PRESCRIPTION GUIDE
Understanding Your Glasses Prescription
Confused by all those numbers and abbreviations on your prescription? You’re not alone. This guide breaks down exactly what each part means – so you can order your glasses online with confidence and make sure you get the right lenses for your eyes.
THE BASICS
What Do Those Numbers Mean?
Your prescription has rows for your right eye (R or OD) and left eye (L or OS). Each row contains numbers under different headings. Here’s what each one means.
SPH (Sphere)
The main strength of your lens, measured in dioptres. A minus number (-) means you’re short-sighted (struggle to see far away). A plus number (+) means you’re long-sighted (struggle to see close up). The bigger the number, the stronger the prescription.
CYL (Cylinder)
This indicates astigmatism – where your eye is shaped more like a rugby ball than a football. CYL shows how much correction you need for this. If this column is blank or shows 0.00, you don’t have astigmatism.
AXIS
Only used when you have astigmatism (a CYL value). It’s a number between 1 and 180 that shows the angle of correction needed. Without an AXIS value, the CYL correction can’t work properly.
ADD (Addition)
The extra magnifying power needed for reading. Only appears if you need bifocals or varifocals. It’s always a positive number and is the same for both eyes. Common values are +1.00 to +3.00.
PD (Pupillary Distance)
The distance between the centres of your pupils, measured in millimetres. This makes sure your lenses sit perfectly in line with your eyes. Adult PDs are usually between 54mm and 74mm. Most opticians measure this – but you may need to ask for it.
PRISM & BASE
Only included if your eyes don’t work well together (e.g. double vision). PRISM is the amount of correction, BASE is the direction (Up/Down/In/Out). Most prescriptions don’t have these at all – don’t worry if yours doesn’t.
EXAMPLE
What a Prescription Looks Like
Here’s an example of a typical UK glasses prescription. Yours might look slightly different depending on your optician, but the key information will be the same.
| SPH | CYL | AXIS | ADD | PD | |
| Right (R) | -2.00 | -0.75 | 180 | +2.00 | 32 |
| Left (L) | -1.75 | -0.50 | 175 | +2.00 | 32 |
Reading this prescription: This person is slightly short-sighted in both eyes (-2.00 right, -1.75 left), has mild astigmatism, and needs reading correction (+2.00 ADD). Their pupillary distance is 32mm per eye (or 64mm total).
Tip: Always check the date on your prescription. Most UK prescriptions are valid for 2 years. If yours is older, you’ll need a fresh eye test before ordering new glasses.
LENS TYPES
Single Vision, Bifocals or Varifocals?
The lens type you need depends on your prescription. Here are the three main options and when each is right for you.
Single Vision
The most common type. One prescription strength across the whole lens. Used to correct either distance vision (most common) or reading vision – but not both. If your prescription has no ADD value, you need single vision lenses.
Best for: Most people under 40
Bifocals
Two prescriptions in one lens, with a visible line dividing them. The top half is for distance, the bottom half is for reading. Less popular today as varifocals offer the same benefit without the visible line.
Best for: Those who prefer a defined reading area
Varifocals
A smooth gradient from distance vision at the top to reading at the bottom – with intermediate vision (computer distance) in between. No visible lines. The most natural option, but takes a few days to adjust to.
Best for: Most people over 45 with an ADD value
Not sure which type you need? Look at your prescription. If there’s an ADD value (and you’re over 40), you’ll likely benefit from varifocals or bifocals. If there’s no ADD value, single vision is what you need.
COMMON QUESTIONS
Prescription FAQs
How often should I get my eyes tested?
The NHS recommends an eye test every two years for adults, or more often if your optician advises it. Children and people over 70 should generally be tested annually. If your vision changes suddenly, book an eye test sooner rather than later.
How long is my prescription valid for?
Most UK eye prescriptions are valid for two years from the date of your eye test. After this, you’ll need a fresh test before ordering new glasses. Some opticians may give a one-year prescription if your eyes are changing rapidly.
What’s the difference between a glasses prescription and a contact lens prescription?
They’re not the same. A contact lens prescription includes additional measurements (like base curve and diameter) because contacts sit directly on your eye. You’ll need a separate contact lens fitting and prescription if you want to wear contacts – even if you already have a glasses prescription.
Do I need my pupillary distance (PD) to order glasses online?
Yes – most online retailers will ask for your PD to ensure your lenses are correctly aligned with your eyes. Some opticians automatically include it on your prescription, others don’t. If yours is missing, you can ask your optician to measure it (they’re legally required to provide it on request) or measure it yourself at home using an online guide.
Can I use an old prescription to order new glasses?
Only if it’s still in date (within 2 years). Even then, your eyes may have changed since your last test. We recommend getting an up-to-date eye test to ensure your prescription is still accurate, it’s also the best way to spot any underlying eye health issues early.
What if my prescription has different numbers for each eye?
That’s completely normal! Most people have slightly different prescriptions in each eye. As long as you enter the values for your right eye (R/OD) and left eye (L/OS) correctly when ordering, your glasses will be made up specifically for each eye.
What if my prescription has different numbers for each eye?
That’s completely normal! Most people have slightly different prescriptions in each eye. As long as you enter the values for your right eye (R/OD) and left eye (L/OS) correctly when ordering, your glasses will be made up specifically for each eye.
READY TO COMPARE?
Got Your Prescription? Find Your Glasses
Now you understand what’s on your prescription, you can confidently compare prices across UK retailers. GlassesCompared finds you the best deal in seconds.