Aug
How to
How to Read Your Eyeglass Prescription
Your eyeglass prescription is a set of measurements, not a code. OD is your right eye, OS is your left. SPH is the lens power that corrects short or long sight. CYL and AXIS together correct astigmatism. ADD is extra power for reading. Once you know those five terms, you can read any prescription in about thirty seconds.
Key takeaways
- OD = right eye, OS = left eye. Some prescriptions use RE and LE instead.
- A minus number in SPH means short-sighted. A plus number means long-sighted.
- CYL and AXIS only appear if you have astigmatism. Blank means you do not.
- ADD appears on reading, bifocal and progressive prescriptions.
- PD (pupillary distance) is often missing from the prescription and must be measured separately.
- Most prescriptions are valid for one to two years depending on where you live.
What do OD and OS mean on a glasses prescription?
OD stands for oculus dexter, Latin for right eye. OS stands for oculus sinister, left eye. If you see OU, that means both eyes together. Some opticians drop the Latin entirely and write RE and LE. They mean the same thing.
The right eye is almost always listed first. If your prescription only has one row, you are being corrected in one eye only.
What does SPH mean?
SPH, or sphere, is the main lens power, measured in dioptres. It corrects short-sightedness (myopia) or long-sightedness (hyperopia).
A minus sign means you are short-sighted: distant objects blur. A plus sign means you are long-sighted: close objects blur. The further the number from zero, the stronger the correction. A -0.75 is mild. A -6.00 is strong and will usually benefit from a high-index lens to keep the lens thin.
What do CYL and AXIS mean?
CYL, or cylinder, corrects astigmatism, where the cornea is shaped more like a rugby ball than a football, so light focuses unevenly. AXIS is a number between 1 and 180 that tells the lab which orientation that correction sits at.
They always appear together. A CYL value with no AXIS is an incomplete prescription. If both boxes are empty or marked DS, you do not have a measurable astigmatism.
What does ADD mean on a prescription?
ADD is additional magnifying power for close work, and it appears once near vision starts to change, typically from the mid-forties. It is always a plus number and is usually the same for both eyes.
If your prescription has an ADD value, you are a candidate for reading, bifocal or progressive lenses. For anyone working across a laptop, a phone and a printed document all day, progressives or a dedicated computer lens are usually the more comfortable answer than swapping pairs.
What is PD and why is it not on my prescription?
PD, or pupillary distance, is the distance in millimetres between the centres of your pupils. It decides where the optical centre of each lens sits, and getting it wrong is one of the most common causes of a new pair feeling subtly wrong.
It is a fitting measurement rather than a clinical one, so many opticians leave it off the prescription. You are entitled to ask for it. You can also measure it yourself with a ruler and a mirror, or use the free VisionCraft Center measurement tool.
Prescription terms at a glance
| Term | What it means | Typical range |
|---|---|---|
| OD / RE | Right eye | — |
| OS / LE | Left eye | — |
| SPH | Main lens power | -8.00 to +8.00 dioptres |
| CYL | Astigmatism correction | -4.00 to +4.00, often blank |
| AXIS | Orientation of CYL | 1 to 180 degrees |
| ADD | Reading power | +0.75 to +3.00 |
| PRISM | Corrects eye alignment | Uncommon |
| PD | Pupillary distance | 54 to 74 mm typical |
How to read your prescription in five steps
- Find the OD row. That is your right eye.
- Read SPH first. Note whether it is plus or minus.
- Check whether CYL and AXIS are filled in. If they are, you have astigmatism.
- Look for ADD. If present, you need reading correction as well.
- Look for PD. If it is missing, measure it or request it before ordering.
What if the numbers do not match between eyes?
Different powers in each eye is completely normal. A large difference between the two (clinically called anisometropia) can affect which lens material and thickness will be most comfortable, and is worth mentioning when you order.
How long is a prescription valid?
Validity varies by country, and typically runs one to two years. An expired prescription is not a safety issue in itself, but it may no longer be accurate, and many retailers will not fill it. If yours has expired, book a fresh test before ordering.
What does PRISM mean on a prescription?
