Pupillary Distance (PD) is the distance between the centers of your pupils, typically recorded in millimeters. This measurement dictates where the optical center of each lens must be placed within the frame. Accurate alignment ensures your line of sight passes directly through the clearest point of your prescription lens. If the PD is measured incorrectly by even a few millimeters, it can introduce unwanted prismatic effects, leading to visual discomfort, eye strain, or headaches. This measurement is essential when ordering prescription glasses online, as professional, in-person measurement is not always readily available.
Understanding Binocular and Monocular PD
The Pupillary Distance measurement is expressed in two primary ways. Binocular PD, often called Single PD, is the total distance measured from the center of one pupil to the center of the other. This single number, typically ranging from 54 to 74 millimeters for adults, is sufficient for most standard, single-vision prescriptions.
Monocular PD, also known as Dual PD, separates the total distance into two numbers: the distance from the center of the bridge of the nose to the center of the right pupil, and the distance to the center of the left pupil. For example, 32/31 represents 32 mm for the right eye and 31 mm for the left. This segmented measurement is useful for individuals with asymmetrical facial features or those requiring high-power or progressive lenses, as it allows for more precise alignment. The measurement for near-vision glasses is often slightly smaller than the distance PD, typically reduced by about 3 mm, because the eyes naturally converge when focusing on close objects.
The Self-Measurement Method Using a Mirror or Camera
Camera Scaling Method
The most accurate method for self-measuring PD involves using a digital camera or smartphone and a known-size reference object for scaling. For this procedure, you will need a smartphone with a camera and a standard magnetic-strip card, such as a credit card, which has a known width of approximately 85.6 millimeters. The card acts as a physical scale factor within the image, allowing digital measurement applications or manual scaling to determine the distance between your pupils.
Remove any glasses and ensure you are in a well-lit area with a plain background. Hold the card horizontally against your forehead, positioned just above your eyebrows, ensuring the entire card is visible. Look directly into the camera lens, keeping your head and eyes perfectly straight, and capture a straight-on photograph. Looking straight ahead is crucial, as looking sideways can cause the eyes to converge, resulting in an artificially narrow PD measurement.
After capturing the photo, use the known card dimensions to determine the pupil distance within the image. You can upload the image to a specialized PD measurement application or use photo editing software to manually scale the measurement. Measure the distance between the pupils in pixels and compare it to the pixel width of the card to calculate the distance in millimeters through a simple ratio. For instance, if the card measures 500 pixels and the pupil distance measures 300 pixels, the PD is calculated as (300/500) 85.6 mm.
Mirror Method
A simpler alternative is the mirror method, which uses a dry-erase marker on a mirror’s surface. Stand 12 to 16 inches from a mirror and hold a millimeter ruler flat against your brow. Close your right eye and align the zero mark of the ruler with the center of your left pupil. Without moving your head or the ruler, open your right eye and close your left eye, noting the millimeter mark that aligns with the center of your right pupil. This measurement represents your Binocular PD. Repeat the process multiple times to ensure consistency, aiming for a measurement within a 1-millimeter range across all attempts.
Estimating PD Using Existing Eyewear
A secondary method estimates your PD using existing glasses, assuming the optical centers of the current lenses were correctly aligned to your pupils. This technique focuses on locating the optical center of the lenses—the point that provides the least distortion and clearest vision. You will need a fine-tipped, non-permanent marker and a distant object, at least 20 feet away, to maintain a natural, non-converged gaze.
While wearing your glasses, focus on the distant object and use the marker to place a small dot on the lens directly over the object, without shifting your gaze. Repeat this process for the other lens, ensuring you look only at the distant target. When you remove the glasses, the two marks represent the approximate optical center of each lens as positioned for your eyes.
To obtain the PD estimation, use a soft measuring tape or a ruler to measure the distance in millimeters between the two dots on the lenses. This distance represents the Binocular PD the glasses were manufactured to accommodate. To estimate Monocular PD, measure the distance from the center of the bridge of the frame to each individual dot. Remember that this method is an estimation, as the original glasses may not have been manufactured with a perfectly accurate PD.