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Measure pupillary distance online
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Measure pupillary distance online as a rough millimeter estimate from one front photo. The browser finds both iris centers and uses visible iris width as an assumed scale rather than a physical ruler. It draws a center-to-center line and two short iris-width guides. The displayed uncertainty covers only the assumed iris-size variation; an optician can obtain a calibrated distance for lens fitting.
Pupil centers and an assumed iris scale
Pupillary distance, or PD, joins the centers of the pupils. The photo analysis also finds a short horizontal span across each visible iris; it averages those two widths to reduce reliance on either side alone. If one iris is obscured or the short guide sits off its edge, both the scale and final estimate deserve doubt. The plotted guides let you see those assumptions in the submitted image.
How pixels become millimeters
The formula divides center-to-center pixels by mean visible iris-width pixels, then multiplies by an assumed 11.7 mm iris diameter. The separate ± value propagates a 0.5 mm uncertainty in that assumed diameter. It does not include face turn, gaze or a missed iris edge. Because no physical ruler is calibrated to the person, the millimeter result is approximate even when the arithmetic itself is exact.
Reading the uncertainty correctly
Read the main millimeter value with its limited uncertainty. If a short iris-width guide misses the visible iris or one center point looks displaced, the scale or numerator may be wrong. The ± figure does not include every source of error: a turned head, off-axis gaze and perspective add uncertainty. Repeating clear images can reveal an unstable estimate but cannot replace a fitting measurement.
As a hypothetical calculation, a 250-pixel pupil-center span divided by a 50-pixel average visible iris width is five iris widths. Multiplying by the assumed 11.7 mm iris diameter gives 58.5 mm. The displayed scale-only uncertainty would be about ±2.5 mm from the half-millimeter iris assumption; gaze, camera angle and detection can add more. That arithmetic cannot turn a photo estimate into a prescription fitting; an optician measures the wearer for the chosen lenses.
Two images can produce the same center-to-center pixel distance but different millimeter estimates if one has a narrower detected iris width. That is why the short iris guides are as important as the long pupil line. To measure pupillary distance online for a rough comparison, inspect both guides and retake a photo if glare hides one edge. The printed decimal should not be treated as more certain than those visible landmarks.
Gaze and iris visibility
Reflections, tinted lenses and off-axis gaze can obscure an iris edge and shift the estimated center. Near-camera perspective also distorts the relation between the two eyes. To measure pupillary distance online, check the cyan center-to-center line and the two short iris-width guides after processing. For general framing, read how to take a photo for a PSL rating.
Why the estimate is not a fitting measurement
This page cannot measure near PD for reading glasses, monocular left and right fitting distances, or where a lens should sit in a chosen frame. It uses a population average iris width and a two-dimensional image. Do not order prescription eyewear solely from this estimate. For a general face comparison, use the eye-spacing ratio, which does not claim a millimeter scale.
The page does not compare a result with male or female reference values or prescribe a target distance. It also cannot split the estimate into separate monocular fitting values for each lens. The practical improvement is a clearer image for curiosity or a calibrated in-person measurement for eyewear; neither is an attempt to change the person’s actual pupil spacing.
Eye spacing without an assumed millimeter scale
The eye type detector uses eye corners to classify spacing relative to eye width, whereas this page estimates center-to-center millimeters from an assumed iris scale. The face proportions calculator likewise uses a different eye-spacing denominator. Those numbers should not be converted into one another without their endpoint definitions.
Measure pupillary distance online FAQ
Can I use a pd ruler online from a selfie?
This page uses iris width as a scale, but a selfie can distort the result. Use it only as an estimate.
Is a printable pupillary distance ruler more accurate?
A correctly printed and used ruler may provide a different reference, but professional fitting remains preferable.
Why do I need both irises visible?
The tool averages their spans to reduce reliance on one possibly noisy outline.
Does looking sideways affect PD?
Yes. An off-axis gaze or turned head changes where the pupil centers appear in the photo.
Can I use this for glasses prescriptions?
No. This estimate is not for prescriptions or lens fitting.
Is the photo uploaded?
No. Iris landmarks and calculations run in your browser.