A Simple Guide to Measuring Your Pupillary Distance at Home
Online Glasses has changed the way glasses are purchased, providing people with trendy frames that cost a fraction of what they would in a physical store. But there is one thing that customers find problematic when making a purchase, namely, Pupillary Distance (PD). It is a small figure that is never included on a prescription document, which creates a problem. The best part is that a doctor's consultation is not necessary to get a PD measurement. By following a simple process with a couple of easy-to-find instruments, a PD measurement can easily be taken at home.
Why Does Accurate PD Matter for Online Glasses?
Pupillary Distance (PD) is the distance, in millimeters, from the center of your pupils. There is a "correct" optical center on every prescription lens, which is the most accurate part of your prescription. In order for your prescription glasses to function optimally, your optical center must line up exactly with your pupil. If your PD is not accurate, you are essentially looking through the side of the lens rather than the center. It can induce prismatic effects, leading to eye strain, headaches, and distorted vision.
The common belief is that people who purchase glasses from online stores run the risk of the glasses not fitting properly. This is not true, as the glasses usually fit fine; it is the PD measurement that is one problem. If the PD measurement is accurate, online stores can precision-cut the glasses the same as an optician in your area. It is all dependent on that one detail.
Know the Difference: Single vs. Dual PD
For instance, when submitting your details on a retailer's online platform, there are two options that are commonly available: "Single PD" and "Dual PD." It is essential to know whether it is single PD or dual PD.
Single PD Explained
Single PD is one number representing the total distance from one pupil to the other (e.g., 63mm). This is the standard measurement used for most single-vision prescriptions. It assumes that your nose is perfectly centered between your eyes. For most people with low to moderate prescriptions, a Single PD is sufficient for a comfortable fit.
The Precision of Dual PD
Dual PD consists of two numbers (e.g., 32/31). This measures the distance from the center of the nose bridge to the right pupil and the left pupil independently. This is crucial because very few human faces are perfectly symmetrical. One eye might be slightly further from the nose than the other. If you have a high prescription or require progressive lenses, using a Dual PD ensures the most accurate focal point alignment.
Gather Your Tools for Measuring
You do not need expensive medical equipment to get a reliable reading. However, you do need a specific type of measuring tool. A standard tape measure from a toolbox is generally not accurate enough because the hook at the end obscures the view, and the increments are often in inches
The gold standard for home measurement is a simple millimeter ruler. A rigid plastic or wood ruler is best. Avoid metal rulers if possible, as you will be holding them close to your eyes, and a slip could cause injury. Ensure the ruler has clear, easy-to-read markings.
Many online eyewear retailers offer a printable PD ruler. This is a convenient option, but it comes with a warning. You must print the document at "100%" or "Actual Size" in your printer settings. If the printer scales the image down even slightly, your measurement will be wrong. Always verify the printout against a real ruler or a credit card before using it.
Measure Your PD with a Mirror
If you are alone, you can easily measure pupillary distance at home using a mirror. This method requires a steady hand and good lighting.
Set Up Your Stance
Stand about 8 inches (20cm) away from a well-lit mirror. Hold the ruler flat against your eyebrows or resting on the bridge of your nose. Keep your head straight and level; tilting your head will skew the measurement.
The Measurement Process
- Close your right eye.
- Align the 0mm mark of the ruler exactly with the center of your left pupil.
- Without moving your head or the ruler, close your left eye and open your right eye.
- Look straight ahead and read the millimeter line that aligns with the center of your right pupil.
- Repeat this process at least three times to ensure consistency. The average of these attempts is your Single PD.
Ask a Friend to Help for Better Accuracy
When you look at yourself in a mirror, your eyes naturally converge slightly, which can reduce the measurement by a millimeter or two. A friend can help you keep your gaze parallel.
Sit opposite your friend at eye level, about knee-to-knee distance. Have them hold the ruler against your forehead. Here is the critical trick: do not look at the friend's eyes. If you look at their eyes, your eyes will converge.
