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Diameter (DIA)

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The diameter (DIA) is the cross-sectional diameter of a contact lens, measured in millimetres. This value determines the size of the lens and is always stated on your contact lens prescription.
Contact lenses

What is diameter (DIA)?

The diameter of a contact lens is the distance from one side of the lens to the other, measured straight across the centre. This value is expressed in millimetres and abbreviated as DIA. You will always find the diameter listed on your contact lens prescription, alongside other values such as power and base curve.

The diameter determines the physical size of the lens and how it fits on your eye. Soft contact lenses usually have a diameter of between 13.8 and 14.5 millimetres, whilst rigid lenses are smaller, typically between 9.0 and 10.0 millimetres. The correct diameter ensures that the lens sits properly on your cornea and has sufficient room to move when you blink.

The diameter is determined by your optician or optometrist during the fitting process. This choice is based on the size and shape of your eye, the type of lens and your comfort whilst wearing them. A diameter that is too large or too small can affect how the lens sits and moves.

Good to know

How is the diameter determined?

During your fitting, your optician or optometrist will measure the dimensions of your eye and determine which diameter is best suited to you. This is done using specialised equipment and is tailored to the type of lens you will be wearing. The diameter is stated on your lens prescription and is specific to each lens brand and type.

The difference between soft and rigid lenses

Soft lenses have a larger diameter because they cover a larger area of the cornea and also overlap slightly onto the white of the eye. Rigid gas-permeable lenses, on the other hand, are smaller and rest only on the cornea, leaving more room for movement beneath the eyelid. This difference in diameter is related to the material and the way the lens fits on the eye.

Can I adjust the diameter myself?

No, the diameter is a fixed specification that you cannot change yourself. When ordering lenses, you must use the exact diameter stated in your prescription. A different diameter may result in the lens not fitting properly, feeling uncomfortable or not moving correctly on your eye. If you’re unsure about your size, please contact your optician.

At SMEYEL , you’ll find contact lenses in various diameters, so you can always order the size that matches your prescription.

Frequently asked questions

If the diameter does not match your prescription, the lens may be too tight or too loose. A lens that is too large may be restricted in its movement and feel uncomfortable, whilst a lens that is too small may move around too much or slip off your eye. Always use the diameter prescribed by your optician.

Yes, the diameter is stated on the packaging of your contact lenses, usually abbreviated as DIA. You’ll find this value alongside other specifications such as power, base curve and the brand. When ordering, always check that the DIA value matches your prescription.

Yes, different brands and types of contact lenses can have different diameters, even if they have the same prescription. Each lens brand has its own design and fit. You should therefore never simply switch brands without consulting your optician, as the diameter and other parameters may differ.

Not necessarily. The diameter must match the shape and size of your eye and the type of lens. A larger diameter does not automatically mean greater comfort; the key is that the lens fits properly and can move freely. Your optician will determine which diameter suits you best based on a fitting.

Author & updates

Author: Mv-support
Publication date: 13 July 2026
Last updated: 13 July 2026