OPTOMETRIST EXPLAINS HOW TO READ A GLASSES PRESCRIPTION

11/12/2020 | Dr. Sara Frye | 5 Min Read

Ever wondered what all those letters and numbers mean on that slip you get when you leave your eye exam? This article will review how to read a glasses prescription. It’s actually very simple once you break it down into its key components. It may look like Greek to you (and some of it does come from Latin!) but it’s really not that complicated. Once you’re through with this article, you’ll be able to understand exactly how to read your glasses prescription. Every time you get a new prescription, you’ll be able to understand what’s changed before deciding whether to buy new glasses.

What Does an Eyeglass Prescription Look Like?

The format of your glasses prescription will vary depending on the person that wrote it for you. Some are computerized, while others are hand-written.

Some will use a grid format, where boxes are filled in, while others will be written on a blank pad.

Standard Grid Glasses Prescription

Most prescriptions are written from left to right, but you may also see a vertical prescription, which is written from top to bottom.

Vertical Prescription of Glasses

You’ll be able to identify if your prescription is for a bifocal or progressive lens if it contains an “add”, which stands for lens addition.

Some prescriptions also incorporate prism, in which case you’ll find both the amount of prism and the base on the prescription. Some components will be found on every glasses prescription, while others will only apply in certain cases.

What Do The Numbers On The Prescription Mean?

When you’re learning how to read a glasses prescription, you’ll need to understand each component. Here, we’ll break down each element into simple terms.

OD and OS

Most glasses prescription will include the terms OD and OS. These are abbreviations for the Latin words oculus dextrus and oculus sinister, respectively. OD means right eye and OS means left eye. You may also see the abbreviation OU, for oculus unitas, which means both eyes.

Sphere

The term sphere stands for the spherical part of your glasses prescription. If you are myopic, or nearsighted, this number will be negative. If you are hyperopic, or farsighted, this number will be preceded by a positive sign. Nearsighted individuals have more trouble with their distance vision while farsighted people have more problems seeing up close. The unit for this measurement is Diopters, so you’ll sometimes see a D after the value. Spherical values are reported in 0.25 Diopter increments, such as –2.25. Sometimes, the notation DS, for Diopters Sphere, will appear after the sphere value. This indicates there is no astigmatism. Another way this can be specified is by writing the word “sphere” after the spherical value.

Example: -2.25 DS or -2.25 sphere

Cylinder

Cylinder refers to the astigmatism portion of the glasses prescription. Some eye care professionals will report this number with a minus sign in front of it, while others will report it with a plus sign preceding it. Either way, this value refers to the amount of astigmatism you have. Astigmatism is often due to the shape of the cornea, where its form resembles a football, rather than a basketball. This value is reported in 0.25 Diopter increments, like the sphere value. The higher this number, the more astigmatism you have.

Example: -2.25-1.00x180

If you do not have astigmatism, the cylinder field may be left blank. Sometimes, the notation DS, for Diopters Sphere, will appear after the sphere value. This indicates there is no astigmatism. Another way this can be specified is by writing the word “sphere” after the spherical value.

Glasses Prescription

Axis

The axis refers to the astigmatism orientation. Every cylinder must have an accompanying axis. This specifies how to orient the cylinder in the lens. This number is preceded by an “x”, which means axis. The axis is a number between 1 and 180. If you have a glasses prescription with a negative cylinder value and another with a positive cylinder value, the axes will be 90 degrees apart but the prescription will be equivalent. Two prescriptions may look different due to the convention in which they are written, but they could actually be the same.

Example: -2.25-1.00x180 is equivalent to –2.25+1.00x090

If you do not have astigmatism, the axis may be left blank. Sometimes, the notation DS, for Diopters Sphere, will appear after the sphere value. This indicates there is no astigmatism. Another way this can be specified is by writing the word “sphere” after the spherical value.

Prism

Some people require prismatic power to better align the image to their eyes. Prism helps with both vertical and horizontal eye alignment problems. This problem is relatively uncommon, but if you require prism you’ll see both the magnitude and the orientation specified on your glasses prescription. The value is reported in 0.25 Diopter increments. The orientation could be in one of four directions: base up (BU), base down (BD), base out (BO) or base in (BI).

Example: -2.25-1.00x180 prism: 2.00 BD

Add

The add power specifies an added reading prescription that can be put into a bifocal or progressive lens. This is most commonly used for people with presbyopia, which is a condition that develops in your forties where the eyes have difficulty focusing on near objects. Although add powers are most commonly used in presbyopes, they are occasionally used to help children or young adults to read more comfortably. An add power is reported in 0.25 Diopter increments, most commonly between 1.00 to 2.50, though you can see values above or below that. An add power is always positive. You may see a plus sign in front of the value, or it could be left off. Either way, it means the same thing.

Example: -2.25-1.00x180 add: +2.00

PD

Sometimes the pupillary distance, or PD, is added to a glasses prescription. This refers to the distance between the two eyes, or more specifically the pupils, in millimeters. If this value is not on your prescription, it will need to be measured by an optician to ensure that your prescription glasses are centered appropriately over your eyes.

Example: -2.25DS PD: 63.0

That is how to read a glasses prescription!

 
Writer Picture
Dr. Sara Frye,
Optometrist and Medical writer

Dr. Sara Frye (née Gaib) is a highly skilled optometrist and medical writer. She was born in France, grew up in Canada, and now lives in the United States. She did her Bachelor of Science at the University of British Columbia and her Doctor of Optometry at Nova Southeastern University. She then completed her residency in Cornea and Contact Lens at the University of California, Berkeley. Her passion for learning drove her to pursue further studies; she completed a Master of Public Health in Health Services Administration at the University of Arizona.
Dr. Frye has years of experience in teaching, earning the rank of Associate Professor at the Arizona College of Optometry at Midwestern University. She has since been in clinical practice. With experience as both a health care professional and an academic, Dr. Frye has developed unparalleled skills in medical writing and editing.
 
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Check size on your frame

Most frames have size displayed on the inside of the frame arm

Or use a ruler

Use a ruler to measure your existing frame as shown below. Frame sizes are measured in millimeters, so you need to use a ruler that is marked in centimeters and millimeters. If you don't have millimeter ruler, you can click here to print it, or take measurements in inches. Note that measurements in inches need to be taken with the precision of 1/16 (one half of 1/8 if your ruler does not have more precise markings)

I got it! Here is My Size

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Measure your PD

In some cases your vision correction doctor may forget to give you your PD (pupillary distance). The best option will be to ask your doctor to measure it. It is part of your prescription and an eye doctor needs to provide it.

If this is not possible, you may use the methods listed below.

The easiest way:
1) Wear any glasses with clear lenses.
2) Use a mirror or ask a friend or family member to mark your pupil location on the clear lenses using a dry erase marker. They must stand in front of you and be at the same height as yours. If you use a mirror make sure you stand 3ft from the mirror and your general stare is directed to the center of the glasses.
3) Using a ruler with a millimeter range, measure the distance between the two dots and that’s your PD

If you don’t have in your possession glasses with clear lenses, you may measure your PD using this method:
1) Obtain a ruler with millimeter values
2) Place the ruler horizontally on your nose bridge, zero slightly below your right eye;
3) Ask a friend or use your mirror to see the distance on the roller. The distance measured is your PD.

Although we don’t recommend this option, if you are unable to use the methods above, you may use the general PD for: Women 62 and Men 64.