Instead, look past them at a distant object on the wall, or look specifically at their ear. When they measure your right eye, look at their left ear. When they measure your left eye, look at their right ear. This keeps your eyes looking straight ahead (infinity focus), providing the most accurate "Distance PD."
Check Your Numbers: Average PD for Men and Women
Once you have your number, it is helpful to know if it falls within the typical range. This serves as a "sanity check" to ensure you haven't made a major error, such as reading the wrong side of the ruler.
Typical Ranges for Adults
The average PD for men and women varies slightly due to skull size.
- Women: The average PD usually falls between 58mm and 62mm.
- Men: The average PD typically ranges between 60mm and 66mm.
- General Range: Most adults will fall somewhere between 54mm and 74mm.
Variances in Children
If you are measuring for a child, the numbers will be significantly smaller. Children's PDs generally range from 43mm to 58mm. Remember that a child's PD changes as they grow, so you must re-measure every time you order new glasses for them.
Use Apps and Digital Tools for Verification
In the age of smartphones, technology offers an alternative to the analog ruler. Several apps and online tools utilize facial recognition to calculate PD.
How Digital Tools Work
Most of these tools require you to hold a magnetic strip card (like a library card or gift card) against your forehead while looking into the selfie camera. The app knows the standard size of the card and uses it as a reference scale to calculate the distance between your eyes.
Pros and Cons
These apps are incredibly convenient and can be a great way to verify your manual measurement. However, lighting conditions and camera angles can affect accuracy. It is recommended to use an app as a secondary confirmation method rather than your sole source of data, especially for high prescriptions.
Adjust Your PD for Different Lens Types
Your PD is not a static number; it changes depending on what you are looking at. Your eyes naturally move closer together (converge) when you focus on objects near your face.
Reading vs. Distance
The measurement you take looking in a mirror or at a distance is your "Distance PD." This is used for driving, TV, and general walking around. However, if you are ordering dedicated reading glasses, the optical center needs to be moved inward.
The Conversion Rule
To convert a Distance PD to a "Near PD" for reading glasses, the general rule is to subtract 3mm from your Single PD. For example, if your Distance PD is 63mm, your Near PD is 60mm. If you are using Dual PD, subtract 1.5mm from each eye's measurement. Most online retailers will do this calculation for you if you select "Reading Glasses" at checkout, but knowing this rule helps you verify the order. Understanding these details will help you select the perfect pair for your needs.
Conclusion
Calculating your Pupillary Distance is a simple process that will give you the convenience of shopping Online Glasses. You can find that number with a tool such as a mirror, a friend, or even a computer application. The most important part is to make sure that your Pupillary Distance is accurate, which means that when you buy your glasses, not only will they look fantastic, but they’ll also give you the crystal-clear vision that you deserve.
FAQs
Can I locate my PD on my old glasses?
No, the PD is not written on the frame. The numbers that are printed on the inside of the temple arm (for instance, 55-18-140) relate to the size of your frame (Lens Width, Bridge Size, Temples), but not the pupillary distance.
Why doesn't my eye doctor include PD on my prescription?
For many regions, the PD is not part of what is considered a prescription. The eye test is centered on eye health as well as refractive power. The measurement of PD is part of fitting a lens, which is why most clinics withhold this measurement so that you order your glasses from them.
Is the PD the same for contact lenses and glasses?
No. Contact lenses are in direct contact with the cornea, which means that the optical center is always centered on the pupil, irrespective of where your gaze is directed. The prescription for contact lenses, therefore, does not use PD.
What happens if my PD measurement is slightly off?
In low prescriptions, a tolerance level of 1 to 2 mm is generally imperceptible to the brain. For high prescriptions or in progressive lenses, even a 1 mm discrepancy may result in considerable eye strain, dizzy spells, and inability to focus.
