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Glasses Frame Size Calculator

Every frame carries three numbers on the inside of the arm. Enter them along with your own two measurements and this works out whether that frame fits your face and sits correctly in front of your eyes.

The measurement tool

Where to measure

Diagram of a face showing four measurement positions A simplified front-facing head outline. A vertical line runs from the hairline to the chin. Three horizontal lines cross the forehead, the cheekbones, and the jaw corners. Each is numbered and explained in the list below the diagram. 1 2 3 4
  • Face length. From your hairline down to the bottom of your chin, straight down the centre. If your hairline is uneven, use the middle of where it sits.
  • Forehead width. Straight across the widest part of your forehead, roughly halfway between your brows and your hairline.
  • Cheekbone width. From the outer corner of one eye, across the top of the cheekbone, to the same point on the other side. This is usually the widest measurement on the list.
  • Jawline width. From the corner of your jaw below one ear across to the other. Find the corner by clenching your teeth: the muscle that moves is sitting on it.

Measure in front of a mirror with your hair pulled back, keeping the tape flat against your skin rather than following every curve. Measuring each one twice and using the second value removes most of the error, because the first attempt is where the tape is usually in the wrong place.

How the fit is worked out Landmarks, formula, tolerance
What the three numbers mean
A marking like 52-18-140 is lens width, bridge width, and temple length, all in millimetres. Lens width is one lens across. Bridge width is the gap between the two lenses. Temple length is the arm, measured from the hinge to the tip.
Frame centre distance
Lens width plus bridge width gives the distance between the centres of the two lenses. This is exact arithmetic, not an estimate. It is the number that decides whether the optical centre of each lens ends up in front of your pupil.
Decentration
Half the difference between the frame centre distance and your PD. It is how far each lens has to be shifted inward so the optical centre lands on your pupil. Small amounts are routine and every lab does them. Large amounts add thickness at the edge and can introduce unwanted prism, and how much is too much depends on how strong your prescription is, which this tool does not know.
Total frame width
Two lens widths plus the bridge gives the front of the frame. The hinges and end pieces add roughly 4mm to 10mm on each side depending on the design, and manufacturers do not publish it, so the total is reported as a range. A range is what the arithmetic supports.
Fit against your face
A frame front that matches your face width across the cheekbones sits correctly. Noticeably wider and it slides down and the arms splay. Noticeably narrower and it pinches at the temples and leaves marks.
What is not calculated
Temple length. The arm has to reach from the hinge, past your temple, and around your ear, and that depends on the depth of your head from front to back. A photograph taken from the front cannot see depth, and neither can a tape measure across your face. The figure is repeated back to you and left unjudged rather than estimated.

Every figure above is checkable against the measurement engine itself, which is plain JavaScript served with this page: fc-engine.js

Questions people ask

Where do I find the numbers on my glasses?

Inside one of the arms, usually the left, printed small. You will see something like 52-18-140 or 52 with a small square then 18. Some frames put them on the bridge between the lenses instead. Online listings almost always publish them in the specification section.

What is lens decentration and should I worry about it?

It is how far the lab has to move each lens inward so its optical centre sits in front of your pupil rather than in the middle of the frame. Every pair of glasses involves some, and small amounts make no practical difference. It matters more with a strong prescription, because shifting a strong lens adds edge thickness and can introduce prism you notice. If the figure this tool gives you looks large and your prescription is strong, that is a question for your optician rather than a reason to avoid the frame outright.

Why is the total frame width a range instead of one number?

Because the hinges are not part of the printed measurements. A thin metal frame adds a few millimetres each side, a chunky acetate one adds noticeably more, and no manufacturer publishes the figure. Giving you a single total would mean picking a hinge width and hiding the choice inside the answer. The range is what the numbers actually support.

Can I use my old glasses to work out what size to buy?

Yes, and it is the most reliable shortcut there is. Read the numbers off a pair that already fits you well, enter them here to see how they relate to your measurements, and then look for frames close to those figures. A frame that fits is better evidence than any calculation.

Do I need my PD to use this?

For the fit check across your face, no, face width is enough. For the decentration figure, yes, because that is the whole point of it. If you do not have your PD, measure it first with the pupillary distance tool on this site or ask your optician, who already has it on file.

Where to go next