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How to

How to measure your pupillary distance? And how to understand your prescription?

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1.Stand 8 in. away from the mirror.


2. With your face straight,hold the ruler against your brow.


3.Close your right eye and align the ruler's zero to the center of your left pupil.


4. While looking straight,close your left eye and open your right eye.


5. Read the mm line that lines up with the center of your right pupil. This number is your PD.


A Few Things to Note

  • Measure your PD 3-4 times to ensure it’s accurate and consistent.
  • Range of Adult PD: 54-74 mm
  • Range of Child PD: 43-58 mm

Understanding your prescription

What do the letters, numbers and symbols on my prescription mean?

what does your prescription mean

Aside from your name and date of birth, you will see that there are many letters, numbers and images on your prescription. They mean: 

‘+’ or ‘-‘

‘+’ – Shows if you are long sighted (longsightedness)

‘-‘ – Shows if you are short sighted (shortsightedness)

SPH or Spherical number

This shows how strong the correction needs to be. A ‘-‘ symbol above the number indicates shortsightedness whereas a ‘+’ symbol indicates longsightedness. Other symbols that can be found in this section are the infinity symbol (which looks like a sideways eight) or the word Plano (or Pl), which are the equivalent of ‘zero’ and are used when no sight correction is required. Another common term that can be found on prescriptions is V/A (or visual acuity) which measures the standard of vision when corrected.

CYL or Cylinder number

This shows how much astigmatism you have and the number can be negative or positive. Astigmatism is when the front part of your eye is shaped more like a rugby ball than a regular circle. If you don’t have an astigmatism, there will be no figures written in the cylinder and axis boxes.


This shows the position of the cylinder. It will be between 1 and 180.

PD or Pupillary distance

This is the distance between your eyes measured between your pupil centre points. Usually, the optician will not add this to your prescription, so you should ask for it as it’s important for those with high-strength prescriptions to have lenses that are centred more accurately. To ask for your PD, you can tell your optician that you will be purchasing an item they do not stock, such as prescription goggles online.


This will indicate if you need a prescription for reading and is applied to bifocal and varifocal lenses to correct presbyopia. You may also have an Inter Add for computers or other tasks like reading music on a stand.


This shows if you have a muscle imbalance in your eye. It’s less common to see this box filled in as it’s fairly rare. The prism is usually written in fractions (for example 1 ½).


This shows the direction of the prism in your lens, for example IN, OUT, UP or DOWN. This is fairly uncommon so the boxes are normally empty.

Common layouts of prescriptions

Below are examples of prescriptions. The first set are standard – look through these to get a feel. It’s mostly handwritten prescriptions that cause problems but typed ones are generally easier to understand. For each prescription, we also included how it would appear when adding it to our website.

At the end of the page are examples of unusual cases you may encounter, with instructions for what to do. Skip straight to the more unusual items.

This is a standard NHS prescription:

NHS prescription

It would be written as follows:

NHS prescription entered on Glasses Direct

NHS Scotland

NHS Scotland prescription

  • Here the signs have been written ABOVE the numbers – they are still just standard +/- signs.
  • It can be hard to tell where one number ends and the next one begins. It helps to know that sphere and cylinder will always be in the format (x)x.yy, with x being a number from 0 to 10 and yy being either 00, 25, 50 or 75. Axis is always a number between 1 and 180.

It would be written as follows (you can click “My prescription is more complex than this…” to expand the menu):

NHS Scotland prescription entered on Glasses Direct


Boots prescription

It would be written as follows:

Boots prescription entered on Glasses Direct


Specsavers prescription

It would be written as follows:

Specsavers prescription entered on Glasses Direct

Vision Express

Vision Express prescription

It would be written as follows:

Vision Express Prescription entered on GD

More unusual figures

If you need a prism for one or both eyes, it will look similar to this:

Prescription with prism

We can provide lenses with a prism correction, but you’ll need to contact us to discuss this.

Sometimes figures like this appear on your prescription. These are not needed to make your lenses.

Figures not needed

Some prescriptions have this figure, which looks like a sideways 8. This means ‘infinity’. And you should select ‘infinity’ from the list.

Infinity example

This prescription also has some figures scrawled underneath – these are additions for both intermediate and near distances. Even though it’s a single figure, it applies to both eyes.

Additions example

To enter this, you’ll need to use the “My prescription is more complex than this…” link to show the more advanced prescription area.

Sometimes opticians write ‘DS’ in the CYL column. This simply means there is no astigmatism, and you can enter either nothing (leave the field blank) or select the phrase DS in the drop down.

No astigmatism example