Free Online Pupillary Distance Measurement

Whether you're ordering glasses online for the first time or just want to understand what PD means, this guide covers everything about pupillary distance measurement.

What is PD Measurement?

PD measurement (short for pupillary distance measurement) is the process of determining the distance in millimetres between the centres of your pupils. This number tells a glasses manufacturer exactly where to place the optical centres of your prescription lenses so they align with your line of sight.

How is PD Measured Professionally?

In an optician's practice, PD is measured using a pupillometer — a device you hold up to your eyes that directly measures the distance. Alternatively, an optician may use a ruler held across the bridge of your nose while you look at a distant target.

By law in most countries, opticians must provide your PD on request. If yours hasn't been included on your prescription, simply ask — it's your measurement.

How Does Online PD Measurement Work?

Online PD tools like this one use AI face landmark detection. The technology:

Detects your face in the photo
Maps 68 precise landmarks including the inner and outer corners of each eye
Calculates the pixel distance between pupil centres
Converts pixel distance to millimetres using facial proportion ratios

Factors That Affect PD Measurement Accuracy

Head angle: Even a slight tilt left/right can skew the pixel distances. Always face straight-on.

Camera distance: Too close and lens distortion exaggerates the apparent eye separation. Arm's length is ideal.

Lighting: Shadows across the face make landmark detection less accurate. Use even, natural light.

Resolution: Higher resolution photos give more precise landmark positions. Most phone cameras are fine.

Understanding Your PD Measurement Result

Your measurement will show three numbers:

63mm

Total PD

Combined binocular distance

31mm

Left PD

Left pupil to nose bridge

32mm

Right PD

Right pupil to nose bridge

Your left and right PD may differ by 1–2mm — this is completely normal. It simply means your eyes are not perfectly symmetrically spaced from the centre of your nose.

Which PD Number Should I Use When Ordering Glasses?

Single vision lenses: Use the total (binocular) PD — e.g., 63
Progressive lenses: Enter monocular PDs separately — e.g., L31 / R32
Reading glasses: Some retailers ask you to subtract 3mm from your distance PD for a near PD

How Often Should You Remeasure Your PD?

Adult PDs are stable. Once measured, you don't need to remeasure unless you suspect the original was inaccurate. Children should have their PD re-checked every 1–2 years as their faces grow.

PD Measurement — The Complete Guide