What Do the Numbers on My Prescription Mean?

Ordering prescription glasses online means entering your prescription into a form. But what does it all mean?

What Do the Numbers on My Prescription Mean?

Prescription glasses are made using numbers designating specific types of correction and measurements to describe the locations of your eyes.

The main numbers on your prescription are corrections for nearsightedness or farsightedness, and astigmatism (or some combination of these). The farther the number is from zero (above or below), the stronger your prescription is (excluding the Axis measurement).

If you’re getting ready to enter your prescription into one of our order forms online, here’s what all the numbers on it mean:

• Your eyes are marked as OD (right eye) and OS (left eye). The right eye is always first on your prescription.
• Sphere, Cylinder, and Axis are the main correction numbers, and at least one is present on just about every prescription.
• Sphere is the correction for nearsightedness (if it’s negative) or farsightedness (if it’s positive). This type of correction needs to be centered on the pupil, but it is the same correction no matter how you turn the lens. In other words, if you were to put your glasses on upside-down, the correction would be the same.
• Cylinder and Axis work together to correct astigmatism. Whereas Sphere is the same correction no matter how it’s turned, Cylinder must be rotated in a specific direction to give the correct prescription. Axis is the degree measurement, between 0 and 180, that the cylinder is turned.
• Pupil Distance, or PD, is the millimeter distance between your pupils. It is sometimes written as a single number, or as two numbers – one from the center to your right eye, one from the center to your left eye. It is also sometimes designated for “distance” and “near” vision seperately, because your pupils are closer together when you look at something up close. PD is not always marked on the prescription.
• Segment Height is the millimeter distance from the bottom of a lens to the top of a bifocal. On a lined bifocal, the “top” is the line. On a progressive, the top is the center of your pupil. Segment Height is different for every frame and is often estimated. Segment Height is not relevant for Single Vision lenses.
• Addition strength, or ADD, is the strength of a bifocal. It is only marked if you need separate correction for reading, and it is always a positive number. It is also almost always the same for both eyes, meaning that if the doctor only wrote it in once, you can assume it’s the same for both.
• Prism is correction for vergence dysfunction and other related conditions where the eyes to not move or sit in proper coordination, resulting in double vision. This is done on an angle, so it is a power plus a direction. For instance, if one eye is always looking up more than the other, your prism may be 1 BD. BD stands for Base Down – it is correcting your vision down.

Your prescription may seem very confusing and unreadable upon first glance, especially because optical professionals rarely label the eyes “right” and “left,” which would at least give you something familiar on the page. If you read through the bullet points above, however, the number on your prescription should make sense, even if you don’t necessarily know how “strong” a certain number is relative to other prescriptions.

When ordering your glasses, you should be especially careful to ensure you put the right sign down for each number (positive or negative) and double check that you’ve filled everything out correctly. It’s also a good idea to ask questions if you’re not sure about something. You can call our customer service department if you’re a little unsure of something, or you are welcome to leave a comment on one of our blog posts.

Let us know what you’re thinking! We’d love to hear your comments. As always, thanks for reading!