AnyTool
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How can I test my vision online with a Snellen chart at home?

Open AnyTool Vision & Eye Test and calibrate first: drag the on-screen card outline until it matches a real bank or ID card (85.6 mm wide) so the tool learns your screen’s pixels-per-millimetre. Pick a viewing distance, then launch a fullscreen test — a shrinking Snellen letter chart sized to true clinical proportions, an astigmatism fan, an Amsler macular grid or a low-contrast letter fade. Cover one eye, read down to the smallest line you can make out, and the tool shows an estimated acuity such as 20/40. It is a screening and awareness tool only — not a medical eye exam — and it runs entirely in your browser, uploading nothing.

  • Calibrate with a credit/ID card so letters render at their true physical size
  • Pick a viewing distance — Snellen letters are sized to 5 arc-minutes for 20/20
  • Four checks: Snellen acuity, astigmatism fan, Amsler grid, contrast fade
  • One eye at a time, fullscreen; results shown as plain-language estimates
  • Screening only — NOT a diagnosis; 100% in your browser, nothing uploaded

What is

Online vision screening (visual acuity test)

An online vision screening estimates how well you can see by rendering standardised eye-chart targets at controlled physical sizes on your own screen. Visual acuity is reported as a Snellen fraction such as 20/20, where a 20/20 letter subtends a visual angle of 5 arc-minutes at the test distance; sizing it correctly on a screen requires calibrating pixels-per-millimetre (typically against a credit card) and fixing a viewing distance. Such screenings can flag possible problems but cannot replace a clinical eye examination.

Calculators

Related terms

Visual acuitySnellen chartlogMARAstigmatismAmsler gridContrast sensitivityScreen calibration

Frequently Asked Questions

No. It is a screening and awareness tool that can flag possible issues, but it cannot diagnose conditions or replace a professional exam.

An online vision test only estimates the smallest line you can resolve on your own screen, which depends on your calibration accuracy, viewing distance, lighting and display quality. It does not measure eye pressure, examine the retina, refract you for glasses or detect many serious conditions, so a normal result is not a clean bill of health. Use it for awareness, then see an optometrist or ophthalmologist for proper testing — and promptly if you notice any change in your vision, new distortion, dark spots, flashes or floaters.

Every screen has a different pixel density, so the tool needs a known-size object to learn how many pixels equal a millimetre.

A Snellen letter is defined by the angle it subtends at the eye — a 20/20 letter is 5 arc-minutes — so to draw it correctly the tool must know the true physical size of a pixel on your display. Matching the on-screen outline to a standard ID-1 card, which is exactly 85.6 mm wide, gives the pixels-per-millimetre figure. Combined with your chosen viewing distance, that lets every acuity line be rendered at its real size. Skip or mis-set calibration and the estimate can be off by whole lines.

You stare at the centre while covering one eye; uneven spokes can hint at astigmatism, and wavy or missing grid squares can hint at macular changes.

The astigmatism fan is a sunburst of evenly spaced lines — if some orientations look darker, blurrier or doubled while others are crisp, that pattern can suggest astigmatism. The Amsler grid is a fine square grid with a central dot you fixate; while staring at the dot, any wavy, bent, faded or missing squares — especially near the centre — can indicate macular problems. Both are awareness screens only; the Amsler grid covers just the central ~10 degrees and can miss real disease (published sensitivity is under 50%), so abnormal or normal results should both be confirmed by an eye-care professional.

Yes — it runs entirely in your browser with no upload, account or tracking, and works offline after first load.

The Vision & Eye Test is 100% client-side: calibration, letter sizing and every chart are computed and drawn in your browser, with nothing sent to a server. It works on phones, tablets, laptops and monitors, though a larger screen is needed to show the smallest lines at longer distances. No images are downloaded — the optotypes and grids are generated locally — so it keeps working offline once the page has loaded.

