Reading a Frame Size Marking: What 56, 18 and 145 Mean

Every frame we make carries three numbers on the inside of a temple. On our C.1102 they read:

C.1102 5618 – 145

Model reference, then lens width, bridge width and temple length, all in millimetres. Those three numbers are the fastest and most reliable way to specify sizing to a manufacturer — more reliable than a description, and far more reliable than a photograph.

The three measurements

Lens width (eye size) — the first number

The horizontal width of one lens opening at its widest point. On C.1102 that is 56mm.

It is the measurement that most affects how wide the frame reads on a face, and the one buyers quote first. Across our range it runs from 49mm on the compact round C.1262 up to 56mm on C.1102 and C.1271.

Bridge width — the second number, inside the

The distance between the two lenses, sometimes written DBL, for “distance between lenses”. On C.1102, 18mm.

This is the measurement most often ignored and the one that most often decides whether a frame is wearable. A wider bridge lifts the frame and suits a lower or broader nose; a narrower bridge sits closer and pulls the lenses in.

Our range spans 17mm to 21mm. The extremes are instructive: C.319 runs a 21mm bridge under a 54mm squared eye, which is what makes a large lens sit high enough to be wearable rather than dropping onto the cheek.

Temple length — the third number

The full length of the arm from hinge to tip, following the bend. On C.1102, 145mm.

140 and 145 are the common adult lengths. Our C.1262 runs 135mm, which is genuinely short and part of what makes it a compact frame rather than a scaled-down adult one.

What the symbol means

The box between the first two numbers is not decoration. It indicates the measurements were taken under the boxing system, the standard method in which each lens is treated as sitting inside the smallest rectangle that contains it.

It matters because a lens is not a rectangle. Without an agreed convention, “lens width” on a round or a cat-eye is ambiguous. The boxing system removes the ambiguity, which is why every reputable manufacturer marks frames this way and why you can compare a marking from one factory against another.

The fastest way to specify sizing

Take a frame that already sells well in your market. Read the marking off its temple. Send us those three numbers.

That removes more ambiguity than a page of description, and it is the first thing we ask for on a sizing enquiry. If you tell us “medium men’s rectangle” we have to guess. If you tell us “5518 – 140” we know exactly what you mean — and can tell you immediately that C.213 and C.243 both match it.

Reading our range as a sizing curve

Model	Shape	Lens	Bridge	Temple
C.1262	Round	49	19	135
C.245	Soft rectangle	52	18	140
C.356	Rectangle	52	17	140
C.215	Rectangle	53	17	140
C.319	Square	54	21	145
C.213	Rectangle	55	18	140
C.243	Rectangle	55	18	140
C.1233	Square	55	19	145
C.1102	Rectangle	56	18	145
C.1271	Rectangle	56	17	145

A few things worth noticing:

• Temple length tracks lens width. Larger fronts get 145mm arms, mid sizes get 140mm. That is not arbitrary — a wide front needs a longer temple to reach the ear at the right angle.
• C.213 and C.243 share a marking. Same fit, different rim character. Useful when a customer likes the size but not the styling.
• C.1102 and C.1271 differ by 1mm of bridge. Same eye, same temple. That millimetre is the whole point: it lets one shape family fit two nose widths.

What a marking does not tell you

Three numbers do not describe a frame completely. They say nothing about:

• Total frame width across both end pieces, which is often what a retailer actually fits to
• Lens height, which decides whether a progressive lens will glaze
• Frame weight, which depends on sheet thickness and section
• Temple shape, and how much it curves at the tip

So the marking is a starting point, not a specification. It gets you to the right shape quickly, and then a physical sample settles the rest — which is why we recommend approving one before any production commitment.

Marking as a quality signal

One last practical use. When you inspect a sample, look at whether the marking is still crisp.

A size marking that has half-polished away tells you the frame spent too long in the barrel, or was buffed carelessly. It is a small detail that reveals how the finishing stage was run — and finishing is the stage most often shortened when a shipment is late.

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Have a frame that fits your market? Read the marking off its temple and send us the three numbers with your quantities. We will tell you which of our ten models match it, and what adapting one would involve. Start an enquiry, or browse the full collection.