Written and maintained by CASRAI Editorial Board
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This page describes products and general use; it is not medical advice.
What is a PD ruler?
A PD ruler, also called a pupillary distance ruler, is a millimeter scale used to measure the distance between the centers of a person’s two pupils. Seller instructions give the result in millimeters. It is a hand tool: the user holds it horizontally across the face and reads the scale at each pupil.
How is a PD ruler built?
The version described in a seller’s published instructions is a printable paper ruler: it is printed, checked against a standard millimeter ruler, and folded along a dotted line (EyeBuyDirect, “How to use the PD Ruler”, checked 2026-10-30). Because it is a printed scale, its accuracy depends on how it was printed.
PD ruler vs pupillary distance meter vs pupillometer
These three names are often mixed up. The U.S. regulation defines a pupillometer as a device that measures, by reflected light, the width or diameter of the pupil of the eye. That is the size of one pupil, not the distance between two. A PD ruler and a pupillary distance meter are named for the distance between the pupils; they are not described by the pupillometer regulation.
| Name | What it measures | Source |
|---|---|---|
| Printable PD ruler | Distance between the pupils, in millimeters, read from a printed scale | EyeBuyDirect instructions, checked 2026-10-30 |
| Pupillometer (AC-powered or manual) | Width or diameter of the pupil, measured by reflected light | 21 CFR 886.1700(a), checked 2026-10-30 |
How is a pupillometer regulated?
Under 21 CFR 886.1700(b), a pupillometer is a Class I (general controls) device. Both the AC-powered and the manual device are exempt from premarket notification, subject to the limitations in 21 CFR 886.9. The manual device is also exempt from the current good manufacturing practice requirements of the quality management system regulation in part 820, except records and complaint files under 820.35. Under 886.9, the exemption does not apply if the device is intended for a use different from that of a legally marketed device of that generic type, or is intended for lay use where the former intended use was by health care professionals only. This regulation covers pupil width or diameter measurement. It does not name pupillary distance or PD rulers, so it should not be read as classifying a PD ruler.
How to use a printable PD ruler
- Print the ruler without scaling, at 100%.
- Check the printed scale against a standard millimeter ruler, or against the credit card diagram on the seller’s page.
- Fold along the dotted line.
- Stand approximately 8 inches (20 cm) from a mirror, or ask another person to measure for you.
- Hold the ruler horizontally over the center of the right pupil; resting it against the forehead adds stability.
- Look straight ahead and read the distance between the pupils in millimeters.
- Repeat the measurement and average the readings.
Who uses a PD ruler?
The tool is used by individuals measuring their own pupillary distance, or by a second person measuring for them, following the seller’s instructions above. Pupillometers, by contrast, are described in the regulation for measuring the pupil itself.
Common mistakes
- Printing at a scaled size. The seller’s instructions require printing at 100% and checking the scale against a millimeter ruler; a scaled print shifts every reading.
- Taking one reading. The instructions say to repeat the measurement and average.
- Confusing pupil size with pupil spacing. A pupillometer reports pupil diameter; a PD ruler reports the distance between pupils.
FAQ
What is a pupillometer?
A pupillometer is an AC-powered or manual device intended to measure, by reflected light, the width or diameter of the pupil (21 CFR 886.1700(a)). It is not the same as a PD ruler.
What is a pupillary distance ruler?
It is another name for a PD ruler: a millimeter scale for the distance between the pupils.
What is a pupillary distance meter?
The name is used for measuring the same pupil-to-pupil distance that a PD ruler gives, as opposed to a pupillometer, which under 21 CFR 886.1700 measures pupil width or diameter.
Related reading
For how interpupillary distance matters at the eyepieces, see microscope ergonomics. For another hand measuring tool, see stadiometer. For the wider category, see what is a medical device, and for eye-examination instruments, what is an ophthalmoscope. Diagnostic equipment is also listed under diagnostic, monitoring and imaging.
Sources
- 21 CFR 886.1700 Pupillometer (https://www.law.cornell.edu/cfr/text/21/886.1700), FDA, checked 2026-10-30.
- 21 CFR 886.9 Limitations of exemptions (https://www.law.cornell.edu/cfr/text/21/886.9), FDA, checked 2026-10-30.
- EyeBuyDirect, How to use the PD Ruler (https://www.eyebuydirect.com/guides/pd-ruler-guide), seller instructions, checked 2026-10-30.
