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How to Read Your Eye Prescription

19 May 2026

If you have ever had an eye examination, you may get an eye prescription for your glasses. This eye prescription often comes with numbers and abbreviations that may be difficult to understand. Here is your guide to reading your eye prescription and what it tells you about your vision.

What Is an Eye Prescription?

An eye prescription is a written order from an optometrist or ophthalmologist that specifies the precise lens power required to help correct an individual’s vision. It is typically given after a regular eye health checkup or a more comprehensive eye screening to assess your eye condition. Individuals with refractive errors, such as nearsightedness, farsightedness, or astigmatism, may need different prescriptions for each eye. Each prescription includes detailed measurements, such as Spherical (SPH), Cylinder (CYL), and Axis. These measurements are applied to corrective lenses to help focus light onto the retina.

Understanding the Numbers and Abbreviations

An eye prescription consists of certain numbers and abbreviations with different meanings. Here is what you may see on your prescription:

OD and OS

The term OD refers to oculus dexter, which is Latin for “right eye”, whereas OS refers to oculus sinister, which means left eye. These terms are typically used to group the prescriptions for each eye, as most individuals need different corrections for each eye. Some doctors may also modernise these terms by changing them to RE (right eye) and LE (left eye).

Sphere (SPH)

Sphere or SPH refers to the amount of lens power prescribed to correct nearsightedness or farsightedness. This correction applies equally across all meridians of the eye. The power of a lens is expressed in diopters (D). The stronger the prescription required, the further the number is from zero.

Cylinder (CYL)

Cylinder indicates the amount of lens power required to correct astigmatism, and is usually written after the sphere power. Unlike a spherical lens, a cylindrical lens does not have the same power across its entire surface. The number in the cylinder column may show a negative sign to correct nearsighted astigmatism or a positive sign for farsighted astigmatism. Like sphere power, cylinder power is expressed in quarter-diopter (0.25 D) increments.

Axis

For individuals with cylinder power in their eye prescription, they will also have an axis value. This refers to the orientation of the cylindrical correction, and is expressed as a number between 1 and 180 in degrees.

Add

Some prescriptions may have the term “add”, which is the added focusing power that needs to be applied for presbyopia conditions. The number typically ranges from +0.75 D to +3.00 D and will be equal in both eyes.

Prism

In a small percentage of eye prescriptions, a prism measurement, or the amount of prismatic power, may be included to help manage diplopia (double vision) due to eye alignment problems. Measured in prism diopters, it is usually listed as “p.d.” or a triangle if written by hand. When prism diopters are expressed in decimal form, they are usually given to 1 decimal place, such as 0.5.

There are four abbreviations used to represent the prism directions:

BU (base up)
BD (base down)
BI (base in, toward the nose)
BO (base out, toward the ear)

Pupillary distance

PD or pupillary distance is the distance between the centre of your pupils, measured in millimetres (mm). This measurement ensures that your glasses sit precisely where you need them. The pupillary distance is often written at the bottom of your eye prescription card.

What Your Eye Prescription Numbers Mean for Your Vision

Now that you are familiar with the different terms used in an eye prescription, here is how you can interpret the numbers you see on your prescription card:

If the number under SPH is negative (-), you are nearsighted.
If the number under SPH is positive (+), you are farsighted.
If you have a number under the CYL column, you have astigmatism. There will also be a value in the Axis column.
If you have a number under ADD, you need additional lens power for reading, which is commonly used in multifocal lenses, reading glasses, or progressive lenses.
For individuals with double vision, you may notice a number under the prism column.

Eye Prescription vs Contact Lens Prescription

An eye prescription is only recommended for purchasing eyeglasses. If you are planning to get contact lenses, you need a contact lens prescription. While glasses lenses help to correct vision by being worn at a certain distance from the eyes, contact lenses sit directly on the eye’s surface. Thus, the power noted in both prescriptions will be different.

In addition to the information you may already get from your eye prescription, a contact lens prescription includes details such as:

The base (central) curve of the back surface of the contact lens
The lens diameter
The specific manufacturer and brand name of the lens

During your eye exam for a contact lens prescription, your optometrist will reassess your eye condition and conduct a contact lens fitting to help get an accurate measurement. They will also help to determine your suitability for wearing contact lenses in general.

Frequently Asked Questions

How often does an eye prescription change?

It is common for an eye prescription to change over time, because the eyes go through the ageing process just like the rest of our bodies. The exact amount of time varies between individuals, depending on their vision condition and lifestyle. It is recommended to get an eye exam every two years to recheck your prescription, or sooner if you experience vision difficulties despite wearing your current prescription. Your optometrist will typically include the date of issue and the expiration date on your eye prescription.

What eye prescription is considered high or severe?

In general, the further the number of your diopters is from zero, the stronger the correction needed to help you see more clearly. For example, a correction for nearsightedness of -5 diopters or more usually falls under “high myopia,” while a prescription of +5.25 or more for farsightedness may be classified as “high hyperopia.”

What to do if I think my prescription is inaccurate?

Some individuals may feel their prescription is incorrect if they experience headaches or blurred vision while wearing glasses. In some cases, it may just mean your eyes are adjusting to your glasses and need more time to get used to the correction. However, if you still experience this after 2 weeks, book an appointment with your optometrist to confirm your prescription.

About the Author

Dip. Optom. | B.Sc. Optom. (USA) | M.Sc. in Clinical Optom. (USA) | MOH Reg. No. E0801695J

Philip David Izaac is a fully registered optometrist who has been in vision care since 1988. He holds a Master of Science in Clinical Optometry from Pennsylvania College of Optometry, Salus University, USA, and practises at Paragon Medical Centre, Orchard Road, Singapore. Registered with the MOH Optometrists & Opticians Board (Reg. No. EO801695J) since 17 March 2008.