Watch glass markings – 304 means 30.4 mm

When purchasing glass for a watch, you may encounter designations such as 280, 285, 304, 325, 350, or 435. For someone outside the industry, such notation may look like a measurement in millimeters, but in watchmaking, a different method of indicating diameter has been adopted.

What does glass 304 mean?

304 means a diameter of 30.4 mm.

The last digit indicates a tenth of a millimeter. In practice, it is enough to insert a comma in the appropriate place.

Examples:

• 280 = 28.0 mm
• 285 = 28.5 mm
• 300 = 30.0 mm
• 304 = 30.4 mm
• 315 = 31.5 mm
• 325 = 32.5 mm
• 350 = 35.0 mm
• 435 = 43.5 mm

Why are such designations used?

This is a notation method adopted in the watchmaking industry and in parts catalogs. It allows the size of the glass to be recorded briefly, without a comma and unit.

Therefore, if you see the designation 304 in the index or product name, it does not mean 304 mm, but 30.4 mm.

Does 304 always mean only diameter?

Not always does the entire product symbol consist solely of size. The name or index may also include designations of the type of glass, thickness, profile, or other manufacturer symbols.

Therefore, when purchasing, it is necessary to check the full product description and ensure that the number refers to the diameter of the glass.

Will glass 304 fit a hole of 30.4 mm?

This cannot be determined based solely on the measurement of the hole.

30.4 mm can be the diameter of:

• the glass itself,
• the seat in the case,
• the inner part of the seal,
• the visible part of the glass after installation.

These are different dimensions.

For glass mounted on an I-ring seal, the diameter of the glass will be smaller than the outer diameter of the entire system of glass + seal.

See: how to select an I-ring seal for glass

Why must the glass be removed before measuring?

Glass mounted in a watch cannot be accurately measured from above, as its edge is partially hidden in the case, under the bezel, or in the seal.

When selecting glass, very small differences in dimensions matter, so the measurement should be taken after removing the glass from the watch.

For measurement, it is best to use calipers with a reading of 0.01 mm.

30.4 mm vs. 30.5 mm – does 0.1 mm make a difference?

Yes, it can make a very big difference.

In the case of pressed or I-ring mounted glasses, a difference of 0.1 mm can determine whether the glass:

• will fit into the case,
• will have the appropriate pressure,
• will not be too loose,
• will not be too tight.

Therefore, it is not worth rounding the measurement unnecessarily.

Does a difference of 0.01 mm also matter?

Yes. Measuring with an accuracy of hundredths of a millimeter is necessary to properly assess the actual dimension of the glass or seat.

Then, the nearest appropriate catalog size is selected according to the method of installation.

How to select glass based on the designation?

If you know the symbol of the old glass and are sure it refers to the diameter, you can easily convert it.

Example:

304 → 30.4 mm

Then you still need to check:

• the material of the glass,
• the thickness,
• the profile,
• the method of installation,
• the type of seal.

The diameter alone is just one of the parameters.

What types of glass can have a size of 304?

The size 30.4 mm can occur in many types of glass, e.g., as:

• flat mineral glass,
• spherical mineral glass,
• lens glass,
• sapphire glass,
• plexiglass.

Therefore, two glasses marked with the size 304 may have the same diameter but be completely different products.

See: flat, spherical, and lens glass – differences

The most common mistake when reading glass designations

The most common mistake is treating the number as a full measurement in millimeters.

Therefore, it is worth remembering a simple rule:

304 = 30.4 mm
325 = 32.5 mm
350 = 35.0 mm

How to properly select glass for a watch?

First, the old glass should be removed, its type and profile identified, and then the diameter and thickness measured accurately.

In the case of lens glass, it is additionally necessary to measure separately the thickness at the edge and the thickness in the center.

See the complete guide to selecting glass for a watch

Glasses for watches at Kurant Wholesale

The watchmaking wholesaler Kurant has in stock over 80,000 pieces of watch glasses in a very large number of diameters, thicknesses, materials, and profiles.

Among the offerings are mineral, sapphire, plexiglass, flat, convex, and lens glasses.

See watch glasses at Kurant