Understanding Your Eye Prescription
Reading Your Eyeglass Prescription
The sphere value (SPH) is the main number on your prescription. It measures the lens power needed to correct nearsightedness or farsightedness, written in diopters. A negative SPH means you are nearsighted: the higher the negative number, the blurrier distant objects appear without correction. A positive SPH means you are farsighted.
According to StatPearls (2024), prescriptions are written in 0.25-diopter increments. A prescription of -2.00 needs a stronger lens than -1.00. If your SPH is close to zero (such as -0.25 or +0.25), your correction is mild and you may not need glasses for every activity.
The cylinder (CYL) and axis values appear together and correct astigmatism. CYL measures how much astigmatism correction you need. AXIS identifies the angle of the uneven corneal curve on a scale of 0 to 180 degrees. According to StatPearls (2024), if a CYL value is present, an AXIS value must also be listed. If no CYL appears on your prescription, you have no significant astigmatism.
The ADD value appears on prescriptions for patients who need help with close-up tasks like reading. Presbyopia (age-related loss of near focus) begins around age 40, and the ADD number increases over time as the lens inside your eye loses flexibility. A typical ADD ranges from +0.75 to +3.00.
Your doctor adds this power to your distance prescription to create bifocal, progressive, or dedicated reading glasses. The ADD value is the same for both eyes in most cases. If your prescription has no ADD, your near focus is strong enough that you do not need reading correction.
Pupillary distance (PD) is the space in millimeters between the centers of your pupils. According to clinical references, the average PD is approximately 62 mm for women and 64 mm for men, with a typical range of 58 to 68 mm. Accurate PD ensures your lenses are optically centered so you look through the clearest part of each lens.
Some prescriptions list a single PD (binocular measurement) while others list a separate measurement for each eye (monocular PD). The monocular measurement is more precise, for progressive lenses. If your PD is not on your prescription, ask your eye doctor or the optical shop to measure it before you order glasses.
What Your Numbers Mean
A negative SPH value means you are nearsighted (myopic). Your eye focuses light in front of the retina, making distant objects blurry. Mild myopia falls around -0.25 to -3.00, moderate myopia ranges from -3.25 to -6.00, and high myopia exceeds -6.00. Higher numbers mean stronger correction and thicker lenses without high-index materials.
A positive SPH value means you are farsighted (hyperopic). Your eye focuses light behind the retina, making close tasks harder. Young patients with mild hyperopia may not need glasses because their focusing muscles compensate. As those muscles weaken with age, the hyperopia becomes more noticeable and correction becomes necessary.
Astigmatism shows up as a CYL value paired with an AXIS angle. The CYL can be written in negative or positive form depending on your doctor's notation preference. Both conventions describe the same correction; the numbers just look different.
A CYL of -0.50 represents mild astigmatism. Values beyond -2.00 indicate more significant astigmatism that may make you more sensitive to lens positioning in your frames. The AXIS tells the lab exactly where to orient the astigmatism correction in your lens. Even a small error in axis placement can cause blurry or distorted vision.
Some patients have astigmatism in one eye and not the other. Your doctor measures each eye independently, so your OD and OS values may look quite different on the same prescription.
Your prescription can shift for several reasons. Children and young adults see their myopia increase as their eyes grow. Adults over 40 develop presbyopia and need increasing ADD power for near vision. Hormonal changes during pregnancy or menopause can temporarily alter your prescription.
Diabetes causes the lens inside your eye to swell when blood sugar runs high, producing temporary but noticeable prescription changes. Cataracts also change how your lens bends light, causing a 'second sight' improvement in near vision before distance vision worsens. Your doctor distinguishes between refractive shifts and signs of disease during your exam.
Eyeglasses vs Contact Lens Prescriptions
According to the AAO, a contact lens prescription is separate from an eyeglass prescription. Contact lenses sit on the cornea rather than in a frame in front of your eyes, so the power values may differ slightly. Your contact lens prescription also includes base curve (the curvature of the back surface of the lens) and lens diameter.
You cannot use your eyeglass prescription to order contact lenses. A contact lens fitting measures how the lens sits on your cornea, checks for adequate tear coverage, and confirms that the chosen lens material works for your eyes. Your doctor may try several lens types before settling on the best option.
The FTC updated the Eyeglass Rule in 2024, requiring eye doctors to provide patients a copy of their eyeglass prescription at the end of every exam. You do not need to ask for it, and your doctor cannot charge extra or require you to buy glasses from their office. Prescriptions are valid for 1 to 2 years, depending on state law.
Contact lens prescriptions follow similar rules under the FTC's Contact Lens Rule. Your doctor must release your prescription after completing the fitting process. You can take both prescriptions to any retailer, online or in-store, to purchase your lenses.
When ordering glasses online or in-store, you need your SPH, CYL, AXIS, and PD at minimum. If you wear bifocals or progressives, you also need your ADD value. Double-check every number before submitting your order, since small errors in CYL, AXIS, or PD can cause headaches and blurry vision in your new lenses.
High-index lens materials produce thinner, lighter lenses for stronger prescriptions. Anti-reflective coating reduces glare and improves clarity. Your optician can recommend lens options based on your prescription strength and how you plan to use your glasses.
Eye Prescription Questions Answered
No. Contact lens prescriptions include base curve and diameter measurements that eyeglass prescriptions do not. The power values may also differ. You need a separate contact lens fitting and prescription from your eye doctor before ordering contacts.
A negative SPH means you are nearsighted. The larger the negative number, the stronger the correction needed. A value of -1.00 is mild; -6.00 or higher is considered high myopia and requires thicker lenses or high-index materials.
Ask your eye doctor or optical shop to measure your PD during your exam. Some online retailers offer tools to estimate PD using a ruler and mirror, but a professional measurement is more accurate. If your prescription does not include PD, call your doctor's office and request it.
Most people have slightly different prescriptions between their two eyes. Each eye develops its own shape and focusing ability. Your doctor measures and corrects each eye independently so both contribute to clear, comfortable binocular vision.
Eyeglass prescriptions are valid for 1 to 2 years, depending on state regulations. Contact lens prescriptions expire after 1 year. After expiration, you need a new exam to get an updated prescription. Wearing an expired prescription will not damage your eyes, but your correction may no longer match your current needs.
A brief adjustment period of 1 to 2 weeks is normal with a new prescription, for first-time progressive lens wearers. If headaches persist beyond two weeks, or if your vision feels worse than with your old glasses, return to your eye doctor. The prescription, lens measurements, or frame fit may need adjustment.
Get Your Prescription Checked
Understanding your prescription helps you make informed choices about your glasses and contacts. Schedule your next eye exam to keep your prescription current and your vision sharp.