Your glasses prescription can look more confusing than it needs to. It may contain plus signs, minus signs, decimals, abbreviations, boxes for right and left eyes, and sometimes extra notes such as ADD, prism or base direction. For most customers, the problem is not that the prescription is unusually complicated. The problem is that nobody has explained what the numbers actually mean in plain English.
This guide explains how to read your glasses prescription without turning it into an optical textbook. It covers the most common terms, what each number does, how plus and minus prescriptions differ, when lens thickness becomes important, why PD is not always written down, and which mistakes to avoid when ordering glasses online.
The simple rule
Your prescription is not just a set of numbers to copy into checkout. It is the optical instruction used to make your lenses. If one value is entered in the wrong place, missed completely or copied with the wrong sign, the finished glasses may not feel right.
What is a glasses prescription?
A glasses prescription is the written result of your eye test. It tells the optical lab what lens power is needed for each eye so your glasses can correct your vision. It is not a diagnosis of eye health, it is not a frame measurement, and it is not the same as your contact lens prescription.
The prescription normally separates your right eye and left eye. You may see them written as R and L, Right and Left, or OD and OS. OD refers to the right eye and OS refers to the left eye. Some UK prescriptions simply use RE and LE. Whatever format is used, the key thing is that each eye must be copied correctly.
A simple prescription may only include one value for each eye. A more detailed prescription may include sphere, cylinder, axis, reading addition, prism and base direction. That does not automatically mean your eyes are unusual. It simply means the lenses need more specific instructions.
The lens power
It explains what optical correction each eye needs, including short-sight, long-sight, astigmatism and reading support where required.
How the frame fits
It does not tell us your frame size, bridge fit, lens height, or whether the frame you have chosen is suitable for your prescription.
Do not treat your prescription like admin.
The prescription is the foundation of the glasses. If it is copied incorrectly, even a beautiful frame and expensive lenses can be wrong. If anything on the card looks unclear, send it for review before ordering.
Why glasses prescriptions look so confusing
Prescriptions look confusing because they are written for optical use, not for customer readability. The abbreviations are efficient for opticians and labs, but they can feel intimidating if you only look at a prescription once every couple of years.
The numbers are also very precise. A small change in prescription can make a noticeable difference to comfort and clarity. This is why values are usually written in quarter steps, such as 0.25, 0.50, 0.75 and 1.00. You may also see plus and minus signs, which are just as important as the numbers themselves.
The main thing to remember is that prescription numbers are not a judgement of how “bad” your eyes are. They are simply instructions. A higher number may mean a stronger lens, but that does not automatically mean the glasses will be difficult to make. The frame choice, lens index and measurements all affect the final result.
“My prescription is complicated, so I cannot buy glasses online.”
Not always. Some prescriptions do need more care, but many can still be ordered online safely when the details are checked properly. The dangerous part is guessing, not the fact that the order is online.
Every prescription abbreviation explained
Most glasses prescriptions are built from a small group of abbreviations. Once you understand these, the prescription becomes much less intimidating. The exact layout may vary between opticians, but the terms usually mean the same thing.
Prescription terms at a glance
The main power of the lens for short-sight or long-sight.
The correction for astigmatism, if needed.
The direction of the astigmatism correction.
Extra power for reading, occupational or varifocal lenses.
SPH / Sphere
SPH stands for sphere. This is the main power of the lens. It may be written with a plus sign or a minus sign. A minus prescription usually corrects short-sight, meaning distance vision is blurred. A plus prescription usually corrects long-sight, meaning near vision may be harder, although this can vary depending on age and focusing ability.
For example, -2.00 means the lens has two dioptres of minus power. +2.00 means the lens has two dioptres of plus power. They are not interchangeable. Missing the sign completely changes the prescription.
-3.00 SPH
A minus prescription usually means short-sight. The glasses help make distance vision clearer.
+2.00 SPH
A plus prescription usually means long-sight. The glasses may support near vision, distance vision or both depending on the person.
CYL / Cylinder
CYL stands for cylinder. This is used to correct astigmatism. Astigmatism is very common and usually means the front surface of the eye or the internal optics are shaped in a way that needs correction in a particular direction.
