Reviewed by Jonathan Miu, B Optom / B Sci, Kellyville Optical
An eye prescription lists the lens power each eye needs to see clearly. OD is your right eye and OS is your left. SPH corrects short- or long-sightedness, CYL and AXIS correct astigmatism, and ADD is extra power for reading. All powers are measured in dioptres, in steps of 0.25.
Prescriptions can look like a string of codes, so this guide explains each term in plain English, with a worked example on how to read your eye prescription.
Your prescription at a glance
- OD (oculus dexter): your right eye. Sometimes written R.
- OS (oculus sinister): your left eye. Sometimes written L.
- OU (oculus uterque): both eyes.
- SPH (sphere): the main lens power. A minus (−) number means short-sighted (myopia), and a plus (+) number means long-sighted (hyperopia).
- CYL (cylinder): extra power to correct astigmatism. Blank or “DS” means no astigmatism correction is needed.
- AXIS: the angle of the astigmatism correction, from 1 to 180 degrees. It only appears when there’s a CYL value.
- ADD (addition): extra magnifying power for close work, used in reading glasses, multifocals and progressives.
- PD (pupillary distance): the distance between the centres of your pupils, in millimetres. It’s used to centre the lenses in your frames.
- PRISM: a correction that helps the eyes work together, written with a base direction (BU, BD, BI or BO). Most people don’t need it.
- Plano (PL) or 0.00: no correction needed for that value.
Frequently asked questions
SPH stands for sphere. It’s the main lens power needed to focus your vision, measured in dioptres. A minus number (e.g. −2.00) means you’re short-sighted and see near objects more clearly than far ones. A plus number (e.g. +1.50) means you’re long-sighted.
CYL stands for cylinder. It’s the extra lens power that corrects astigmatism, where the front of the eye is shaped more like a rugby ball than a soccer ball. If the CYL box is empty or says DS, you don’t need astigmatism correction.
AXIS is the angle, from 1 to 180 degrees, at which the astigmatism correction (CYL) is positioned in the lens. It tells the lab which direction to orient that correction. An axis number always goes with a CYL value, and on its own it doesn’t mean your prescription is stronger or weaker.
OD means your right eye and OS means your left eye. They’re short for the Latin oculus dexter and oculus sinister. OU means both eyes. Many Australian prescriptions simply use R and L instead.
ADD is the extra magnifying power added to the bottom of the lens for reading and close work. It’s usually needed from your 40s onwards as the eye’s focusing becomes less flexible (presbyopia). ADD is used for reading glasses, bifocals and progressive lenses.
PD (pupillary distance) is the distance between the centres of your pupils in millimetres. It makes sure the optical centre of each lens lines up with your eyes. PD is often measured when you order glasses rather than during the eye test, so it may not be written on your prescription.
A minus (−) prescription corrects short-sightedness (myopia), where distant objects look blurry. A plus (+) prescription corrects long-sightedness (hyperopia), where close objects are harder to focus on. The bigger the number, the stronger the lens.
The number alone doesn’t tell you how healthy your eyes are. As a general guide, many optometrists describe a sphere up to about ±2.00 as low, about ±2.25 to ±5.00 as moderate, and above ±5.00 as high. Your optometrist can explain what your prescription means for you, and whether it’s changing
No, a glasses prescription can’t be used for contact lenses. Contact lenses sit on the eye rather than in front of it, so the power can differ, especially for stronger prescriptions. A contact lens prescription also includes the lens brand, curve and diameter, which needs a contact lens fitting.
Most prescriptions last 2 years, some can be shorter. There will be an expiry date on your prescription to let you know when it expires. Medicare covers a comprehensive eye test with an optometrist once every three years if you’re under 65, and every year from 65. See an optometrist any time your vision changes.
How to read your prescription, step by step
To read an eye prescription, work across each line from left to right: which eye it’s for, then sphere, cylinder, axis and any reading addition. Every prescription follows the same order, even if the layout looks different.
- Find the eye. The top line is usually the right eye (OD or R) and the second line the left eye (OS or L).
- Read the sphere (SPH). Check the sign first. Minus means short-sighted, plus means long-sighted. Then the number: the further from zero, the stronger the lens.
- Check for cylinder (CYL). A number here means you have some astigmatism. No number, or “DS”, means you don’t need astigmatism correction.
- Read the axis. If there’s a CYL value, the axis (1 to 180) shows where the correction sits. Treat CYL and axis as a pair.
- Look for ADD. A plus number here is extra power for reading. It’s usually the same for both eyes.
- Note anything else. Prism, PD or notes like “for computer use” or “distance only” tell the optical dispenser how to make your lenses.
Worked example
Here’s how a typical prescription might look:
- R (OD): SPH −2.50, CYL −0.75, AXIS 180, ADD +1.50
- L (OS): SPH −2.00, CYL −1.25, AXIS 170, ADD +1.50
What it means:
- Both eyes are short-sighted, because the sphere values are minus. The right eye (−2.50) is slightly stronger than the left (−2.00).
- Both eyes have some astigmatism. The left eye has a little more (−1.25 compared with −0.75), and the axis values show it sits at a slightly different angle in each eye.
- An ADD of +1.50 means extra power is needed for reading. This person might suit progressive lenses, which blend distance and reading in one lens.
Why do some prescriptions look different?
Prescriptions from different practices use different layouts, but the information is the same. Some write R and L instead of OD and OS. Some leave CYL blank or write DS when there’s no astigmatism. And cylinder can be written as a plus or a minus value. Both describe the same correction, just written differently, so you can’t compare two prescriptions number for number.
Glasses and contact lens prescriptions are different
A glasses prescription and a contact lens prescription aren’t interchangeable. Glasses sit about 12 millimetres in front of your eyes, while contact lenses sit directly on them, so the power you need can change, especially above about ±4.00. A contact lens prescription also lists details a glasses prescription doesn’t:
- Base curve (BC): the curve of the lens, matched to the shape of your eye
- Diameter (DIA): the size of the lens
- Brand and lens type: for example, daily disposable or monthly
These are worked out at a contact lens fitting, where your optometrist checks the lens sits comfortably and your eyes stay healthy. Buying contact lenses without a fit check can lead to eye infections if it is an incorrect fit, similar to how wearing ill-fitting shoes can lead to issues. Read more about glasses vs contact lens prescriptions.
When should you have your prescription checked?
Have your eyes checked if your vision seems blurrier, you’re getting more headaches or eye strain, or you’re squinting to read or see signs while driving. Otherwise, follow the check-up schedule your optometrist recommends. Children and people with conditions such as diabetes may need more frequent checks.
A prescription measures how your eyes focus, but an eye test also checks your eye health. That’s why it’s worth coming in for regular tests, even if your glasses still feel fine.
