Your Glasses Prescription Explained

Glasses Prescription Explained

Picture the scene; you’ve just been to visit the optician to have your eyes tested. All is well as the optician examines your eyes, checking all is well in the optical department. Once you’ve been given the once over, you’re handed a prescription and sent on your way. But what do the numbers on your prescription actually mean?

Being Handed Your Prescription – Things to Consider

Firstly, before we delve into the detail, there is one important thing you need to remember. You are entitled to a free copy of your prescription. Most opticians will hand this over once your examination is complete, but there may be those who are less willing to comply. You should never be refused a prescription, nor should you have to pay extra to receive one.

Secondly, you don’t have to buy prescription men’s or women’s glasses from the optician who dispensed your prescription. You may opt to purchase your frames and/or lenses elsewhere. This will allow you to choose from a much wider range of colours, shapes and styles, including rectangular, round or aviator glasses.

Understanding the Numbers on Your Prescription

To the untrained eye, a prescription is just a series of random numbers. The key is understanding what these numbers mean so that you can request the correct lenses when buying online.

Entering the numbers is super-simple. It’s basically a case of entering each number into the corresponding box on the web page.

If you find that you have too many (or too few) digits for the number of boxes shown, take a moment to review the information below or check your prescription for errors. Bear in mind, some of the numbers listed on your prescription might not be needed.

Your Prescription – The Basics

An eye prescription is basically a series of numbers for your right eye and your left eye. There are indicated by ‘R’ and ‘L’.

The unit of measurement for lenses is dioptres and all numbers are written in increments of 0.25.

Occasionally the Latin terminology will be used for the right and left eyes:

  • OS – (Oculus Sinister) for the left eye
  • OD – (Oculus Dextrous) for the right eye

Explaining Prescription Terminology

Struggling with the terminology printed on your prescription? Here’s a handy breakdown that should clear things up.

SPH – (Sphere):

This box will contain a ‘+’ or ‘– ‘symbol before the number.

The + indicates that you are long-sighted, while the – indicates that you are short-sighted.

The number refers to the power of the lens. The higher the number, the stronger the prescription lens needs to be.

CYL – (Cylinder):

This refers to a condition called an ‘astigmatism’. This is caused by an irregular (often rugby ball-shaped) cornea. The higher the number, the more irregular the shape of your eye. If you have perfectly rounded eyes, this box will be left blank.


The axis is the number which relates to astigmatism. This will be a number between one and 180 degrees and determines how much correction is required.


This indicates that there is a muscle imbalance between your eyes, suggesting that they do not work in tandem with one another.  This number – the prism correction – will correct the imbalance and prevent double vision from occurring.


Relating to the prism, this indicates the position of the lens.


If you require a reading addition, you’ll need to include this number, as ‘add’ means reading addition. This number is usually more prominent on the prescriptions of those over the age of 45, as the eyes lose their ability to focus correctly as we age.

If you have an ‘add’ number, it means that you will require two types of lenses – for distance and for reading.