The screen eye chart computes the physical height of the E optotypes from the one-arcminute principle, offers 14 rows from 4.0 to 5.3 and supports a random-direction test. A browser does not know the real size of the screen, so you have to calibrate first by comparing an 85.6 mm bank card or by entering the screen's diagonal in inches to estimate pixels per millimetre.

1. 01Complete the screen calibration.
2. 02Choose a test distance of 1, 2.5 or 5 metres.
3. 03Browse the whole E chart, or judge the opening direction in the random test with the arrow keys or the buttons.

Row 5.0 is computed from the standard viewing-angle relationship, and changing the distance rescales the E's pixel size on screen proportionally; when a row works out smaller than about 3 px the page warns that the screen's precision is insufficient.

• Card calibration is affected by how carefully you align it, by browser zoom and by system scaling; entering the screen size is also an estimate that assumes the pixels the system reports match the physical screen.
• The E directions are generated at random with Web Crypto.
• Screen brightness, viewing distance, ambient light and display DPI all affect the result.
• This tool is not a medical device and cannot diagnose eye disease or replace refraction with a standard light box.

Calibration parameters are stored in the current browser locally and are never uploaded. A vision result here is only indicative; for any vision problem, have a professional eye examination or refraction test.