How to Read Your Glasses Prescription (SPH, CYL, Axis, Add, PD)
What every number on an eyeglass prescription means, how to get your PD, and what to check before ordering glasses online.
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Your glasses prescription is a short table of numbers, one row per eye. Once you know what each column means, ordering glasses online is mostly a matter of copying them in carefully.
This guide explains the terms; it isn’t medical advice. Always use a current prescription from an optometrist or ophthalmologist, and ask them if anything is unclear.
The rows: which eye is which
- OD – oculus dexter, your right eye.
- OS – oculus sinister, your left eye.
- Some prescriptions write OU for both eyes, or simply “Right” and “Left”.
The columns
| Term | What it means |
|---|---|
| SPH (sphere) | The main lens power, in diopters. A minus sign means nearsighted; a plus sign means farsighted. The further from zero, the stronger the prescription. |
| CYL (cylinder) | The amount of astigmatism correction, if any. A blank column usually means no astigmatism correction. |
| Axis | The direction of the astigmatism, in degrees from 1 to 180. It only appears when there’s a CYL value. |
| Add | Extra magnifying power for close-up reading, used in bifocals and progressives – common after about age 40. |
| Prism | A correction some people need for double vision, which helps merge two images into one. |
Source: American Academy of Ophthalmology.
Example
| SPH | CYL | Axis | Add | |
|---|---|---|---|---|
| OD (right) | −2.25 | −0.75 | 180 | |
| OS (left) | −2.00 | −0.50 | 170 |
This person is nearsighted in both eyes, with mild astigmatism, and needs no reading add.
Pupillary distance (PD)
PD is the distance between the centers of your pupils, in millimeters. Online stores need it to center each lens in front of your eye. Two things to know:
- It’s often not on your prescription. According to the FTC, only a few states require prescribers to include it, and some allow a charge for measuring it. If you were measured but not given the number, you may be able to request it from your records.
- Accuracy matters more for some prescriptions. The AAO notes that a small PD error can cause problems with progressives and high-power lenses, while low-power single-vision glasses are more forgiving.
You can get your PD from your eye doctor or optician, or measure it with the tools most online stores provide (ruler method or a camera-based measurement).
Your rights to your prescription (US)
- Your prescriber must give you a copy after an eye exam that includes a refraction, whether you ask or not, and before offering to sell you glasses.
- They can’t charge extra for the copy, require you to buy glasses from them, or ask you to sign a waiver.
- Eyeglass prescriptions usually last one to two years, depending on state law; you can’t use an expired one to order.
Source: FTC.
Glasses vs. contact lens prescriptions
They’re different. A contact lens prescription includes measurements specific to the lens size and brand and requires a fitting, so you can’t use a glasses prescription to order contacts (or vice versa).
Checklist before ordering online
- Prescription is current (not expired).
- Right (OD) and left (OS) entered on the correct rows.
- Plus and minus signs copied exactly.
- Axis entered only if there’s a CYL value.
- PD entered (single number, or two numbers – one per eye – if that’s how it was given).
- Add included if you’re ordering progressives, bifocals or readers.
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