When choosing contact lenses for vision correction, there are a number of important factors to consider. One of them is optimum fit, which is helped largely by making sure you order the exact prescription your optician has given you: this will ensure that the contact lens fits well on your eye. 

Wearing poorly-fitting contact lenses can damage the eye and cause problems such as blurred vision, headaches, and eye strain. The diameter and base curve of a contact lens are two measurements that affect how well they fit your eye.

What is the base curve?

The base curve of a contact lens is the curvature of the back surface of the lens. It determines the type of fit the lens must have to match the natural curvature of your eye.1 It is usually expressed in millimetres and may be further characterised as steep, median, or flat. Typical base curve values range between 8.0 and 10.0 mm, though it can be flatter (from 7.0mm) if you have a rigid gas-permeable lens. A person with a higher base curve number has a flatter cornea (the clear, front surface of the eye) compared to someone with a lower base curve number, which indicates a steeper cornea.

What is the diameter?

The diameter of a contact lens is the width of the lens from edge to edge. It is also expressed in millimetres. This number is usually between 13 mm and 15 mm, though it can be as small as 9mm if a rigid gas-permeable lens, and it determines where the lens will sit in your eye.1 When you wear a contact lens with the appropriate diameter, the lens will remain stable in your eye, i.e. hold its position. If you wear a contact lens with the wrong diameter, it can cause discomfort and may even fall out.

What impact do base curve and diameter have on comfort?

Contact lenses are not one-size-fits-all. Having the right fit is essential for clear vision and long-term comfort and satisfaction with your lenses. The diameter and base curve are important factors in determining what the optimum fit is for you. A proper fit ensures full coverage of the cornea, optimum edge alignment, and adequate movement of the lens for tear exchange.2 Tear exchange describes the flow of tears at the edge of the lens, that is triggered when we blink.

A well-fitted lens covers the cornea properly and prevents dryness due to an exposed cornea.2Dry eye is a common cause of eye discomfort with contact lens use, and a well-aligned lens does not produce any edge strain (lens tightness) which can lead to discomfort.2 When the base curve and contact lens diameter are appropriate, there is adequate tear exchange from under the lens surface, which helps clear out debris.2 All these factors contribute to your lenses being comfortable to wear and also to preventing eye damage and strain, thereby promoting overall eye health.

Can I order contact lenses with a different base curve?

You should never order contact lenses with a base curve that is different from your prescription. This can damage your eyes and cause problems with vision. This is especially important for people who purchase cosmetic coloured contact lenses, which are often of a standardised size.

Can I order contact lenses with a different diameter?

It is not recommended to wear contact lenses with a different diameter from your prescription. If the diameter is too wide, the lens will be loose in the eye and may slip out of place. If the diameter is too small, the lens will have a tight fit, causing discomfort.

Book an appointment with a Specsavers optician today to learn more about optimum fitting and contact lens base curve and diameter. You can also visit our contact lenses page for answers to your questions about vision correction with contact lenses.


1. Specsavers Australia. (no date). What do the Power and BC measurements mean on my contact lens prescription? [Online]. Available at: https://www.specsavers.com.au/... [Accessed 8 November 2019].

2. The Vision Care Institute – Johnson and Johnson Medical Ltd. (no date). Soft Contact Lens Fitting. [Online]. Available at: https://www.jnjvisioncare.ae/s... [Accessed 8 November 2019].