Your eye doctor hands you a small piece of paper after your exam, and it is covered in abbreviations, plus and minus signs, and numbers that seem to follow no obvious logic. Understanding what a glasses prescription means can help you order the right lenses, compare prices across retailers, and have more informed conversations with your eye care provider.
OD and OS
These Latin abbreviations refer to your two eyes. OD stands for “oculus dexter” — the right eye. OS stands for “oculus sinister” — the left eye. Some prescriptions also use OU, meaning “oculus uterque” or both eyes together. When filling a prescription, always make sure lenses are assigned to the correct eye, particularly if there is a significant difference between the two.
Sphere (SPH)
The sphere value describes the overall refractive power of the lens needed to correct your vision, measured in diopters. A minus sign (–) indicates nearsightedness (myopia): the lens corrects eyes that focus in front of the retina instead of on it. A plus sign (+) indicates farsightedness (hyperopia): the lens corrects eyes that struggle to focus at close distances. The larger the number — in either direction — the stronger the prescription.
A sphere value of –1.00 is mild myopia. A value of –6.00 is considered high myopia and carries a higher long-term risk of retinal complications.
Cylinder (CYL) and Axis
These two values work together to correct astigmatism — a condition where the cornea or lens is slightly oval or irregular in shape rather than perfectly round, causing blur at all distances. The cylinder value indicates the amount of astigmatism correction needed. A minus cylinder is used in most US prescriptions; some European countries use plus cylinder notation for the same eyes. The axis is a number from 1 to 180 that describes the orientation of the astigmatism — which direction the irregular curvature runs. The cylinder and axis values are meaningless without each other.
If your prescription has no cylinder or axis, your vision is free of significant astigmatism, and only the sphere and add (if applicable) matter.
Add Power
Add power appears on prescriptions for patients who need bifocals, progressives, or reading glasses in addition to their distance correction. It represents the extra magnifying power added to the lower portion of a bifocal or progressive lens to help with near vision. Add power is always a positive number and typically ranges from +0.75 to +3.00. The same add power usually applies to both eyes, even if the distance prescription differs significantly between eyes.
Pupillary Distance (PD)
PD is not always included on a prescription, but it is required when ordering glasses. It measures the distance in millimeters between the centers of your pupils and ensures that the optical center of each lens is properly aligned with your eye. PD can be measured as a single number (the total distance) or as two numbers representing each eye’s distance from the nose (monocular PD). An accurate PD is especially important for strong prescriptions — a significant error can cause headaches, eye strain, and prismatic distortion.
Prism
Prism correction is included in some prescriptions to treat binocular vision problems — conditions where the eyes do not work together correctly, causing double vision or significant eye strain. Prism values are expressed in prism diopters and paired with a base direction (BU, BD, BI, or BO — base up, down, in, or out). If your prescription includes prism, ensure that your optician and any online retailer you use is experienced in grinding prism into lenses.
Try our prescription decoder tool to quickly interpret your prescription values. For a full eye examination and updated prescription, visit our eye care services page.