How to Measure Your Child’s Pupillary Distance
Pupillary distance, or PD, is the distance between the centers of your child’s pupils. This measurement helps position the prescription lenses correctly in front of the eyes.
Your child’s PD may already appear on the eyeglass prescription. When it is missing, a parent or another adult can follow the steps below using the printable Potato Glasses PD ruler.
This guide provides general information to help parents record a child’s pupillary distance for ordering prescription glasses. It does not replace a professional PD measurement, eye examination, prescription, or advice from your child’s eye care professional.
PD is normally recorded as one number or two separate numbers.
Do not use an age-based average or another person’s measurement.
Measure several times and confirm that the results closely agree.
What Is Pupillary Distance?
PD is an optical measurement used when positioning prescription lenses inside a frame.
Pupillary distance is measured in millimeters between the centers of the pupils. It helps the laboratory position the optical center of each lens in relation to the child’s eyes.
PD is different from SPH, CYL, and AXIS. Those values describe prescription power, while PD describes lens positioning.
Some prescriptions include PD, while others do not. You may be able to request the measurement from the eye care provider who examined your child.
Learn how to read the other prescription fieldsWhat Is the Difference Between Single PD and Dual PD?
Check the prescription or order form carefully to determine which PD format is required.
Single PD
Single PD is the total distance from the center of one pupil to the center of the other pupil.
Dual PD
Dual PD records the distance from the center of the nose bridge to each pupil separately. The right and left measurements may not be identical.
How to Measure a Child’s PD at Home
An adult helper should take the measurement while the child sits still and looks at a distant target.
Prepare the Ruler Correctly
- Download and print the PD ruler at 100% or actual size.
- Do not select “Fit to page” or allow the printer to resize it.
- Check any printed calibration mark before using the ruler.
- Use the side marked for measurement with another person.
- Measure in a well-lit room.
Seat the Child Comfortably
Ask your child to sit upright with their head level. The adult measuring should sit or stand directly in front of them.
Choose a Distant Target
Ask your child to look straight ahead at an object across the room rather than looking at the ruler or the adult’s face.
Position the Ruler
Hold the ruler level and steady against the child’s forehead or nose bridge, following the instructions printed on the ruler.
Align the Zero Mark
Line up the zero mark with the center of the child’s right pupil without allowing the ruler or the child’s head to move.
Read the Left-Pupil Position
Read the millimeter number that aligns with the center of the left pupil. This is the single PD measurement.
Repeat the Measurement
Remove and reposition the ruler, then repeat the complete process several times. Use the result only when the measurements closely agree.
Parent tip: Keep your viewing position directly in front of the child. Looking from the side can create parallax and change the number you read.
How Can Parents Improve PD Measurement Accuracy?
Stable positioning and repeatable results are more important than completing the measurement quickly.
Before Accepting the Result
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Confirm that the ruler printed at its original scale.
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Keep the ruler straight and level.
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Make sure the child’s head is not tilted or turned.
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Ask the child to continue looking at the same distant target.
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Repeat the complete measurement from the beginning.
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Use a result only when the repeated measurements are consistent.
When Should You Request a Professional PD Measurement?
Home measurement may not be the best option in every situation.
Ask for help when you cannot produce a stable measurement after several careful attempts.
Movement, head turning, or difficulty focusing on the distant target may make the result unreliable.
Prism and other complex prescription requirements should be reviewed carefully before ordering.
Professional lens-positioning measurements may be preferable when greater precision is required.
Confirm whether the number is a single PD, dual PD, distance PD, or another type of measurement.
Arrange an updated eye exam rather than using an old prescription and an old PD without professional guidance.
How Should You Enter Your Child’s PD?
Check whether you have one single PD or two dual-PD numbers.
Enter the measurement in millimeters using the matching order-field format.
Compare the entered value with the prescription or your written measurement.
Contact customer support when the format or measurement remains unclear.