How to choose glass for a watch – a complete guide

Replacing the glass in a watch may seem like a simple task, but proper selection requires precise measurement and recognition of the type of glass. Differences of just fractions of a millimeter can determine whether the glass will be correctly seated in the case.

When choosing, pay attention primarily to:

• the material of the glass,
• the diameter,
• the thickness,
• the profile of the glass,
• the method of installation,
• the type of gasket, if used,
• any additional elements, such as a magnifying glass over the date display.

What does the glass size 304, 285, or 325 mean?

In watchmaking, the diameter of the glass is often recorded without a comma, in tenths of a millimeter.

Therefore:

• 304 means 30.4 mm,
• 285 means 28.5 mm,
• 325 means 32.5 mm,
• 350 means 35.0 mm,
• 435 means 43.5 mm.

The designation 304 does not mean glass with a diameter of 304 mm. This method of notation has become established in the watchmaking industry and is often used in catalog indexes of glasses.

Why should glass not be measured without removing it from the watch?

The correct dimension of the glass cannot be reliably determined by measuring it from above while it is still in the case.

Part of the glass is hidden under the edge of the case, the bezel, or the gasket. Measuring the visible surface may therefore yield a result smaller than the actual diameter.

Before selecting new glass, it is best to remove the old glass and measure it with calipers.

How to measure watch glass accurately?

Use a good caliper, preferably one that allows readings with an accuracy of 0.01 mm.

In watchmaking, differences of hundredths of a millimeter can be significant for how the glass is seated. This is especially true for press-fit glasses and those installed with a gasket.

Even if a given series of glasses comes in sizes of 0.1 mm, accurate measurement allows for a proper decision on which catalog size to use.

How to measure old glass?

After removing the glass from the case, measure its outer diameter in several places. It is worth repeating the measurement, as the old glass may have a damaged edge or slight deviations from previous installation.

Also measure its thickness and determine whether the glass is flat, convex, lenticular, or has another profile.

What to do if the old glass is broken?

If the glass cannot be measured because it is broken or missing, measure the seating area in the case.

Do not measure a random hole visible from above, but the surface where the glass or its gasket is actually seated.

It is also necessary to determine whether the glass was installed directly, glued, pressed, or seated on a gasket.

Types of watch glasses by material

Plexi glass – acrylic

Plexi, also known as acrylic or plastic glass, was commonly used especially in older watches.

It is relatively soft and flexible. It is easier to scratch than mineral or sapphire glass, but many small scratches can be polished out. It is also less prone to shattering than hard glass materials.

Plexi comes in many profiles and is particularly important for repairs of older and vintage watches.

Mineral glass

Mineral glass is one of the most commonly found types of watch glass. It is harder and more scratch-resistant than plexi, while being significantly cheaper than sapphire glass.

It comes in a huge number of diameters, thicknesses, and profiles: from very thin flat glasses to spherical and lenticular versions.

Sapphire glass

Sapphire glass is made from synthetic sapphire. Its greatest advantage is its very high scratch resistance.

However, it is more expensive and harder to process than mineral glass. High scratch resistance should not be equated with indestructibility – with a sufficiently strong impact, sapphire glass can also break.

Plexi, mineral, and sapphire glasses differ primarily in hardness, scratch resistance, behavior during impact, and price. :contentReference[oaicite:1]{index=1}

Types of glasses by shape

Flat glass

Flat glass has a flat surface and is one of the most popular types used in modern watches.

When selecting it, the diameter and thickness are key, e.g., 1.0 mm, 1.5 mm, 2.0 mm, or 2.5 mm.

Convex glass – spherical

Spherical glass is convex, meaning its surface rises towards the center of the glass.

It should not be treated as flat glass of the same diameter. The profile affects the height above the dial and hands, as well as the appearance of the watch.

Mineral and sapphire convex glasses are available in many diameters and profile heights. :contentReference[oaicite:2]{index=2}

Lenticular glass

Lenticular glass is not the same as regular spherical glass.

It has a more pronounced profile resembling a lens and a clear difference between the edge and the central part of the glass. For such glasses, the diameter alone is not sufficient – the profile and its height are also important.

Therefore, two glasses of the same diameter – one spherical and one lenticular – should not be automatically considered interchangeable.

Glass with a magnifying lens over the date display

Some glasses have an additional magnifying lens over the date display.

When selecting such glass, in addition to diameter, thickness, and profile, you must also consider:

• the position of the lens,
• its size,
• its orientation relative to the date window,
• the orientation of the glass during installation.

Glass with a lens should not be chosen solely because it has the same diameter as the original glass.

Glass without a lens

Standard glasses without a lens are more versatile, but still need to be matched in terms of diameter, thickness, material, profile, and method of seating.

How to select an I-ring gasket for the glass?

I-ring is a gasket for the glass with a simple cross-section, placed between the outer edge of the glass and the case. It is responsible for both seating the glass and sealing the connection.

When selecting, you need to know:

• the diameter of the glass,
• the inner diameter of the seating area in the case,
• the thickness of the I-ring wall,
• the height of the gasket.

The I-ring has a specified inner diameter and wall thickness. For example, an I-ring with an inner diameter of 30.0 mm and a wall thickness of 0.40 mm has an outer diameter of approximately:

30.0 + 0.4 + 0.4 = 30.8 mm.

Such catalog dependencies can also be seen in professional gasket sets: for example, for a 30.0 mm glass, an I-ring with a wall thickness of 0.40 mm has an outer diameter of 30.8 mm. :contentReference[oaicite:3]{index=3}

This does not mean, however, that it is always sufficient to subtract twice the thickness of the gasket from the case opening. During installation, there is appropriate pressure, so the entire system is selected: case + I-ring + glass.

How to measure the case for glass with an I-ring?

If the old glass and gasket are missing, you need to accurately measure the inner diameter of the socket in the case where the outer surface of the I-ring is located.

The measurement should be made with internal calipers in several directions.

Then you need to determine the height of the available space for the gasket and select the appropriate wall thickness.

Professional instructions for selecting a gasket also start with an accurate measurement of the inner diameter of the glass seating area in the case. :contentReference[oaicite:4]{index=4}

Can an old I-ring gasket be reused?

When replacing the glass, it is recommended to check the condition of the gasket. The old gasket may be deformed, hardened, or damaged during disassembly.

If the watch is to maintain its declared water resistance, a proper water resistance test should be performed after replacing the glass or gasket.

Why might a 30.4 mm glass not fit where I measure 30.4 mm?

Because the result depends on what was measured.

30.4 mm can mean:

• the outer diameter of the old glass,
• the diameter of the case opening,
• the inner diameter of the gasket,
• the diameter of the visible area above the glass.

These are four different dimensions. Therefore, before ordering, you need to know which element the measurement refers to.

The most common mistake – measuring glass still in the watch

If the glass is still mounted, its actual edge is usually not visible. Measuring from above may yield a result smaller than the actual size.