A reading glasses prescription is a short string of numbers and abbreviations that tells a lens maker exactly how much magnifying power each eye needs for close-up tasks. Once you know what each column means, the whole thing takes about 30 seconds to decode.
OD, OS, and OU: Which Eye Is Which
Every prescription starts by separating your two eyes. OD stands for the Latin “oculus dexter,” your right eye. OS stands for “oculus sinister,” your left eye. If you see OU, that means “oculus uterque,” or both eyes together. Some newer prescriptions skip the Latin and simply label the rows “Right” and “Left,” but OD and OS are still the standard on most forms.
Sphere (SPH): Your Main Lens Power
The sphere column is the core of any prescription. It tells you how much corrective power your lenses need, measured in units called diopters. A plus sign (+) in front of the number means you’re farsighted and need help seeing things up close. A minus sign (−) means you’re nearsighted and need help with distance. For a pure reading prescription, the sphere value is almost always a positive number, because reading glasses are designed to bring close objects into focus.
The number itself reflects the strength of the correction. A prescription of +1.00 is mild, while +3.00 or higher is a stronger lens. Your two eyes don’t have to match. It’s common for one eye to need a different sphere value than the other.
Cylinder (CYL) and Axis: Astigmatism Correction
If your prescription has numbers in the CYL and Axis columns, you have some degree of astigmatism. A normal eye is shaped like a basketball, curving evenly in every direction. An eye with astigmatism is shaped more like a football, with one curve longer than the other. The cylinder value measures how much extra correction that uneven curve needs.
The axis is a number between 1 and 180 that describes the angle of the astigmatism on your cornea. It tells the lab which direction to orient the cylinder correction in the lens. CYL and axis always appear together. If one is blank, the other will be too, and that simply means you don’t have astigmatism in that eye.
ADD Power: The Reading-Specific Number
The ADD value is the number most relevant to reading glasses. It stands for “addition power” and represents the extra magnifying strength layered on top of your distance prescription so you can focus on close-up tasks like reading, sewing, or looking at your phone. ADD exists because of presbyopia, the gradual stiffening of the lens inside your eye that makes near focus harder as you age, typically starting in your early to mid-40s.
ADD is always a positive number, usually falling between +1.00 and +3.50 diopters. A lower ADD means you need only a small boost for reading. A higher ADD means your eyes need more help pulling close objects into focus. The ADD value is normally the same for both eyes, even when the rest of the prescription differs.
How to Calculate Your Reading Lens Strength
If you already wear glasses for distance and want a separate pair of single-vision readers, you can figure out the lens power with simple addition: take your sphere value and add the ADD value.
For example, if your right eye has a sphere of +1.00 and an ADD of +1.75, your reading lens for that eye would be +2.75. If your sphere were −1.00 instead, the math gives you +0.75 for the reading lens. Do the calculation for each eye separately, since the numbers may differ.
This is the same formula optical shops use when making dedicated reading glasses from a full prescription. It’s worth double-checking the math before you order, especially online, because a small error means the lenses won’t feel right.
Pupillary Distance (PD)
Your pupillary distance is the gap, in millimeters, between the centers of your two pupils. It determines where the optical center of each lens sits, so you’re actually looking through the strongest, clearest part of the glass. If PD is off, you can end up with eye strain, fatigue, or blurry vision. The effect gets worse the stronger your prescription is.
PD isn’t always printed on the prescription itself. Some eye doctors include it automatically; others only provide it if you ask. If you’re ordering reading glasses online, you’ll almost certainly need this number. Average adult PD falls roughly between 54 and 74 mm, and your optician can measure it in seconds. You can also measure it at home with a ruler and a mirror, though a professional measurement is more reliable for strong prescriptions.
Prism: A Less Common Addition
Most reading prescriptions don’t include prism, but if yours does, it means your eyes have an alignment issue that makes them point slightly off from each other. Prism correction bends light before it enters the lens so both eyes receive the image in the same spot, reducing double vision or strain.
Prism is measured in prism diopters and paired with a direction: BI (base in), BO (base out), BU (base up), or BD (base down). These labels tell the lab which edge of the lens gets the thicker prism material. If you see prism on your prescription, you need custom lenses. Over-the-counter readers can’t replicate this correction.
Custom Readers vs. Over-the-Counter Readers
Drugstore reading glasses (often called “cheaters”) use the same magnification in both lenses and come in fixed strengths like +1.25 or +2.00. They work fine if both of your eyes happen to need identical correction, you have no astigmatism, and your pupillary distance is close to average. For occasional, short-duration reading, many people get along with them perfectly well.
Custom prescription readers, on the other hand, match the exact sphere, cylinder, axis, and ADD for each eye individually. The lens materials and coatings are typically higher quality, and the optical centers are aligned to your specific PD. If your prescription differs between eyes, includes any cylinder correction, or contains prism, custom lenses will give you noticeably sharper, more comfortable vision. People with very narrow or very wide pupillary distances are especially likely to experience strain or double vision from off-the-rack readers.
A Quick Walkthrough of a Sample Prescription
Here’s how to read a typical prescription line by line:
- OD (right eye): SPH −2.00, CYL −0.50, Axis 90, ADD +2.00
- OS (left eye): SPH −1.75, CYL −0.75, Axis 180, ADD +2.00
This person is nearsighted (minus sphere values) with mild astigmatism in both eyes (the cylinder and axis entries). The ADD of +2.00 means they need moderate magnification for reading. To calculate dedicated reading lens power, you’d add sphere and ADD for each eye: the right eye becomes +0.00 (essentially no power beyond the astigmatism correction) and the left eye becomes +0.25. The cylinder and axis values stay the same and get built into the reading lenses as well.
If the same person grabbed +2.00 drugstore readers, those lenses would ignore the difference between their eyes, skip the astigmatism correction entirely, and assume an average pupillary distance. They might still be usable for a quick glance at a menu, but they’d be a poor match for extended reading.