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How to understand your contact lens prescriptions - Important parameters!

Anyone who wants to buy contact lenses should know the necessary parameters. The contact lens parameters can be found in the prescription written by an eye doctor (or optometrist or ophthalmologist). It is important to note that contact lens prescriptions and eyeglass prescriptions are not the same.

Please also note that the data obtained by the Optician or Ophthalmologist should not be older than one year, as the parameters of the eyes change with age.

The contact lens parameters you need to know

Curvature (BC)

The base curve of a contact lens describes the curvature of the lens in millimeters, to provide the highest comfort while wearing lenses, i.e. how well the lens attaches to your eye. Normally these parameters are between 8–10 mm.

Dioptres / Sphere (D/dpt./S/SPH/PWR)

Sphere is the power of the contact lens. The unit of measure for sphere power is a diopter, or D. In general the spherical power of a lens is indicated with a plus sign (+) for hyperopia and a minus sign (-) for myopia.

Average (DIA)

This value is the diameter (or size) of the lens. Soft contact lenses have a larger diameter from 13.00 to 15.00 mm than hard contact lenses from 9 to 10 mm.

Cylinder (CYL)

The cylinder value is needed for the manufacture of contact lenses for astigmatism. It is a negative value, which makes any correction for the curvature of the cornea.


The axis is required for astigmatism as a supplement to the contact lens with cylinder. It describes the exact position of the curvature on the cornea and is given in a range of 0° – 180°. Please note that a degree of zero is also written as 180. The number 90 means the vertical position of the eye, and the number 180 corresponds to the horizontal meridian.

Addition (ADD)

ADD is the difference between the dioptric value of the far vision and the dioptric value of the near vision. Values up to 1.25 are regarded as low, up to 2.00 as average and up to 3.00 as high.

How to determine a contact lens prescription?

The contact lens values are determined by an Optician or Ophthalmologist using various tests and measuring methods. Thanks to an accurate prescription the patient can buy the correct lenses.It is important that all parameters are checked for accuracy every 6–12 months. Eyesight and eyes can change several times over the course of a lifetime, so adjustment is important. If you feel that your vision has changed, you should immediately re-evaluate your contact lens values.

The difference between contact lens and spectacle prescription

They are completely different because eyeglass lenses are placed roughly 12 millimeters from your eyes, whereas contact lenses are applied directly to the eyes' surface. However it's necessary to determine both values so that a perfect view is possible.

So, if you are also wearing “glasses”, be careful not to mix the prescriptions, when you buy new contact lenses. Furthermore, the measurement method between glasses and contact lenses is slightly different, as eyeglasses prescription needs to measure the distance between the eyes.

Additional information on a contact lens package

In addition to your eyes' essential values, you will also find information on the water content on the packs of contact lenses, as well as the oxygen permeability. The dryer your eyes are, the less water content should be in a particular material. Contacts with a high water content have the negative property of evaporating the water on long wearers, thus removing moisture from the eye. So that means you should pay attention to the appropriate material: hydrogel contact lenses have about a higher water content, which evaporates faster and so requires more liquid from the tear film. The result: the eye dries up faster. If you have dry eyes you should choose silicone hydrogel lenses because they have a good oxygen permeability and thus require 20 percent less water.

The oxygen permeability is given with the value “Dk/t”. The longer the lenses remain on the eyes, the more oxygen permeable they should be. If the value is too low, redness, or itching and burning may occur in sensitive eyes.

Not every contact lens brand suit each patient's needs

Just like with shoes and clothing, the units of measure and condition for contact lenses may vary according to the manufacturer. Therefore it is possible that you might have to re-touch your parameters, when changing brand and you are strongly advised to consult an optician, when doing so.


Contact lens Parameters Unit of measurement Abbreviation More information
Dioptres dioptre PWR, D, SPH, dpt from –30.00 to +30.00
Curvature millimeter BC from 8.00 to10.00
Average millimeter DIA from 13.00 to 15.00
Cylinder dioptre CYL, ZYL from –0.25 to –10.00, toric contact lenses
Cylinder axis degree A, AX, AXIS from 0 to 180, toric contact lenses
Addition dioptre ADD from 1.00 to 3.00, multifocal lenses

Don't hesitate to contact us for advice. Call us for free on 0800 249 4219, send us an email, or write us on facebook. We will reply you as soon as possible!

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