Vision Calculator
Convert between different vision measurement systems. Compare vision prescriptions, analyze eye test results, and track vision changes over time.
Convert between Snellen, decimal acuity, logMAR, and approximate Jaeger notation.
Decimal acuity, such as 1.00 for 20/20 or 0.50 for 20/40
Calculate spherical equivalent from SPH and CYL. When the numbers fit this simple model, the calculator also gives a rough uncorrected-acuity range.
Vision testing and measurement have come a long way since Dutch ophthalmologist Herman Snellen developed his major eye chart in 1862. This landmark invention - featuring a large letter "E" at the top and progressively smaller letters below - established the foundation for standardized vision assessment that we still use today. Before Snellen's innovation, vision evaluation was largely subjective and inconsistent, relying on a patient's ability to read text at various distances without any standardized methodology.
Throughout the 20th century, vision measurement evolved from this simple beginning into detailed systems used by eye care professionals worldwide. Our calculator bridges these various notation systems that emerged across different regions and medical traditions, helping patients understand the relationships between these seemingly different ways of expressing the same visual capability.
Visual acuity measures how small a detail you can resolve at a set distance under controlled conditions. It is the familiar 20/20 style number, and it is useful, but it is not the whole eye exam. A person can read the 20/20 line and still have dry eye, eye teaming problems, glaucoma risk, color-vision differences, or trouble focusing up close.
That is why a calculator can translate notation but cannot replace an optometrist or ophthalmologist. Use the result to understand paperwork, compare old and new prescriptions, or talk more clearly with an eye-care professional. If vision changes suddenly, if one eye is worse than the other, or if you see flashes, floaters, pain, or a curtain-like shadow, seek medical care promptly.
Testing conditions affect acuity. Lighting, chart distance, pupil size, fatigue, uncorrected dry eye, and guessing can all move the result by a line or two. Small differences between tests are common, so trends over time are usually more useful than a single number.
Sphere, cylinder, and axis describe different parts of the optical correction. Sphere handles the overall focusing error. Cylinder adds power in one direction to correct astigmatism. Axis tells the lens maker where that cylinder power sits. A small axis change can be noticeable when cylinder power is high, but may barely matter when cylinder power is low.
Prescriptions can be written in plus-cylinder or minus-cylinder form. Both can describe the same lens if they are transposed correctly. That is a common source of confusion when comparing prescriptions from different offices or countries. The signs may look different while the optical correction is equivalent.
Add power is another separate value, usually used for reading or progressive lenses. It is not the same as sphere. It tells how much extra plus power is needed for near work. Contact lens prescriptions may also differ from glasses prescriptions because the lens sits directly on the eye instead of several millimeters away.
Snellen, decimal, and LogMAR notation all describe acuity, but they feel different because their scales move in different directions. Higher decimal values mean better acuity. Lower LogMAR values mean better acuity. Snellen fractions are easy to recognize but awkward for statistics, which is why research often uses LogMAR.
A conversion is most helpful when it prevents misunderstandings. If a European report lists 0.5 decimal acuity, that corresponds roughly to 20/40. If a study reports 0.3 LogMAR, that is also around 20/40. The numbers look unrelated until they are put on the same scale.
Do not round too aggressively when tracking medical changes. A one-line change may matter in some conditions and not in others. Keep the original notation from the exam record, then use conversions as a plain language aid.
A small prescription change does not always require new glasses right away. The decision depends on symptoms, job needs, driving, screen use, headaches, and how old the current lenses are. Someone who drives at night or reads fine detail all day may notice a change sooner than someone with lighter visual demands.
Children need closer monitoring because vision can change quickly and because uncorrected vision problems can affect schoolwork and eye development. Adults with diabetes, glaucoma risk, previous eye surgery, or sudden changes should follow the schedule set by their clinician rather than waiting for a routine exam.
Contacts add fit and eye-surface health to the equation. A contact lens prescription includes brand, base curve, diameter, material, and wearing schedule. Those details are not interchangeable with a glasses prescription, even when the power looks similar.
People often compare prescriptions by looking only at the sphere value. That can miss changes in cylinder, axis, or add power. A prescription with a slightly weaker sphere but more cylinder may still feel stronger or clearer because it corrects a different blur pattern.
Another source of confusion is acuity with correction versus without correction. "20/40 unaided" and "20/20 with glasses" describe different conditions. Keep the labels attached to the numbers when you compare exam notes.
