The Three Numbers Inside Your Glasses Arm, Explained
Why a frame carries measurements at all
A spectacle frame is not one size. It is a set of dimensions, and those dimensions decide not only how the frame fits but what the lenses inside it have to be. The numbers stamped inside the temple arm exist so that anyone - you, or a different optician three years later - can order an equivalent frame without measuring it by hand.
The system behind them is called the boxing system, and it is worth knowing because it is the vocabulary every optician uses. A rectangle is imagined around the lens shape, with its sides touching the widest and tallest points of the rim. Everything is then measured from that box.
- A (eye size) - the horizontal width of the box in millimetres. This is the first number on the arm.
- B - the vertical height of the box. Usually not stamped, but it matters: it controls how much depth of lens you have for a progressive corridor.
- DBL (distance between lenses) - the gap between the two boxes, which is what most people call the bridge width. The second number.
- ED (effective diameter) - the diameter of the smallest circle that fully contains the lens shape. Not the same as A in any frame that is not a perfect circle.
- Temple length - the third number, measured from the frame front to the tip of the side arm.
The number that actually changes your lenses
Add A to DBL and you get the frame PD - the horizontal distance between the geometric centres of the two lens shapes. It is the frame's own pupil distance, and it is rarely the same as yours. That difference is the decentration, and it is where frame choice quietly becomes an optical decision.
Decentration per eye = (frame PD - your PD) / 2
Two consequences follow. First, thickness: the further the lens has to be shifted to centre it on your pupil, the further off-centre your pupil sits within the blank, and the more edge thickness you end up with at a given power. Second, and less obvious, the minimum blank size the lab must start from:
Minimum blank size = ED + (2 x decentration) + 2 mm
That 2 mm is edging allowance. If the required blank is larger than the blanks made in your chosen material and index, the job cannot be produced in that material at all - which is why a large, angular frame combined with a narrow pupil distance sometimes forces a change of index or a different frame entirely.
Why this is not just a cosmetic detail
A frame that is too wide for your pupils does not merely look off-centre; it changes the optics. Unwanted prism increases with both decentration and the power of the lens, which is one reason a strong prescription can feel uncomfortable in a large frame that behaves perfectly well with a mild one. And because decentration interacts with index and thickness, the thin-lens upgrade that solved a thickness problem in one frame may be unnecessary in a narrower one.
It is also why moving lenses into a new frame is not the same as re-ordering the same frame. If A and DBL change, the decentration changes, and with it the fit of the lens to your pupil. You can estimate the thickness consequence of that change before committing to an index upgrade using our free lens thickness calculator.
Temple length is about fit, not optics
Temple length is the one number among the three with no optical role. It controls where the arm ends behind your ear and therefore whether the frame sits at the right height and angle on your face. A frame that is optically well matched but two millimetres short on the temple will slide forward - and a frame that slides changes its own optics, because the vertex distance and the effective power of the lens change as it moves away from the eye. When you compare a frame's numbers against one you already own, match the temple length first; A and DBL are the numbers that change the lenses.
Common misconceptions
- "54-18-140 means the frame is 54 mm wide." No. 54 is the width of one lens box, not the frame. The total front width includes the bridge and the end pieces and is usually much larger.
- "Any frame can take my prescription." Not automatically. Minimum blank size, decentration and the blanks available in a given material all constrain what can be made, most sharply at high powers.
- "The numbers are universal." The boxing system is standard practice, but not every frame is marked, and cheaper frames sometimes omit a number. If one is missing, have the frame measured rather than guessed.
- "A bigger frame gives more room for progressives." More B measurement does help a fitting height, but a wider frame increases decentration, which adds thickness and can add unwanted prism. Size is a trade-off in both directions.
When to ask a dispensing optician
Ask before you buy if your prescription is strong in either direction, if your pupil distance is notably narrow or wide for your face, if you are choosing a large or heavily curved frame, or if you wear a progressive and need a usable fitting height. Ask as well if the same prescription has felt different in different frames - that is usually decentration and vertex distance at work, not imagination. And bring the frame's numbers with you when comparing options online: A, DBL and temple length are the only reliable way to compare two frames across sellers, because a frame described only as medium tells you nothing you can check.