What is Pupillary Distance (PD) and How Do You Measure It?
Pupillary Distance (PD) is the horizontal distance between the centers of your pupils. It is a crucial measurement for ensuring that your lenses are aligned correctly with your eyes, which helps optimize your vision. Getting an accurate PD ensures the optical center of your lenses corresponds with your pupils, resulting in maximum clarity.
Why Does PD Matter?
- PD ensures proper lens alignment: When you wear glasses, the optical center of each lens should be directly in front of your pupils to achieve optimal vision. If the alignment is off, it can cause discomfort, strain, and blurry vision.
- It’s required for making prescription glasses: Opticians use your PD to center the prescription lenses accurately in the frame. Without it, the lenses might not align properly with your eyes, affecting how you see.
How to Measure Your PD at Home
You can measure your PD at home with just a ruler and a mirror. Here’s a simple step-by-step guide:
- Stand in front of a mirror, about 20 cm (8 inches) away.
- Hold a ruler horizontally above your eyes, close to your face.
- Close your right eye and align the 0 mm line of the ruler with the center of your left pupil.
- Look straight ahead, then close your left eye and open your right eye.
- Measure the distance between the center of your left pupil and the center of your right pupil. This value is your PD.
- Repeat the process two or three times to make sure the measurement is consistent.
Average PD Values
- For females, the average PD is about 62 mm (2.44 inches).
- For males, the average PD is slightly larger at about 64 mm (2.52 inches).
Single PD vs. Dual PD
- Single PD: This is the total horizontal distance between the centers of your pupils. It’s the most common PD measurement.
- Dual PD: This is more specific and refers to the distance from the center of your nose bridge to each pupil. Most people’s faces are not perfectly symmetrical, so dual PD measurements provide greater accuracy.
PD for Reading Glasses
Your PD changes depending on the distance you’re looking at. For reading glasses, the PD is generally smaller than for distance glasses:
- To find your PD for reading glasses, subtract 3 mm from your single PD or 1.5 mm from each of your dual PD measurements.
For example:
- If your single PD for regular glasses is 63 mm, it would be 60 mm for reading glasses.
- If your dual PD measurements for regular glasses are 33 mm (right) and 30 mm (left), they would become 31.5 mm and 28.5 mm for reading glasses.
PD vs. Segment Height
- PD is the horizontal distance between your pupils, whereas segment height refers to the vertical distance from the bottom of your lens to the center of your pupil for progressive lenses. Segment height is particularly important for multifocal lenses to ensure the right areas of the lens align with your eyes.
Apps to Measure PD
You can also use apps to measure your PD:
- EyeMeasure
- GlassesOn
These apps can guide you through the process using your mobile phone and camera, making it easier to get a precise measurement without the need for a ruler.