Contact lens prescriptions: What is power (PWR/SPH)?

Learn how to read your contact lens prescription and what abbreviations like PWR/SPH (power), BC (base curve), and DIA (diameter) mean.

Published on September 8, 2025

Understanding the power of your lenses

When it comes to contact lenses, one of the most important terms you'll encounter is "power." This term is represented by the "PWR" or "SPH" (sphere) value on your prescription, and it refers to the lens strength necessary to correct vision problems such as nearsightedness (myopia) or farsightedness (hyperopia). This power is measured in diopters, a unit that quantifies the ability of the lens to redirect light. It’s either a positive or negative number, but your eye care professional will refer to it as “plus” or “minus” rather than “positive” or “negative.”

Nearsightedness (Myopia)

If you have myopia, your prescription will include a negative number (for example, -1.50). Myopia means you have trouble seeing distant objects clearly. The higher the negative number, the stronger the lenses you'll need to help you see things that are far away.

Farsightedness (Hyperopia)

For hyperopia, your prescription will have a positive number (such as +2.00). This means you have difficulty seeing near objects, but can see distant objects more clearly. Again, higher positive numbers indicate a stronger degree of correction is needed.

A look at more prescription numbers

When you receive a contact lens prescription, it will include a bunch of numbers and abbreviations. It’s no problem if you don’t understand them, your eye care professional writes it so that wherever you buy your contact lenses will understand. But knowing how to read your prescription can help you double-check that the lenses you receive are correct. If you have any questions about your prescription, don’t hesitate to ask your eye care professional.

Basic prescription numbers

Power/Sphere (PWR/SPH):

We’ve already covered this, but to drive home the point, this value tells you the strength needed to correct your vision. It begins with a "+," or “plus” for farsightedness and a "−," or “minus” for nearsightedness, followed by a number that describes the strength in diopters. For example, a power of -3.00 indicates a stronger correction for nearsightedness compared to -1.00. It’s important to note that you’ll usually see contact lens prescriptions written in “steps” of 0.25D. That’s because this is in line with the way most contact lenses are manufactured.

Base Curve (BC):

This indicates the curvature of the lens and is measured in millimeters. A correct base curve helps the lens fit properly on your eye for comfort and effectiveness.

Diameter (DIA):

The diameter measurement refers to the full width of the contact lens. It ensures that the lens fits correctly over your cornea, typically ranging from 13 to 15 mm

More prescription numbers for astigmatism

If you have astigmatism, you’ll notice extra measurements on your prescription. Astigmatism happens when you have a cornea that is not perfectly round, and specific numbers will help in correcting this condition.

Cylinder (CYL)

This is usually a “minus” number for contact lenses (though it may be a 'plus' or a 'minus' number for eyeglass prescriptions). It’s a measurement of the extra lens shape needed to address the astigmatism.

Axis (AX)

The axis value is measured in degrees and describes the orientation of the astigmatism within the eye. This number ranges from 0 to 180 and tells you where to place the cylindrical correction relative to the lens.

More prescription numbers: presbyopia (age-related farsightedness)

If you are over the age of 40, it’s very common to experience presbyopia. This hardening of the lens inside your eye as you age affects your near vision. Your contact lenses can help correct this, and if it does, your prescription may include a few more numbers.

Near addition (ADD):

The addition value indicates the extra magnification needed for reading or close-up tasks. This value generally ranges from +0.50 to +3.00.

Dominant (D/N):

In a multifocal prescription, the dominant figure relates to which eye has the primary focus for distance (dominant eye) and which assists with near vision (non-dominant eye). This distinction enhances the effectiveness of multifocal lenses.

Understand Presbyopia

Keeping your prescription current matters

If you haven’t visited your eye doctor to renew your prescription in more than a year, it’s time to schedule an appointment. Visiting your eye doctor regularly is a simple way to help maintain your eye health and ensure your prescription is accurate. Your eyesight can change over time, and on top of that, an eye exam can help detect underlying health issues, like diabetes or high blood pressure, which can affect your vision and more.

Find an Eye Care Professional

FAQs

Frequently asked questions

What is power in contact lenses?

Power is the lens strength required to correct your vision. On your prescription, it’s a number that can be positive or negative depending on whether you are nearsighted or farsighted.

How do I know what my power means?

Your power values tell you how much correction you need. A negative value means you are nearsighted, while a positive value indicates you are farsighted. The larger the number, the stronger the correction required.

Can I use my glasses prescription for contact lenses?

No, you cannot use a glasses prescription for contacts, as they require different measurements due to their direct contact with the eye. Always consult with an eye doctor for a specific contact lens fitting and separate prescription for contact lenses.

How often should I get my eyes checked?

It's recommended to have routine eye exams every 1-2 years, depending on your age and eye health. If you have a contact lens prescription, you ideally should have that renewed every year to help ensure your eyes are staying healthy with contact lens wear and that your prescription is up to date and helping you see as well as possible.

What if my contact prescription has expired?

If your prescription has expired, you will need to schedule an eye exam. Wearing expired contact lenses can lead to discomfort and potential eye damage, so it's essential to have an updated prescription for your safety.