PRISM corrects eye alignment. If your eyes do not point at exactly the same spot, the brain has to work to fuse the two images, which can cause double vision, eye fatigue or headaches. A prism value shifts the image slightly so both eyes can work together comfortably.
It appears as a number in prism dioptres, followed by a base direction: base in, base out, base up or base down. It is far less common than the other values, and it is not something to guess at or copy from an old pair. If PRISM appears on your prescription, mention it when you order, because it affects which frames and lens types will work.
Which lens index do I need for my prescription?
Lens index describes how strongly a material bends light. A higher index means the same correction can be achieved with a thinner lens. For a strong prescription this is the difference between a lens that sits flush in the frame and one that bulges noticeably at the edges.
These are the general guidelines opticians work to. Your dispenser may recommend differently depending on frame shape and your PD.
| Prescription strength (SPH) | Typical lens index | What it means in practice |
|---|---|---|
| Up to about ±2.00 | 1.50 standard | Thin enough in most frames |
| About ±2.00 to ±4.00 | 1.59 to 1.60 | Noticeably slimmer edges |
| About ±4.00 to ±6.00 | 1.67 high index | Worth the cost at this strength |
| Above about ±6.00 | 1.74 high index | Thinnest available, best in smaller frames |
Two things matter alongside index. Smaller frames produce thinner lenses at any prescription, because there is less lens to taper. And an anti-reflective coating matters more as index rises, since higher-index materials reflect more light.
How do I read a child’s prescription?
The terms are identical: OD, OS, SPH, CYL, AXIS. What differs is how quickly the numbers change. Children’s prescriptions are typically reviewed more often than adults’, so a prescription that is a year old may already be out of date.
Children’s PD also changes as they grow, so a measurement taken eighteen months ago should not be reused. If a prescription includes a prism value or notes about patching, that is a clinical treatment plan rather than a simple correction, and should be dispensed by the practice managing it.
What changes between one prescription and the next?
Small movements of 0.25 dioptres are common and often not noticeable. Larger jumps, a new or changed CYL and AXIS, or a first ADD value are the changes you will actually feel.
A useful habit: photograph each prescription when you receive it. Comparing two or three over time tells you whether your eyes are stable or trending in one direction, which is more informative than any single reading.
Common mistakes when ordering from a prescription
- Guessing the PD. Using a population average instead of your own measurement is the single most common cause of a new pair feeling subtly wrong.
- Reading the rows in the wrong order. OD is almost always first. Swapping the two eyes produces a technically perfect pair that is unwearable.
- Dropping the minus sign. A -2.00 and a +2.00 are opposite corrections. Copy the sign exactly.
- Entering CYL without AXIS. The two only work together. One without the other is an incomplete order.
- Using an expired prescription. Legal in some places, but the numbers may no longer reflect your vision.
- Ignoring the ADD value. If it is on the prescription, single-vision distance lenses will not solve your close work.
Frequently asked questions
Can I use my contact lens prescription for glasses?
No. Contact lenses sit directly on the eye and glasses sit roughly 12 mm in front of it, so the powers are calculated differently. The two are not interchangeable.
What does DS or PL mean on my prescription?
DS means dioptre sphere and PL means plano. Both indicate zero correction in that box.
Does a higher number mean worse eyesight?
It means more correction is needed, not that your eyes are unhealthy. Prescription strength and eye health are separate things.
Can I order glasses without a PD?
You can, but you should not guess it. Measure it properly or ask your optician. An inaccurate PD is one of the commonest reasons a technically correct pair still feels wrong.
When should I see an eye-care professional?
If you notice sudden vision changes, persistent headaches, double vision, flashes or floaters, book an appointment rather than reaching for a new prescription. An eye test checks eye health, not just focus.
Once you can read it, ordering gets simple
A prescription is five pieces of information and one measurement. Once you can read them, buying online stops being a leap of faith and becomes a specification exercise, which is exactly how eyewear should be bought.
Ready to put it to use? Browse OpteLux titanium eyeglasses or prescription sunglasses, and measure your PD free at the VisionCraft Center before you order.