Cylinder values may be written as plus or minus depending on how the optician records the prescription. When ordering glasses, the sign must be copied exactly as written unless a professional has transposed the prescription correctly. Do not change a cylinder from minus to plus yourself.
AXIS
Axis tells us the direction of the cylinder correction. It is written as a number between 1 and 180. Axis is not a lens strength. It is an angle. This is a common point of confusion because customers sometimes think every number on the prescription is a power.
If your prescription has a cylinder value, it should also have an axis value. The two work together. A cylinder without the correct axis is incomplete.
ADD / Addition
ADD, or addition, is extra plus power used for near vision. It is commonly needed for reading glasses, varifocals or occupational lenses. If you are over your early forties, you may start seeing an ADD on your prescription because the eyes naturally become less able to focus up close with age.
The ADD value is not normally used for standard distance-only glasses. It becomes important when you want reading help, a pair for close work, or varifocals that combine distance and near vision in one lens.
PRISM and BASE
Prism is a correction used to help with eye alignment. It is not present on every prescription. If your prescription includes prism, it should be reviewed carefully before lenses are made. Prism may be written with a base direction such as base in, base out, base up or base down.
Prism is one of the areas where online ordering needs particular care. The values must be copied exactly, and the frame choice may also matter. If you have prism and are unsure, do not guess your way through checkout.
PD / Pupillary distance
PD stands for pupillary distance. It is the distance between the centres of your pupils. Strictly speaking, PD is a measurement rather than part of the refractive prescription, but it is essential for making glasses because it positions the optical centres of the lenses correctly.
Many prescription cards do not include PD. That does not mean it is optional. It simply means it was not written on the copy you were given. For single vision lenses, PD matters. For varifocals, PD and fitting height matter even more.
| Term | Plain English meaning | Common mistake |
|---|---|---|
| SPH | Main power of the lens. | Missing the plus or minus sign. |
| CYL | Astigmatism correction. | Entering it in the sphere box or changing the sign. |
| AXIS | Angle of the astigmatism correction. | Treating it like lens strength. |
| ADD | Extra near power. | Using it when ordering distance-only glasses. |
| PRISM | Eye alignment correction. | Leaving it out because it looks unfamiliar. |
| PD | Measurement between pupils. | Guessing it from a mirror or old guesswork. |
If there is a number on the prescription, it is there for a reason.
Do not ignore prism, ADD, cylinder or axis because you are unsure where they go. A clear photo of the prescription is usually much safer than trying to interpret a complicated card alone.
Plus vs minus prescriptions
The plus or minus sign before the number is one of the most important parts of the prescription. It tells the lab what type of lens power is needed. A plus lens and a minus lens behave differently, look different at the edges, and correct different focusing needs.
Minus lenses are usually thinner in the centre and thicker towards the edges. This is why stronger minus prescriptions can show more edge thickness, especially in large frames. Plus lenses are usually thicker in the centre and thinner towards the edges. In stronger plus prescriptions, frame size and lens design still matter, but the thickness pattern is different.
How plus and minus lenses differ
Usually correct short-sight. Often thicker at the lens edge, especially in larger frames.
Usually correct long-sight. Often thicker through the centre, depending on power and frame choice.
This is why two people with the same frame may not get lenses that look the same. One person may have a low prescription and standard lenses that look very neat. Another may have a stronger prescription that needs a smaller frame or thinner lens material to achieve a better result.
How strong is my glasses prescription?
Customers often ask whether their prescription is “strong”. There is no single answer that works for everyone, because the final lens thickness and comfort depend on more than the number alone. Frame size, frame shape, lens material and how the frame sits all matter.
As a general guide, prescriptions around +/-1.00 to +/-2.00 are often considered lower. Prescriptions around +/-2.00 to +/-4.00 are often moderate. Prescriptions above this may need more careful frame and lens selection. But these are not hard rules. Astigmatism, prism and differences between eyes also affect lens choice.
