Understanding Your Prescription

What do all the numbers and words mean on your prescription? Make sense of it all with our guide below...

P.S Don't be put off, you can always email a copy of your prescription and we'll add it to your order!

How to read your prescription

All Opticians work to the same parameters when writing an eye prescription. Here are some terms commonly used:

SPH (Sphere)

The Sphere describes the strength your glasses need to be to correct your focus:

A + value in your SPH box means you are long-sighted (Hyperopic), a difficulty in seeing objects close to you. A - value means you are short-sighted (Myopia) and struggle to focus on distant objects.

This is measured in Dioptre's (an optical unit of measurement - 1 dioptre = 1m^-1). The values usually range from 0.00 to +/- 20.00 in steps of 0.25. You may also see the word Plano PL or an infinity sign written here, this means that you are neither long nor short sighted with perfect focus. However, you may still have Astigmatism - A CYL and AXIS value on your prescription.


CYL (Cylinder)

The Cylinder value (along with your AXIS) corrects for an astigmatism - Where the eye is shaped more like a rugby ball than a football, the slight change in shape causes a focusing of light at more than one place in the eye. Astigmatism normally occurs alongside short sight or long sight. 

If you don't have astigmatism, this box will be empty. If you do, there will be a plus or minus value. Again this is measured in Dioptre's and ranges from +/-0.25 to +/-4.00 in steps of 0.25 where the higher the number, the more oval the eye. 

It's important to note it's common to have astigmatism in just one eye, so don't worry if on your prescription you only have this value in one eye. For an empty field please enter 0.00 on our online form when ordering. You could also have "DS" written under your CYL value, this just indicates no astigmatism and is the same as leaving the box empty.


You will only see an AXIS value if there is a CYL value present. This value tells us where the astigmatism is on your eye, the direction of the astigmatism - measured in degrees. Values range from 0 to 180 and usually go up in steps of 1/2,1,5 or 10 degrees.

Prism & Base

A Prism in your prescription tries to correct a lazy eye, indicating that your eyes don't work well as a pair (strabismus). This is measured in Prism Dioptre's with values from 0.5 to 10 and usually go up in steps of 1/2 or 1 Prism Dioptre's.

The Prism value is always accompanied with a Base direction, this is to tell the lab where to put your Prism. This can be IN, OUT, UP and DOWN. An example above shows Prism 0.5 Base UP. On the rare occasion, some people can have a prism in two directions in one eye.

We can provide glasses for customers needing a Prism in their lens, however we require you select our Gold Package or above during the order process.

ADD (Addition)

This is short for a reading addition (Add/Near) and describes how much additional correction you need for reading or close up work, this will always be the same in both eyes. If you have a value in this field you have different prescriptions for distance and reading glasses.  This is also measured in Dioptre's and usually ranges from +0.50 to +4.00 in steps of 0.25.

If you require reading, bifocal or varifocal glasses please include the Add/Near values, if you require distance /driving glasses then you may omit this. 

Pupillary Distance (PD)

Your pupillary distance is the distance (usually measured in millimeters) between the centers of the pupils of the
eyes and is usually between 50 and 70mm. This measurement is different for each person and depends on whether they are looking at near or far objects. 

The PD is used as an indicator to where the "optical centre" of the lens should be; in front of the pupils for minimal distortion when looking through your glasses. If not correctly set, the incorrect PD can cause blurred vision and/or headaches.

Some prescriptions have the PD for each eye, for example: R 32 L 31, you would enter this as 63mm on the online form. It is not routinely given on every prescription but you can always ask your Optician to add it to your prescription. For varifocal orders we send you a PD ruler for us to accurately measure this. 

For more help visit our guide on how to measure your PD.

Other terms you may see...


Indicates that the eye is not sighted and the same values should be entered in both eyes, so that both the lenses look and feel (weight) the same.

OD (Oculus Dexter)

OD is simply used to refer to the right eye.

OS (Oculus Sinister)

OS is used when referring to the left eye.

The important points to note are...

  • Make sure you do not confuse + and – values on your sphere and cylinder values.
  • Make sure you enter the axis values exactly as they appear – do not enter 12.5 if it states 125.
  • If ordering reading glasses, don’t forget to enter your reading addition.
  • If you have been prescribed computer use glasses, you will have an intermediate addition written down, and this may also appear as INT ADD and again may be a single figure such as +1.00.

Please don’t be put off by your prescription – just remember you can email it to help@glasses2you.co.uk!

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