Detailed Explanation

Methodology

How On-Screen Vision Testing Works: Calibration and Letter Sizing

AnyTool Vision & Eye Test computes everything in the browser. The core problem with any on-screen acuity check is that a Snellen letter is defined by the visual angle it subtends — a 20/20 letter spans 5 arc-minutes at the test distance — so the tool must render it at a precise physical size. It solves this with a two-part calibration: the user drags an on-screen outline to match a standard ID-1 bank card (exactly 85.6 mm wide) to derive the screen’s pixels-per-millimetre, then picks a viewing distance. For each 20/D acuity line the engine converts the line’s total angle (5 × D/20 arc-minutes) to a physical height with height = 2 · distance · tan(angle ⁄ 2), then to pixels using the calibrated px/mm. The letters are generated locally — no images are downloaded — and reported with logMAR and 6/x metric equivalents.

  • A 20/20 letter subtends 5 arc-minutes; a 20/D line subtends 5 × D/20
  • Calibration matches an on-screen outline to a 85.6 mm ID-1 card → px/mm
  • Letter height = 2 · distance · tan(angle ⁄ 2), then × px/mm to pixels
  • A chosen viewing distance fixes the second unknown in the sizing
  • Optotypes are generated in-browser — no images, nothing uploaded
How It Works

The Four Checks: Acuity, Astigmatism, Amsler and Contrast

The Snellen check renders a chart from a large 20/200 line down to 20/10; the user covers one eye, reads down and taps the smallest line they can resolve, yielding an estimated acuity such as 20/40 with a plain-language band. The astigmatism fan is a 36-spoke sunburst — orientations that look darker, blurrier or doubled while others stay crisp can hint at astigmatism. The Amsler grid is a fine square grid with a central fixation dot for spotting wavy, faded or missing squares that can indicate macular changes. The contrast-sensitivity check fades letter rows from bold toward faint so the user can mark the faintest row they can still read. Each test opens fullscreen, one eye at a time, with an on-screen cover-one-eye reminder.

  • Snellen chart 20/200 → 20/10, self-reported smallest readable line
  • Astigmatism fan: 36 evenly spaced spokes for uneven-focus screening
  • Amsler grid with central dot for macular distortion / scotoma
  • Contrast fade rows from bold to faint for contrast sensitivity
  • Fullscreen, one eye at a time, with cover-one-eye reminders
Limitations

Why This Is Screening, Not a Medical Eye Exam

Every result here is an estimate that hinges on calibration accuracy, the measured viewing distance, room lighting and display quality — a small calibration error can shift the acuity result by whole lines. The Amsler grid checks only the central ~10 degrees and has published sensitivity under 50%, so it can miss real macular disease, and a normal screen of any kind is not a clean bill of health. The tool does not measure intraocular pressure, examine the retina, refract for a glasses prescription or detect many serious eye and systemic conditions. It is explicitly a screening and awareness aid: users are told to get regular professional eye exams and to seek care promptly for any sudden change in vision, new distortion, dark spots, flashes or floaters.

  • Calibration / distance errors can shift acuity by whole lines
  • Amsler grid covers only central ~10°; sensitivity is under 50%
  • Does not measure eye pressure, refract, or examine the retina
  • A normal result does not rule out disease
  • Screening and awareness only — see a professional for real testing
Privacy & Security

Privacy and Compatibility

The Vision & Eye Test is 100% client-side: calibration, letter sizing and every chart are computed and drawn in the browser with no server, account or tracking, and it works offline after first load. It runs on phones, tablets, laptops and monitors — though longer viewing distances need a larger screen to display the smallest acuity lines. Because the optotypes and grids are generated locally rather than fetched as images, nothing about your test or your vision ever leaves your device.

Online vision screening: AnyTool vs typical web eye-chart pages
CapabilityAnyToolTypical online eye charts
ProcessingRuns in your browserRuns in your browser
Screen calibrationCredit-card → px/mm, true-size lettersOften none — fixed pixel sizes
Acuity sizingAngular (5 arc-min at 20/20) + distanceOften approximate
Astigmatism checkYes — 36-spoke fanSometimes
Amsler gridYes — with fixation dotRarely
Contrast sensitivityYes — fading letter rowsRarely
Result detailSnellen + logMAR + metricSnellen only, if any
Honest "not a diagnosis" framingStated prominently up frontOften understated
Cost / signupFree, no signupOften ad-heavy or gated

AnyTool computes calibration and every chart locally and uploads nothing — but it is a screening aid, not a medical eye exam.