Do not use prescription strength alone to choose lenses
A -4.00 prescription in a small, round frame may look much neater than a -3.00 prescription in a very oversized frame. The number matters, but the frame can make or break the result.
| Prescription range | What it often means | What to consider |
|---|---|---|
| Up to around +/-2.00 | Often a lower prescription. | Standard lenses may be suitable depending on frame choice. |
| Around +/-2.00 to +/-4.00 | Often a moderate prescription. | 1.6 lenses may be worth considering for neater, lighter lenses. |
| Around +/-4.00 to +/-6.00 | Often a stronger prescription. | Frame size and higher-index lenses such as 1.67 may become more important. |
| Above around +/-6.00 | Often a high prescription. | 1.74 lenses, smaller frames and professional review are strongly worth considering. |
When should you consider thinner lenses?
Thinner lenses are often described by index. The index number tells you how efficiently the material bends light. Higher index lenses can be made thinner than standard lenses for the same prescription. Common options include 1.5, 1.6, 1.67 and 1.74.
A 1.5 standard index lens can be excellent for many lower prescriptions. A 1.6 lens is thinner and lighter than standard and is often a strong all-round choice for moderate prescriptions. 1.67 high-index lenses are usually considered for stronger prescriptions where thickness and weight matter more. 1.74 ultra high-index lenses are commonly used for very strong prescriptions where the thinnest possible plastic lens is preferred.
However, lens index should not be treated like a simple upgrade ladder where the most expensive option is always best. A higher index can help, but it does not override poor frame choice. A very large frame can still create thick-looking lenses even when a thinner material is used.
| Lens option | Often suitable for | What to consider |
|---|---|---|
| 1.5 standard index | Lower prescriptions and everyday single vision use. | Good value and optical clarity, but may look thicker in stronger prescriptions or larger frames. |
| 1.6 thin index | Moderate prescriptions or customers wanting a neater, lighter lens. | A strong all-round option, often around 20% thinner than standard lenses. |
| 1.67 high index | Stronger prescriptions where edge thickness and weight are more noticeable. | Can produce a neater result, especially when paired with a sensible frame shape. |
| 1.74 ultra high index | Very strong prescriptions where the thinnest plastic lens is preferred. | Premium option. Best considered alongside professional frame and prescription review. |
Do not buy lenses like upgrades on a takeaway menu.
The best lens is not automatically the cheapest or the most expensive. It is the one that suits your prescription, your frame and how you use your glasses every day. A 1.74 lens is not automatically better than a 1.6 lens, and paying for unnecessary upgrades wastes money. Equally, choosing a lens that is too thick for a stronger prescription can leave you with heavier glasses and thicker lens edges than necessary.
For a deeper explanation, read the Burghley & Co prescription lenses page or continue to the lens thickness guide when it is available.
When does a prescription mean you need varifocals?
A prescription does not always directly say “you need varifocals”. Instead, it may include an ADD value for near vision. If you need help for both distance and reading, there are several possible options: separate distance and reading glasses, occupational lenses, bifocals or varifocals.
Varifocals are designed to give distance, intermediate and near vision in one pair of lenses. They are useful when you do not want to keep switching between pairs. But they need a suitable frame and accurate fitting measurements, especially fitting height.
One main viewing distance
Suitable for distance, reading or screen use depending on what the prescription is written for.
Multiple distances in one lens
Useful when you need distance and near correction in one pair, but requires more accurate fitting.
Ordering distance glasses when you actually need reading support
If your prescription includes an ADD, make sure you understand what the glasses are being made for. Distance-only glasses, reading glasses and varifocals are not the same product.
Common mistakes when entering your prescription online
Most prescription entry mistakes are simple, but the consequences can be frustrating. They usually happen because the customer is trying to translate an optical document into online boxes without help.
Missing the sign
+2.00 and -2.00 are completely different. The sign is not optional.
Mixing up right and left
Always check whether the prescription uses right/left, RE/LE or OD/OS.
Putting axis in the wrong box
Axis is an angle, not a power. It should only be used with cylinder.
Ignoring prism
If prism is on your prescription, it needs to be included and checked.
Using an old prescription
An old prescription may no longer be accurate, even if your old glasses still feel familiar.
