A vision test produces a set of numbers and abbreviations that describe how well each eye sees and what correction it needs. Once you know what each value means, the whole printout becomes straightforward. Here’s how to decode every part of your results, from the eye chart score to the finer details of a glasses prescription.
The 20/20 Number: Visual Acuity
The most familiar result from an eye exam is the Snellen fraction, written as something like 20/20, 20/40, or 20/200. The first number is always 20, representing the distance in feet at which you read the chart. The second number tells you how far away a person with normal vision could stand and still read that same line. So if your result is 20/40, you need to be at 20 feet to read letters that someone with standard vision could read from 40 feet away. A result of 20/100 means you need to be at 20 feet to see what others see clearly at 100 feet.
A lower second number is better. 20/20 is considered normal. 20/15 means your vision is sharper than average. If you see results written as 6/6 instead, that’s the metric version used in countries like the UK and Australia, where the test distance is measured in meters rather than feet. 6/6 equals 20/20.
For legal and practical context: visual acuity of 20/200 or worse (with best correction) is the threshold for legal blindness in the United States. Acuity worse than about 20/60 generally means you cannot pass a driver’s license vision screening without corrective lenses.
OD, OS, and OU: Which Eye Is Which
Your results will list each eye separately using Latin abbreviations. OD (oculus dexter) is your right eye. OS (oculus sinister) is your left eye. OU (oculi uterque) means both eyes together. These labels appear on everything from your acuity scores to your glasses prescription, so the right eye’s numbers are always on the OD line and the left eye’s numbers are on the OS line.
Sphere (SPH): Your Basic Prescription Power
The sphere column is the core of your glasses or contact lens prescription. It measures lens power in units called diopters (D) and tells you how much correction your eyes need to focus light properly.
The sign in front of the number matters. A minus sign means you’re nearsighted: you see things up close fairly well but distant objects look blurry. A plus sign means you’re farsighted: distance vision may be okay, but close-up work is harder. The farther the number is from zero, the stronger the correction you need. A prescription of -1.00 is mild nearsightedness, while -6.00 or beyond is classified as high myopia and typically means significantly blurred distance vision without glasses. Most nearsighted prescriptions fall between -0.50 and -6.00, a range called low myopia.
If you see a number like +2.25, that’s moderate farsightedness. If the SPH column reads “PL” or “Plano,” it means zero correction is needed for that component.
Cylinder (CYL) and Axis: Astigmatism Correction
These two columns always go together. If they’re blank, you don’t have astigmatism and can skip this section.
Astigmatism means your cornea (the clear front surface of your eye) isn’t perfectly round. Instead of being shaped like a basketball with the same curve in every direction, it’s more like a football, with one curve steeper than the other. This causes light to focus unevenly, making vision blurry or slightly doubled at any distance. The cylinder number, written in diopters with a minus or plus sign, tells you how much extra correction is needed to compensate for that uneven curve. A CYL of -0.75 is mild. A CYL of -3.00 or more is significant.
The axis is a number between 1 and 180 that pinpoints where on your cornea the astigmatism sits. Think of your eye like a clock face: an axis of 90 degrees runs vertically (top to bottom), while 180 degrees runs horizontally (side to side). The axis doesn’t describe severity. It simply tells the lab which angle to set the cylindrical correction in your lenses so it lines up with the actual shape of your cornea.
ADD Power: The Reading Correction
If you’re over 40, your prescription may include an ADD (addition) value. This is extra magnifying power added to the bottom portion of bifocal, progressive, or reading glasses so you can see things up close, like books or phone screens. It compensates for presbyopia, the gradual stiffening of the lens inside your eye that makes it harder to shift focus to nearby objects.
ADD is always a plus number, typically ranging from +0.75 in the early stages to +2.50 or +3.00 for people in their 60s and beyond. The same ADD value usually applies to both eyes, though it can occasionally differ. This number is listed separately from your distance prescription because it’s only used for near-vision tasks.
Pupillary Distance (PD)
Pupillary distance is the measurement, in millimeters, between the centers of your two pupils. It’s not a measure of eye health or vision quality. It exists so the optical center of each lens lines up precisely with each pupil when your glasses are made. If the alignment is off, you may get eye strain or blurry vision even with the right prescription power.
PD is especially important if you order glasses online, since the lab needs it to cut and position your lenses. Your eye care provider is required by federal law to give you your written prescription at no charge, but PD is not always included automatically. Some states require it; others don’t, and providers may charge a small fee to measure it separately. The average adult PD falls roughly between 54 and 74 mm. You may see a single number (binocular PD) or two numbers (monocular PD), one for each eye measured from the bridge of your nose.
Eye Pressure (IOP)
During a comprehensive exam, your eye doctor typically measures the pressure inside your eyes, called intraocular pressure or IOP. This is the “puff of air” test or the probe-touch test you may remember. The result is recorded in millimeters of mercury (mmHg), and normal pressure falls between 10 and 21 mmHg. It’s normal for pressure to fluctuate slightly throughout the day, with readings tending to be a bit higher in the morning.
Elevated pressure doesn’t automatically mean you have glaucoma, but it’s a risk factor your doctor monitors over time. Conversely, some people develop glaucoma with pressure in the normal range. The IOP number is one piece of the puzzle, usually interpreted alongside other tests like optic nerve imaging and visual field testing.
Prism: Eye Alignment Correction
Most prescriptions don’t include prism, so if you don’t see it on yours, your eyes are aligning well on their own. When it does appear, prism correction helps redirect light so both eyes work together properly. It’s prescribed for people who experience double vision or eye strain caused by one eye pointing slightly higher, lower, or off to one side compared to the other.
Prism is measured in prism diopters and includes a base direction: BU (base up), BD (base down), BI (base in, toward the nose), or BO (base out, toward the ear). The base direction tells the lab which edge of the prism should be thickest. For example, if one eye drifts upward, the prescription might read something like 2.0 BD over that eye, meaning base-down prism is needed to shift the image downward so both eyes see the same thing in the same place.
Putting a Sample Prescription Together
Here’s what a typical prescription looks like when you read it left to right:
- OD: -2.50 -0.75 x 180
- OS: -3.00 -1.25 x 015
- ADD: +2.00
- PD: 63
This person is nearsighted in both eyes (the minus SPH values), with some astigmatism in each eye (the CYL values). The right eye needs less correction than the left. The axis tells the lab the angle of the astigmatism for each eye. The ADD means they also need reading power for close-up tasks, and their pupils are 63 mm apart. The left eye’s prescription is a bit stronger overall, which is completely common. Perfect symmetry between the two eyes is the exception, not the rule.
One detail worth noting: a glasses prescription and a contact lens prescription are not interchangeable. Contacts sit directly on your eye rather than a short distance in front of it, so the power values are often adjusted. Your doctor will write a separate contact lens prescription if needed, which may also include a base curve and lens diameter.