Getting the correct eyeglass size is fundamental for comfort and ensuring vision correction functions as intended. A frame that is too wide will constantly slip, while one that is too narrow can cause pressure headaches and distort the lens’s optical center. Proper sizing relies on a combination of numerical measurements found on the frame and specific measurements taken from your face. Understanding these figures guarantees a precise fit for clear, comfortable vision.
Deciphering Standard Frame Measurements
Eyeglasses typically carry a sequence of three numbers printed on the inside of the temple arm, separated by a dash or a small square (e.g., 54-18-140). These measurements are given in millimeters and represent the foundational sizing of the frame.
The first number is the Lens Width (or eye size), which measures the horizontal width of the lens at its widest point. This measurement is generally between 40 mm and 60 mm and influences the overall field of view.
The second number is the Bridge Size, which indicates the distance between the two lenses where the frame rests on the nose. Ranging from 14 mm to 24 mm, this measurement determines how the glasses sit. If this number is too small, the lenses will be too close together, and if it is too large, the glasses will sit too low on the face.
The third number is the Temple Length, which measures the arm from the hinge all the way to the tip that curves behind the ear. Common lengths include 135 mm, 140 mm, 145 mm, and 150 mm. The correct temple length ensures the arm extends far enough to hook comfortably over the ear without causing pressure at the side of the head or behind the ear.
Measuring Your Pupil Distance (PD)
The Pupil Distance (PD) is a measurement of the wearer, representing the space between the centers of your pupils in millimeters. This figure dictates where the optical center of your prescription lenses must be placed to align with your line of sight. An inaccurate PD can lead to eye strain, fatigue, or headaches, especially with higher-powered prescriptions.
The measurement is often provided by an eye care professional, but it can be taken at home using a millimeter ruler. Stand about eight inches away from a mirror and hold the ruler flat against your browline. Close your right eye and align the ruler’s zero mark with the center of your left pupil.
Keeping your head and the ruler still, close your left eye and open your right eye. Read the measurement that aligns with the center of your right pupil. This single number is your binocular PD, which is suitable for most distance prescriptions. A more detailed approach uses dual or monocular PD, which measures the distance from the center of the nose bridge to the center of each pupil separately, providing a more precise optical alignment.
Choosing the Total Frame Width
The total frame width is the overall horizontal measurement of the glasses from one side to the other. This dimension is not usually printed on the frame but is calculated by summing the Lens Width (multiplied by two) and the Bridge Size. This sum provides the distance between the hinges, though the full width also includes the small endpieces where the frame front meets the temple arms.
Ideally, the total width of the frame should be close to the width of your face measured across your temples. The frame should not extend much wider than the face at the temples to ensure stability and comfort.
Frames that are too narrow will press uncomfortably into the sides of your head, which can lead to constant pressure and misalignment of the lenses. Conversely, a frame that is significantly too wide will constantly slip down your nose and may look disproportionate on your face. When the width is correct, your eyes should appear centered within the lens openings, which is structurally beneficial for vision clarity.
Checking for Optimal Comfort and Alignment
Once you have selected a size based on the numerical measurements, the final check involves a physical assessment of the frame’s contact points on your face. A correctly sized frame should rest securely on the bridge of the nose without causing painful pressure or leaving deep red indentations when removed. If the glasses constantly slide down, the bridge is likely too wide or the nose pads need adjustment.
The temple arms should extend straight back and only begin to curve downward just past the top of the ear, fitting snugly without digging into the side of the head. If the bend is too early or the arms are too short, they can cause discomfort at the side of the head or behind the ear.
Visually, the top of the frame should sit no higher than the line of your eyebrows, and the bottom should avoid resting on your cheeks to prevent rubbing and irritation.
The vertical alignment is also important, particularly with multifocal lenses, where the lens height must be sufficient to accommodate different viewing zones. For all lens types, the center of your pupils should align horizontally and vertically with the optical center of the lens. Even with the right measurements, minor adjustments to the nose pads or temple tips by an optician can fine-tune the fit for optimal comfort and visual performance.