If you've ever gotten partway through ordering glasses online and hit a field asking for your "PD," you're not alone in wondering what that number even is — or how you're supposed to find it without a trip to the eye doctor. Here's how to measure it accurately at home, using nothing but a mirror and a card you already have in your wallet.
What Is Pupillary Distance (PD)?
Pupillary distance is the space between the centers of your two pupils, measured in millimeters. It's what lets an optical lab position the optical center of each lens directly in front of your eye — get it wrong, and even a perfectly accurate prescription can cause eye strain, headaches, or subtly blurred vision, because you're looking through the wrong part of the lens.
Most adults have a PD between 54mm and 74mm, though it varies by person. It's a completely different measurement from the numbers on your prescription (like SPH, CYL, or AXIS) — your eye doctor measures your vision, but PD is a physical measurement of your face, and it's not always included on a written prescription. That's why most online eyewear retailers, Aldermiller included, ask for it separately.
The Card Method: The Most Reliable At-Home Approach
You don't need a specialized ruler app for this — you need a standard credit card, debit card, or ID card. Nearly every card issued in the US is manufactured to the same fixed width: 85.60mm. That consistency is what makes this method accurate.
What you'll need:
- A standard-width card (credit, debit, gym membership, driver's license — any of these work)
- A mirror
- A friend to help (optional, but improves accuracy)
- Good, even lighting
Steps:
- Stand about 8 inches from a mirror, facing it directly, in a well-lit room. Avoid backlighting (like a window behind you), which can throw off shadows and make your pupils harder to see clearly.
- Hold the card horizontally against your brow line, just below your eyebrows, so the edge of the card runs across your face at pupil height.
- Close your right eye. Line up the "0" edge of the card (the left edge) with the center of your left pupil, as reflected in the mirror.
- Without moving the card, open your right eye and close your left eye. Look at where the center of your right pupil lines up against the card now.
- Read the measurement at the point where your right pupil center falls on the card. Since the card's total width is 85.60mm, you can estimate the distance proportionally — for example, if your right pupil lines up at roughly the halfway point, your PD is close to 43mm (which would be unusually low; most people fall in the 54–74mm range, so double-check your alignment if your number looks far outside that).
- Repeat 2–3 times to confirm consistency. If your measurements vary by more than 1–2mm between attempts, try again with better lighting or ask a friend to hold the card steady while you look straight ahead.
A more precise variation: if you have a millimeter ruler instead of a card, the same left-eye/right-eye technique works — just line up the ruler's zero mark with your left pupil center and read the millimeter mark at your right pupil center directly, without needing to estimate proportionally.
Single PD vs. Dual PD — Which Do You Need?
Most people only need a single, overall PD number (the total distance between pupil centers), and that's what the card method above gives you. However, if you're ordering progressive lenses, some labs request a dual PD — the distance from the center of your nose bridge to each individual pupil (e.g., 32mm / 33mm instead of one combined 65mm number). This split measurement matters more for progressives because the lens has multiple distinct optical zones that need to be positioned precisely relative to your nose, not just your face's overall width.
If you're ordering progressives and unsure which format is needed, it's worth submitting both — most prescription intake forms, including Aldermiller's, will ask specifically if a dual measurement is required.
When Self-Measurement Isn't Precise Enough
Self-measured PD is reliable for most single-vision glasses, but it does carry a margin of error — typically within 1–3mm using the card method, which is acceptable for most everyday prescriptions. If you have a strong prescription, a significant difference in strength between your two eyes, or you're ordering progressives, a small PD error has a bigger impact on comfort and clarity. In those cases, an in-person measurement by an optometrist or optician (often free, even without booking a full exam) will be more precise than any at-home method.
Submitting Your PD to Aldermiller
Once you have your measurement, you'll submit it through the secure prescription intake link sent to you in your order confirmation email — the same place you'll upload your written prescription. If you're at all unsure about your self-measured number, note that in the submission; our licensed reviewing optometrist checks every submission before lenses go into production, and will flag anything that looks inconsistent with your prescription before fabrication begins.
Have questions about your specific prescription or measurement? Reach out to hello@aldermiller.com — we're happy to help before you place an order, not just after.
[Shop Prescription Eyeglasses →]