Pupillary distance, or PD, is the measurement in millimeters between the centers of your pupils. This measurement is necessary when ordering prescription eyewear, especially online. An accurate PD ensures the optical center of each lens aligns perfectly with the center of your pupil. This alignment provides the clearest vision correction and prevents visual discomfort. If the PD measurement is incorrect, even slightly, it can lead to eye strain, headaches, or blurred vision.
Professional Measurement Options
The most reliable and accurate way to obtain your pupillary distance is through a trained eye care professional, such as an optometrist, ophthalmologist, or licensed optician. These professionals use specialized devices, like a digital pupilometer, which is considered the gold standard for PD measurement. The pupilometer provides a highly precise measurement, typically with variations of less than 1 millimeter.
You should specifically request your PD measurement after a comprehensive eye examination, as it is often not automatically included on the written prescription. Many optical shops or eye doctors will provide this measurement at no extra charge if you are purchasing new eyewear from them. If you are only requesting the measurement, some providers may charge a small administrative fee. Relying on a professional measurement is highly recommended, especially for complex prescriptions or multifocal lenses, where precision is paramount.
How to Measure Pupillary Distance Yourself
If a professional measurement is not immediately available, you can perform a self-measurement using a simple millimeter ruler. This approach requires you to stand about eight inches (20 cm) away from a mirror or have a friend assist you. The average adult PD typically falls between 54 mm and 78 mm.
To measure on your own, hold the ruler horizontally across the bridge of your nose, ensuring the millimeter marks face the mirror. Close your right eye and align the zero mark of the ruler exactly over the center of your left pupil. While keeping the ruler perfectly still, close your left eye and open your right eye, then read the measurement that aligns with the center of your right pupil. This single number represents your total, or binocular, pupillary distance.
For increased accuracy, repeat this process two or three times and use the average of the measurements. Some prescriptions, especially for progressive lenses, may require a monocular PD, which is the separate distance from the center of each pupil to the bridge of your nose. You would measure the distance from the center of your nose to the center of the right pupil and then repeat the process for the left pupil, resulting in two separate numbers, such as 32/30. Self-measurements carry a higher risk of error, showing variations that can be several millimeters off, which may compromise the effectiveness of your new glasses.
Using Digital PD Measurement Tools
A modern alternative is the use of digital pupillary distance measurement tools, typically available as smartphone applications or integrated online features. These tools utilize your device’s camera to capture an image and calculate the distance between your pupils. The process often involves taking a photo or short video while holding a standard-sized object, such as a credit card, as a reference scale.
The application uses the known dimensions of the reference object to calibrate the measurement, providing a calculated PD value. Some advanced apps may leverage depth-sensing technology for more precise results. While these digital methods are convenient for at-home use, they are not as precise as a professional pupilometer measurement.
The accuracy of digital tools can vary significantly, with some demonstrating an error of around 0.5 to 1.5 millimeters compared to professional measurements. This level of accuracy is often sufficient for most single-vision prescriptions, but the result is an estimation. Using a digital tool offers a quick and easy way to obtain a PD, but if your prescription is high or complex, seeking a professional measurement remains the better choice for optimal visual comfort.