When comparing two prescriptions, look at one eye at a time and keep the notation format the same. A change from -2.00 to -2.25 sphere is a small shift. A change in cylinder or axis may feel larger if you have astigmatism. If the prescription was transposed between plus-cylinder and minus-cylinder notation, convert it before deciding what changed.
Pupillary distance, segment height, frame fit, lens material, and progressive lens design can affect comfort even when the prescription is correct. If new glasses feel wrong, the issue may be measurement, adaptation, lens design, or an error in fabrication. Give your eyes the adaptation period recommended by the clinic, but return for a recheck if symptoms are strong or do not improve.
Keep copies of exam results. They make it easier to spot trends, explain changes to a new provider, and order replacement eyewear. They also help separate acuity changes from lens comfort issues.
Eye numbers are helpful, but symptoms should guide urgency. New double vision, sudden blur, eye pain, flashes, a shower of floaters, or a dark curtain in vision needs prompt medical attention. Those signs are not solved by converting Snellen to decimal notation.
Milder symptoms still deserve context. Eye strain after long screen sessions may involve dry eye, lighting, ergonomics, focusing effort, or an outdated prescription. Trouble with headlights at night may come from uncorrected astigmatism, cataracts, dry eye, scratched lenses, or pupil size. The same acuity score can feel different in real life.
Bring specific examples to an eye exam: which distance is blurry, when it happens, whether one eye is worse, and what glasses or contacts you were using. That information is often more useful than saying only that your vision is "worse."
Eye charts use high-contrast letters under controlled lighting. Real life is messier. Rainy roads, dim restaurants, glare from screens, and low-contrast print can expose problems that a bright exam lane does not reproduce. That is why someone can test well and still struggle in specific situations.
Contrast sensitivity, tear-film quality, pupil size, and lens clarity all affect how sharp the world feels. If the converted acuity number looks fine but daily overall practical vision does not, describe the situation instead of focusing only on the chart score.
This is also why updated lenses may feel better in one setting before they feel better everywhere. Night driving, computer work, reading, and sports all stress different parts of the visual system.
If you use several pairs of glasses, compare them in the same lighting and distance. Older scratched lenses can make a prescription seem worse than it is.
Small comfort details matter too. Clean lenses, a well-adjusted frame, proper screen distance, and breaks during near work can make the same prescription feel clearer in daily use.
An eye prescription typically includes three main numbers: sphere (SPH), cylinder (CYL), and axis. The sphere number indicates the lens power needed to correct nearsightedness (-) or farsightedness (+), measured in diopters. The cylinder number describes the lens power needed to correct astigmatism. The axis (0-180 degrees) shows the orientation of the astigmatism correction. For example, a prescription of -2.50 SPH -1.25 CYL x180 means you need -2.50 diopters for nearsightedness correction and -1.25 diopters of cylindrical correction at 180 degrees for astigmatism.
20/20 vision (Snellen fraction) and decimal notation are different ways to express the same visual acuity. In 20/20 vision, the first number represents the testing distance (20 feet), and the second number indicates the distance at which a person with normal vision can read the same line. In decimal notation, 20/20 becomes 1.0, 20/40 becomes 0.5, etc. For example, if you have 20/100 vision, that's 0.2 in decimal notation, meaning you must be 20 feet away to see what someone with normal vision can see at 100 feet. European countries typically use decimal notation, while the US commonly uses the Snellen fraction.
Astigmatism occurs when your cornea or lens has an irregular shape, causing light to focus on multiple points instead of one. The cylinder and axis values in your prescription correct this. The cylinder (CYL) indicates the additional power needed in a specific meridian, while the axis shows the angle (in degrees) where this power should be applied. For instance, a prescription with -1.00 CYL x90 means you need an additional -1.00 diopter of correction along the 90-degree meridian. This creates a toric lens shape that compensates for your eye's irregular curvature, bringing light to a single focus point and providing clearer vision.
No. 20/20 means standard distance visual acuity under test conditions. It does not measure peripheral vision, depth perception, color vision, eye teaming, or eye health. You can have 20/20 acuity and still need an eye exam for other issues.
A minus sphere corrects nearsightedness by moving the focus point back onto the retina. A plus sphere corrects farsightedness by moving the focus point forward. The sign tells the lens maker which type of correction is needed.
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