Free Online Eye Test: Vision Check on Your Screen
A calibrated letter chart for distance or near reading and a contrast check. Add the color blind test and the astigmatism dial for a quick all-round look.
This is a screening tool, not a medical diagnosis. Results depend on your screen, brightness and lighting. See an eye doctor for a proper exam. Medical disclaimer
Last updated: . Based on the sources listed below.
Step 1: Set the size with a bank card
Hold a standard bank card (85.6 mm wide) against the screen. Drag the slider until the grey box is exactly as wide as the card. This tells the page how many screen pixels make a millimetre.
Step 2: Pick your distance and read the chart
Cover one eye. Keep the distance exactly. Rows too small to draw on your screen are left out.
Contrast check
Letters get fainter from top to bottom. Pick the faintest row you can still read. This depends heavily on your screen and the room light, so use it only to compare with yourself.
This is a screening aid, not a medical diagnosis. Results depend on your screen, your distance and the lighting. See an eye doctor for a proper exam.
How does this online eye test work?
Snellen letters are built so that the stroke and gap are one fifth of the letter height, and the whole letter subtends 5 minutes of arc at the distance it is meant for (Webvision: Visual Acuity, EyeWiki: Visual Acuity Testing in Adults). Rows above the 20/20 row are larger: 20/40 letters are twice as tall as 20/20 letters, and so on. We work out each letter’s height in millimetres as 2 × distance × tan(2.5′ × row ÷ 20), then use your card calibration to draw it. Our letters are simple block shapes drawn by us on that 5 by 5 grid.
| Row | Metric | Letter height at 1 m | at 40 cm |
|---|---|---|---|
| 20/200 | 6/60 | 14.55 mm | 5.82 mm |
| 20/100 | 6/30 | 7.27 mm | 2.91 mm |
| 20/70 | 6/21 | 5.09 mm | 2.04 mm |
| 20/50 | 6/15 | 3.64 mm | 1.45 mm |
| 20/40 | 6/12 | 2.91 mm | 1.16 mm |
| 20/30 | 6/9 | 2.18 mm | 0.87 mm |
| 20/25 | 6/7.5 | 1.82 mm | 0.73 mm |
| 20/20 | 6/6 | 1.45 mm | 0.58 mm |
Computed with the formula above. On many screens the smallest rows are too small to draw sharply at long distances, which is why some rows are left out.
What it cannot do
- It cannot measure your prescription or find eye disease, and it cannot replace a comprehensive eye exam (EyeWiki: Visual Acuity Testing in Adults).
- Screen size, pixel density, brightness, glare, your distance and the card calibration all change the result. Illumination and contrast both affect measured acuity (Webvision: Visual Acuity).
- The number of letters per row and how close they sit can make some rows easier to read than others (EyeWiki: Visual Acuity Testing in Adults).
Also try the color blind test, the astigmatism test, or print the Snellen eye chart.
Frequently asked questions
Is an online eye test accurate?
No. It gives a rough idea at best. Clinics use standard charts, set distances and controlled lighting, and EyeWiki says standardized testing conditions help ensure an accurate visual acuity. Screen size, your distance and the room all change the result here. Only an eye exam is reliable.
What does 20/20 vision mean?
The Snellen fraction is the distance tested divided by the distance at which the letter subtends 5 arcminutes, so at 20/20 you can read, at 20 feet, letters that subtend 5 arcminutes there. EyeWiki calls it the theoretical norm for best-corrected acuity in a healthy eye under optimal conditions.
Why do I need to cover one eye?
Clinicians test each eye separately, right eye first by convention, covering the other eye with an opaque occluder, says EyeWiki. One eye can be better than the other, and the better eye hides the weaker one when both are open.
Does this test replace an eye exam?
No. EyeWiki describes visual acuity as one measure within a comprehensive medical eye evaluation, and says all adults need such evaluations, more often with age, to catch eye diseases early. Sharp letters on a screen do not rule out an eye condition.
Sources
- American Academy of Ophthalmology EyeWiki. Visual Acuity Testing in Adults. https://eyewiki.aao.org/Visual_Acuity_Testing_in_Adults
- Kalloniatis M, Luu C. Visual Acuity. Webvision, University of Utah (also on NCBI Bookshelf). https://www.webvision.pitt.edu/book/part-viii-psychophysics-of-vision/visual-acuity/
All sources were read on October 11, 2026. We only state what the sources say, and we do not invent numbers. These pages have not been reviewed by an eye care professional. Spot a mistake? Email info@colorblindcheck.app.