OD is your right eye and OS your left. SPH (sphere) is the main lens power: minus numbers correct nearsightedness, plus numbers correct farsightedness. CYL and AXIS correct astigmatism: CYL is the strength and AXIS is the angle from 1 to 180. ADD is extra magnifying power for reading, used in progressive and bifocal lenses. To order glasses online you also need your PD, the distance between your pupils in millimeters.
A glasses prescription looks like a table of codes, but each number has a simple job. Here is how to read yours. Always get your prescription from a qualified eye care professional.
A sample prescription
| SPH | CYL | AXIS | ADD | |
|---|---|---|---|---|
| OD (right) | -2.25 | -0.75 | 180 | +2.00 |
| OS (left) | -2.00 | -0.50 | 175 | +2.00 |
What each term means
- OD / OS / OU: right eye, left eye, both eyes.
- SPH (sphere): the main correction in diopters. Minus corrects nearsightedness; plus corrects farsightedness.
- CYL (cylinder): extra power to correct astigmatism. Blank or “SPH” means no astigmatism.
- AXIS: the angle (1 to 180 degrees) where the CYL correction sits. It only appears with CYL.
- ADD: added magnifying power for close work, used in progressive, bifocal and reading lenses. Usually +0.75 to +3.00.
- Prism and base: corrects eye alignment issues; less common. Listed with a direction (base up, down, in or out).
- PD (pupillary distance): the distance between your pupils in mm. Needed to center the lenses.
How strong is my prescription?
As a rough guide for SPH:
| SPH | Description |
|---|---|
| ±0.25 to ±2.00 | Mild |
| ±2.25 to ±5.00 | Moderate |
| ±5.25 and up | High |
Higher prescriptions benefit from high-index lenses, which are thinner and lighter.
Ordering glasses online
- Use a current prescription, typically valid for one to two years depending on where you live.
- Enter values exactly as written, including minus and plus signs.
- Add your PD. If you have two numbers (for example 31/32), that is a monocular PD for each eye.
- Choose lenses: single vision, progressive or reading, plus coatings.
- Check the return and remake policy in case the prescription feels wrong.
When to get a new exam
Get an eye exam if your vision changes, you have headaches or eye strain, or your prescription has expired. An eye exam also checks eye health, not just vision.
Common questions
What does OD and OS mean on a prescription?
OD (oculus dexter) is the right eye and OS (oculus sinister) is the left eye. OU means both eyes.
Is my glasses prescription the same as my contact lens prescription?
No. Contact lenses sit on the eye, so the power can differ, and a contact lens prescription also includes base curve, diameter and the specific lens brand. You need a separate contact lens prescription from your eye care provider.
What is PD and how do I find it?
PD (pupillary distance) is the distance between the centers of your pupils in millimeters, usually 54 to 74 mm for adults. Ask your optometrist to include it, or measure it with a ruler and mirror or an online tool.
What does a minus sign in SPH mean?
A minus number means you are nearsighted (distance is blurry). A plus number means you are farsighted (close-up is harder). The bigger the number, the stronger the correction.