Accurately measuring your face and current eyewear is fundamental when purchasing glasses, especially through online retailers. These measurements ensure that prescription lenses are correctly centered and that the frame fits comfortably. An accurate fit prevents visual distortion and discomfort, providing the best outcome from your lenses. This process determines frame fit and lens alignment only; it is not a substitute for a comprehensive eye examination required to determine your lens prescription strength.
Essential Tools and Setup
You will need a few common household items to take the necessary measurements. A ruler or measuring tape marked in millimeters is required, as all optical measurements are standardized in this metric unit. A small, millimeter-scale ruler provides the most precision for fine measurements. Ensure you are in an area with bright, consistent lighting to clearly see your pupils and the ruler markings. You will also need a mirror and a non-permanent fine-tipped marker, or the assistance of a friend.
Measuring Pupillary Distance (PD)
Pupillary Distance (PD) is the measurement, expressed in millimeters, between the centers of your pupils. This measurement dictates where the optical center of each lens must be placed in the frame. An incorrect PD means the lens centers will not align with your pupils, which can cause eye strain, headaches, and distorted vision. Adult PD measurements usually fall between 54 mm and 74 mm.
The most straightforward method is measuring your binocular PD, which is the total distance between the centers of both pupils. It is highly recommended to have a friend assist you for the most accurate result. Sit facing your helper at arm’s length and focus your gaze on a distant object (at least 10 to 20 feet away) to relax your eye muscles. Your helper should align the zero mark of the ruler over the center of one pupil and measure straight across to the center of the other.
For greater precision, especially with high prescription strength or bifocals, monocular PD is preferred. Monocular PD is a pair of measurements detailing the distance from the center of the nose bridge to the center of each pupil individually. This accounts for eyes that may not be perfectly symmetrical relative to the center of the face.
Your helper will align the zero mark of the ruler with the center point of your nose bridge. They will measure the distance to the center of your right pupil and record that number. Without moving the zero mark, they will repeat the process for your left pupil, recording the second measurement. These two numbers (e.g., 32 mm and 31 mm) must be listed separately to ensure precise placement of the lens optical centers. Repeat the measurement process several times to ensure consistency and reliability.
Understanding and Measuring Frame Dimensions
The physical dimensions of the eyeglass frame are important for ensuring a proper and comfortable fit. Frame sizes are almost always stamped in a series of three numbers on the inside of the temple arm or bridge, measured in millimeters. For example, 52-18-140 represents the lens width, bridge width, and temple length, respectively. These three metrics determine how the frame sits on your face.
Lens Width
The first number is the lens width, also known as eye size, which is the horizontal measurement of the lens opening at its widest point. This measurement typically ranges from 40 mm to 60 mm. To measure this on an existing frame, use your millimeter ruler to find the horizontal distance across the lens from the inner edges of the frame.
Bridge Width
The second number is the bridge width, which is the distance between the two lenses and represents the size of the piece resting across your nose. This measurement usually falls between 14 mm and 24 mm. It is a significant factor in preventing the frames from sliding down or pressing uncomfortably on your nasal bridge. You can measure this by finding the distance between the two closest points of the lens openings.
Temple Length
The final number is the temple length, which is the length of the arm piece from the hinge to the tip that curves behind your ear. Standard temple lengths are commonly 135 mm, 140 mm, 145 mm, or 150 mm. To measure this manually, measure in two parts: from the hinge to the point where the arm begins to bend, and then from the bend to the tip, adding the two segments together for the total length. These three frame numbers allow you to select a frame that provides the correct physical fit and comfort.