Interpupillary distance: what it is and how to measure it

November 11, 2026 · 2 min read

Quick facts

Definition: Distance between the center of one pupil and the center of the other.
Reference line: Bipupillary line, which joins the two centers and guides the horizontal plane of the face.
How to measure: On a frontal photo without distortion, with the head straight and the gaze forward.
Use in facial harmonization: Assessing symmetry and alignment of the brows, mouth, and chin.
Caution: Head tilt and lens distortion skew the reading.

Interpupillary distance is the distance between the center of one pupil and the center of the other. The line joining the two points is the bipupillary line, one of the most widely used horizontal references for assessing the face from the front. In facial harmonization, the measurement itself matters less than what the line reveals: whether the structures of the face run parallel to it.

How is it used in the assessment?

With the head straight and the gaze forward, the bipupillary line is usually compared with the brow line, with the line between the mouth corners, and with the base of the chin. If one of them tilts significantly relative to the others, this may point to structural facial asymmetry or just the head posture in the photo.

The reading combines well with the facial midline and with the rule of fifths, which divide the face into vertical and horizontal measurements.

How do you measure without error?

Why is it useful in planning?

When the patient complains that "one side is different from the other," the bipupillary line helps show, visually and objectively, where the difference really is. Often the misalignment is slight and natural, and the conversation about what to treat becomes more honest.

In Harmy

The marking works with 1 frontal photo. Straight photos, with the gaze forward and no tilt, make detection and comparison with the projection more faithful.

Also called: IPD, distance between the pupils.

Frequently asked questions

What is the bipupillary line for?

It serves as a horizontal reference to compare the alignment of the brows, the mouth corners, and the base of the chin. When those planes diverge from the parallel, it is worth investigating asymmetry.

Does interpupillary distance change with a procedure?

No. It is a skeletal, stable measurement. What can change is the perception of the gaze, through changes in the eyelids, brows, and periorbital region.

Terms mentioned

Bipupillary line Facial asymmetry Rule of Fifths Facial anthropometry Facial midline Focal length and distortion

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