How to Read Your Glasses Prescription

Your glasses prescription can look like a jumble of letters, numbers, and symbols. But once you understand what each value means, it becomes simple. Here's your complete guide.

OD and OS

**OD** stands for *Oculus Dexter* — Latin for Right Eye.

**OS** stands for *Oculus Sinister* — Latin for Left Eye.

Some prescriptions also include **OU** (*Oculus Uterque*) — both eyes.

SPH (Sphere)

The sphere value indicates the lens power needed to correct your vision. The unit is diopters (D).

The higher the number (positive or negative), the stronger the prescription.

CYL (Cylinder) and Axis

These two values are always paired and are used to correct **astigmatism** — an irregular curvature of the cornea.

If your prescription has no CYL and Axis values, it means you don't have significant astigmatism.

ADD (Addition)

The ADD value is used in **reading glasses** and **progressive lenses**. It indicates extra magnifying power added to the bottom portion of multifocal lenses for near vision.

ADD values are always positive and typically range from +0.75 to +3.50.

PD (Pupillary Distance)

PD is the distance between your pupils in millimeters, measured from the center of one pupil to the center of the other. This is critical for correct lens alignment.

Converting Glasses to Contact Lens Prescription

If your SPH is stronger than ±4.00, your contact lens prescription will be slightly different from your glasses prescription due to **vertex distance** — the 12mm gap between glasses and your eye.

Use our [free Prescription Converter tool](/prescription) to get your contact lens equivalent.