Spectacles require a measurement, also called a fitting, to ensure that your prescription fits in the frame and also to make sure the frame fits you properly. Care must be taken when measuring for eye glasses.
Select a frame style. Eye glasses come in two materials: plastic and metal. Plastic is lighter and available in many colors. Metal is more durable in most cases and usually has adjustable nose pieces for a more customized fit.
Decide on size. Frames are marked with sizes, usually on the bridge (nose piece) of the frame or the temple, also called arm piece. Frame sizes are measured in millimeters and average 50 to 56 millimeters per lens and 14 to 20 millimeters for bridge size. Frame size is not like shoe size. If you wear a 52 in one style, you may wear a 56 in another. Your frame size depends on the style of the frame, where your eyes are centered and your prescription.
Note the size that fits you best in the style you like. Pick a frame that fits so your eyes are centered in the lens, not too close to the nose or ear side of the frame. This ensures your best vision.
Measure the frame front with a ruler with millimeters if you cannot read the numbers on the frame or if they have worn off the bridge or temples. Start by placing the zero line of the ruler on the outer edge of the lens eyewire (the part that holds the lens in) by the temple and at the widest point, measuring across the widest part of the lens to the inside edge of the eyewire near the bridge. Note the number.
Measure the bridge. Place the ruler on the inside edge of the eyewire and measure across the nosepiece to the inside edge of the other eyewire at the point on the frame that comes in contact with the nose. This is the bridge size. The lens measurement combined with this number is considered the frame size and is written separated by an x; for example, the size might be 54 x 18. This is how an optician would write the measurement to order lenses.
Measure the temple length. The part that goes over your ear, the temple, is sometimes referred to as an arm or arm piece and is also measured in millimeters. Temples average 135 to 150 millimeters in 5 millimeter increments. To accurately measure a temple, bend the curve up so the temple is straight and measure from the hinge to the tip of the temple.