Confused by the numbers on your glasses prescription? Learn what OD, OS, SPH, CYL, AXIS, ADD and PD mean, what plus and minus powers show, and why a prescription is not the same as a contact lens one.
You have had your eyes tested and received a slip full of numbers and abbreviations. What do they actually mean? Understanding your eyeglass prescription helps you know your eyes better, compare prescriptions over time and order the right lenses.
A Sample Prescription
| Eye | SPH | CYL | AXIS | ADD |
|---|---|---|---|---|
| OD (Right) | -2.00 | -0.75 | 180 | +1.50 |
| OS (Left) | -1.75 | -0.50 | 170 | +1.50 |
PD: 63 mm
What Each Term Means
OD, OS and OU
OD (oculus dexter) is the right eye, OS (oculus sinister) is the left eye, and OU means both eyes. Some prescriptions simply write RE and LE.
SPH (Sphere)
The main lens power in dioptres (D).
- Minus (–) means short-sightedness (myopia): difficulty seeing far. See myopia in children.
- Plus (+) means long-sightedness (hyperopia): near work is harder, and strong amounts affect distance too.
CYL (Cylinder)
The amount of astigmatism. If this box is empty, you have little or none.
AXIS
The direction of the astigmatism, from 1 to 180 degrees. It only appears when there is a CYL value.
ADD (Addition)
Extra plus power for reading, usually after 40, used for reading glasses, bifocals or progressive lenses.
PRISM
Occasionally prescribed to help eye alignment or double vision.
PD (Pupillary Distance)
The distance between your pupils in millimetres. It is needed to centre the lenses correctly. Progressives need a separate PD for each eye.
How Strong Is My Prescription?
| Power (SPH) | General description |
|---|---|
| Up to ±2.00 | Mild |
| ±2.25 to ±5.00 | Moderate |
| Above ±5.00 | High; thinner high-index lenses are worth considering |
Glasses vs Contact Lens Prescription
They are not the same. Contact lenses sit directly on the eye, so for higher powers the value is adjusted, and a contact lens prescription also includes base curve, diameter and brand. A contact lens fitting is needed to get one.
Does a Higher Number Mean Unhealthy Eyes?
Not necessarily. A prescription describes focus, not eye health. Someone with –6.00 can have healthy eyes, while someone with no power can have glaucoma. That is why a full eye exam includes a health check.
How Long Is a Prescription Valid?
We recommend a re-check every one to two years for adults, yearly for children, and sooner if vision changes. People with diabetes should have a yearly eye exam.
Frequently Asked Questions
Can I order glasses online with my prescription?
Yes. Share your prescription and PD when ordering from our shop, or WhatsApp a photo of it and we will help.
My two eyes have different numbers. Is that normal?
Small differences are very common. A large difference should be monitored, especially in children, because of the risk of lazy eye.
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Railway Station Road, Central Jail Bazar, Haripur · Open 7 days, 10am – 8:30pm · Free eye camp every Sunday, 10am – 2pm
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Medically reviewed by
Optometrist at Sardar Eye Clinic & Optics, Railway Station Road, Haripur, BS Optometry (Khyber Medical University) and hospital training at HMC Peshawar and Abbottabad Medical Complex, experienced in eye examinations, glasses and contact-lens prescriptions, and children’s eye care. This article is for general information — book an eye test for advice about your own eyes.
