted, or the card slipped. The tool averages repeated captures and discards outliers, so take at least three and compare them. If the three values vary by more than about 1 mm, retake them in better light.

What the software does with the photo

1. Finds your face and both irises.
2. Finds the card and measures its width in pixels.
3. Converts pixels to millimetres using the card's known width.
4. Adjusts for the fact that your pupils sit slightly farther from the camera than your forehead does.
5. Adjusts for the small inward turn of the eyes when they focus on a nearby camera.
6. Shows your PD, the left and right values, and an estimated error.

Step 4 is the subtle one. The card rests on your forehead; your pupils are a couple of centimetres behind that plane. Ignoring the difference would make the pupils look slightly bigger in the photo than they should, so the software applies a correction.

What the tool rejects, and why

The tool refuses to produce a number when the photo makes a trustworthy result impossible, for example if:

• your head is turned more than about 6° (turning shrinks the horizontal gap between your eyes in the image);
• your head is tilted sideways or your chin is raised or lowered;
• the card is angled, partly hidden, or cannot be found;
• your eyes are closed, glare covers the pupils, or you are looking away from the lens;
• the result falls outside a believable adult PD range.

If you hit one of these, see PD tool not working: fixes for common errors.

How close will the result be?

Honest answer: close, but not clinical. A bank-card method can be good enough to order basic single-vision glasses, and the tool shows an estimated error in millimetres so you can judge for yourself. We have tested the maths on simulated faces, but we have not yet completed a study comparing the tool with an optician's pupillometer on real people, so we do not publish an accuracy percentage. The factors that limit precision are explained in what limits online PD accuracy.

For progressive lenses, strong prescriptions or children's glasses, ask an optician to measure you. See PD for progressive lenses.

Quick checklist

• Card flat, all four corners visible
• Looking straight at the lens
• Head level and not turned
• Even light, no glare on your eyes
• Three photos, values within about 1 mm

Sources and further reading

• ISO/IEC 7810 defines the ID-1 card size (85.60 × 53.98 mm) used by bank and ID cards. The standard itself is sold by ISO.
• MediaPipe Iris: real-time iris tracking and depth estimation, Google Research. It describes estimating distance from the roughly constant size of the human iris, which is one way the tool cross-checks the card.
• Related guides: how to measure PD at home, common PD measurement mistakes, what is pupillary distance.