How to read your eyeglass prescription
Regular eye checks are an important part of keeping your eyes healthy and vision clear. If your vision needs to be corrected, your eye care professional (ECP) will give you a prescription for eyeglasses or contact lenses and recommend the degree of the lens that you need to see clearly. Eyeglass prescriptions contain various numbers and letters that can be confusing at first. This article will help you make sense of these numbers and letters and guide you on how to read your eyeglass prescription.
Dioptres
Dioptres (D) is the unit used to measure the degree or power of the corrective lens required for clear vision. The further away a number is from 0, the more correction or stronger prescription is needed to produce clear vision for distance and reading. Sphere power, cylinder power, and add power are written in decimal form in quarter dioptre increments (0.25D).
OU: an abbreviation for oculus uterque, Latin phrase for both eyes
OD: an abbreviation for oculus dexter, Latin phrase for right eye
OS: an abbreviation for oculus sinister, Latin phrase for left eye
Instead of OD and OS, some ECPs use RE (right eye) and LE (left eye).
Sphere
Sphere (SPH) indicates the amount of lens power prescribed to correct myopia (short-sightedness) or hypermetropia (long-sightedness). A minus sign (-) means that you are short-sighted and require concave lenses to correct your vision. A plus sign (+) means you are long-sighted and require convex lenses to correct your vision.
For example,
-2.00: represents 2 dioptres of short-sightedness
-6.25: represents 6 and ¼ dioptres of short-sightedness
+3.00: represents 3 dioptres of long-sightedness
Cylinder
Cylinder (CYL) indicates the amount of lens power needed to correct astigmatism. CYL always follows the SPH power on an eyeglass prescription. The number in the cylinder column can be negative (minus) or positive (plus). When a prescription for astigmatism is written in a positive or plus format, the ECP will convert it into a minus cylinder format by using a formula.
For example:
-2.00×180: represents 2 dioptres of astigmatism at an axis of 180 degrees
+5.00×90: represents 5 dioptres of astigmatism at an axis of 90 degrees
Axis
Axis is a number between 0 and 180 degrees that indicates the orientation of the astigmatism. For people with astigmatism, there will be 3 numbers in your prescription: Sph / Cyl x Axis. For example, -2.00/-1.00×180 means 2D of short-sightedness, 1D of astigmatism at an axis of 180 degrees. If you don’t have astigmatism, the cylinder and axis boxes will be empty or only one value is written on the prescription. For example, -1.00D indicates the presence of only 1 dioptre of short-sightedness with no astigmatism.
Figure 1. Vision of a person with and without astigmatism.
Near addition
Near addition (ADD) indicates the additional lens power needed to correct presbyopia. The value appearing in this section is always a plus power, even if there is no sign to indicate that. ADD is usually the same for both eyes. You may also have an Intermediate ADD, indicating the lens power needed for computer vision or tasks performed at that distance. While an ADD lens power can differ between individuals, ECPs use an age-based table as a guide when doing a refraction test, as seen below:
| Age (years) | Near addition at 40cm |
| 40 to 45 | Less than +1.00D |
| 45 to 50 | +1.25 to +1.50D |
| 50 to 55 | +2.00D to +2.25D |
| 55 to 60 | +2.25D to +2.50D |
| 60 to 65 | +2.50D to +2.75D |
| More than 65 | More than +3.00D |
Prism
Prism indicates the amount of prismatic power that your eyeglasses need to compensate for a difference in the alignment of your eyes. If present, along with the amount of prism, the direction of the prism is indicated by noting the position of the base.
Here are the four abbreviations used to indicate prism direction:
- BU=base up
- BD=base down
- BI=base in
- BO=base.
For example, -2.00D with prism 0.50BD means 2 dioptres of short-sightedness and 0.5 or half a dioptre of prism power at a base down orientation.
Inter-pupillary distance
Inter-pupillary distance (PD) is the distance (in millimeters) between both pupils in the eyes. Monocular PD indicates the distance from your pupil to the middle of your nose. Binocular PD indicates the distance from one pupil to the other.
Visual acuity
Visual acuity (VA) indicates how well you can see with the degree of lenses written in the prescription.
Can I use an eyeglass prescription to buy contact lenses?
The short answer is no. An eyeglass prescription does not contain information for a contact lens prescription. Eyeglasses are positioned at a distance from the eyes, while contact lenses sit directly on the surface of your eyes. The distance affects the power of the lens required to produce clear vision. A contact lens prescription must specify the base curve and diameter of the contact lens.
Example of how to read your eyeglass prescription
Compiling all the components mentioned above, here is an example of what an eye prescription looks like:
| SPH | CYL | Axis | ADD | Prism | |
| OD | -3.00 | – | – | +2.00 | 1BD |
| OS | -2.00 | -1.00 | 180 | +2.00 | 1BU |
In the right eye (OD), the ECP prescribed:
- -3.00D for the correction of short-sightedness
- There is no cylinder power or axis, which means that there is no astigmatism, as indicated with a dash symbol.. Some ECPs may choose to leave it empty or write SPH, indicating that there is only sphere power and no cylinder
In the left eye (OS), the ECP prescribed:
- -2.00D for the correction of short-sightedness
- -1.00D cylinder for the correction of astigmatism
- A cylinder power with an axis of 180 degrees
Both eyes are prescribed with:
- Near addition or ADD of +2.00D for the correction of presbyopia
- Prism correction of 0.5 prism dioptre in each eye. In the right eye, the prism is base down (BD) and in the left eye, the prism is base up (BU)
We hope that this article has been helpful for you to better understand how to read your eyeglass prescription. This article is only a guide and is not intended to replace the information that you may have already received from your ECP. As always, it is always best to seek advice directly from your ECP if you have any queries about your eye check visit.
Tools Designed for Healthier Eyes
Explore our specifically designed products and services backed by eye health professionals to help keep your children safe online and their eyes healthy.