Guessing PD
PD affects optical positioning. Do not estimate it just to complete checkout.
Prescription entry checklist
- I have checked which eye is right and which is left
- I have copied every plus and minus sign correctly
- I have entered sphere, cylinder and axis into the correct boxes
- I have not ignored ADD, prism or base direction
- I have used an up-to-date prescription
- I have not guessed my PD
- I have sent the prescription for review if anything looks unclear
Example prescriptions explained
Examples can make prescriptions easier to understand. The following are simplified examples only, but they show how the numbers work together.
R: -2.00 / L: -2.25
This is a straightforward minus prescription. It would usually be used to help distance vision. Lens thickness would depend on frame size and lens choice.
R: +1.50 / L: +1.25
This is a plus prescription. Depending on age and symptoms, it may help with near vision, distance comfort or general focusing support.
-1.00 / -0.75 x 90
This includes cylinder and axis, meaning astigmatism correction is needed. The axis must be copied with the cylinder.
ADD +2.00
This shows extra reading power. It may be used for reading glasses, occupational lenses or varifocals depending on the order.
Do not self-diagnose from examples.
Examples help you understand the layout, but they do not replace your own prescription or professional advice. Your frame choice, lens type and measurements still need to match your actual order.
When should you ask for help?
You should ask for help whenever something on your prescription card does not make sense. That includes missing values, unclear handwriting, prism, different signs in each eye, a large difference between the two eyes, a strong prescription, or any note you do not understand.
You should also ask if you are changing lens type. Moving from single vision to varifocals, choosing thinner lenses for the first time, ordering prescription sunglasses or selecting a much larger frame are all moments where advice can prevent problems.
Ask before ordering if:
- Your prescription includes prism or base direction
- Your prescription is strong or very different between eyes
- You are unsure whether you need distance, reading or varifocals
- You do not know your PD
- You are choosing a much larger frame than usual
- You have struggled to adapt to glasses before
- The prescription card is unclear or hard to read
Final checklist before using your prescription online
Before ordering glasses online, use this final checklist. It is much easier to check now than after lenses have been made.
Prescription checklist
- I am using my latest glasses prescription, not an old one
- I know which values belong to the right eye and left eye
- I have copied every plus and minus sign correctly
- I understand whether the prescription includes cylinder and axis
- I know whether an ADD value is needed for this order
- I have not ignored prism or base direction
- I understand that PD may be needed even if it is not written on the prescription
- I have chosen a lens type that matches how I will use the glasses
- I have considered thinner lenses if my prescription or frame choice needs them
- I will ask for help before ordering if anything is unclear
The safest order is the one that gets checked before glazing.
There is no prize for guessing your prescription correctly. If the card is unclear, upload or send it for review. A short check before the lenses are made can save time, money and frustration.
Frequently asked questions
What does SPH mean on my prescription?
SPH stands for sphere. It is the main lens power and may be written with a plus or minus sign.
What does CYL mean?
CYL stands for cylinder and corrects astigmatism. If a cylinder is present there should also be an axis.
What does AXIS mean?
Axis is the angle of the astigmatism correction between 1 and 180. It is not a lens power.
What does ADD mean?
ADD is the extra reading power used for reading glasses, occupational lenses and varifocals.
Is my contact lens prescription the same as my glasses prescription?
No. Contact lens and glasses prescriptions are not interchangeable because contact lenses sit directly on the eye.
Why is my PD not on my prescription?
PD is a dispensing measurement and is not always written on your prescription, but it is still needed to make glasses correctly.
Can I order glasses if I have prism?
Yes, but prism prescriptions should be checked carefully before glazing.
Are higher prescription numbers always worse?
No. They simply indicate stronger lens powers. The right frame and lens choice remain just as important.
When should I choose 1.67 or 1.74 lenses?
Higher-index lenses are generally recommended for stronger prescriptions where reducing thickness and weight is beneficial.
What should I do if I cannot read my prescription card?
Do not guess. Upload a clear photograph of the prescription so it can be reviewed before ordering.
Need help?
Unsure what to choose?
If you are not sure about your prescription, frame size or lens choice, ask before you order.