Visual acuity measures how clearly you can see fine detail at a given distance. The Snellen chart — the classic row-of-letters poster in every optician's office — is the world standard for measuring it. This free online tool simulates that test by calibrating letter sizes to your specific screen, then showing you Sloan letters at progressively smaller sizes to estimate your approximate Snellen score for each eye.
Step 1: Calibrate Your Screen
Drag the right edge of the white card below until it matches the width of a physical credit card (or any standard bank/ID card — 85.6 mm / 3.37 in wide). This lets the tool calculate accurate letter sizes for your screen.
Standard credit card width: 85.6 mm (3.37 in)
Detected: — px/mm | Drag card to calibrate
Step 2: Select Which Eye to Test
Test each eye separately for the most useful result. Cover the opposite eye with your hand or a card — do not close it (closing slightly shifts focus). You can also test both eyes together.
Sit 60 cm / 24 inches from your screen. Keep your screen brightness at a comfortable level and ensure normal room lighting.
Visual Acuity Test
Left Eye
Snellen equivalent
20/200
Which letter do you see? (tap or type)
Level Complete
Left Eye Result
Estimated Snellen Acuity
Snellen Scale Reference
Medical Disclaimer
This is a rough screen-based estimate only. Results are affected by your screen resolution, viewing distance, ambient lighting, and monitor calibration. This tool cannot diagnose any eye condition. For accurate measurement and diagnosis, please consult a licensed optometrist or ophthalmologist.
How to Use the Visual Acuity Test
This free visual acuity test uses a screen-calibrated simulation of the classic Snellen eye chart. Unlike a real optician's chart at a fixed 20-foot distance, this tool adjusts letter sizes based on your screen's pixel density so the angular size of each letter matches clinical standards at a 60 cm viewing distance. The test uses the same standardized Sloan letter set (C, D, H, K, N, O, R, S, V, Z) used in professional optometry.
Step 1: Calibrate Your Screen
Accurate results depend on knowing your screen's pixel density. Drag the white card on screen until it matches the physical width of a credit card, debit card, or any standard bank card (all are 85.6 mm / 3.37 inches wide). The tool then calculates how many pixels equal one millimeter on your device. If you know your screen's PPI (pixels per inch), you can enter it directly instead.
Step 2: Set Up Your Viewing Distance
Sit or hold your device so your eyes are approximately 60 cm (about 24 inches) from the screen. This is the reference distance the tool uses for letter sizing. Closer or farther will skew the Snellen estimate. Make sure your room has normal ambient lighting — neither very dark nor lit by direct sunlight on the screen.
Step 3: Choose Which Eye to Test
Select Left Eye, Right Eye, or Both Eyes. When testing one eye, cover the other with your palm or a piece of card — avoid closing it, as this can shift focus slightly. Testing each eye separately reveals whether one is significantly weaker than the other, which would be masked by binocular viewing.
Step 4: Identify the Letters
The test shows one Sloan letter at a time at decreasing sizes across 8 levels (20/200 down to 20/20). Each level shows 5 letters. Click the letter button that matches what you see. You need to get 3 or more correct to pass that level and advance to the next. If you genuinely cannot make out the letter, look carefully for a few seconds — but don't guess repeatedly. The test stops advancing when you can no longer identify letters reliably.
Understanding Your Snellen Score
A Snellen fraction like 20/40 means: at 20 feet (your viewing distance, scaled), you can read what a standard eye reads at 40 feet. Bigger bottom numbers mean worse acuity. 20/20 is the clinical norm for "normal" vision. 20/200 or worse is the legal definition of blindness in many countries. Most adults with glasses or contacts test at 20/20 or 20/25 when corrected; unaided acuity varies widely.
Tips for Best Results
- If you wear glasses or contacts, test both with and without correction for a complete picture.
- Take the test in a well-lit room but avoid glare on your screen.
- Don't squint — squinting artificially sharpens focus and inflates results.
- Complete the calibration carefully — a 10% error in calibration translates directly to an off-by-one-level result.
- If you fail 20/40 even with correction, book an eye appointment soon.
Frequently Asked Questions
Is this visual acuity test free?
Yes, the test is completely free with no signup, account, or payment required. Open the page and start the calibration immediately.
Is my data safe and private?
Yes. The entire test runs locally in your browser. Your results are never sent to any server or stored anywhere outside your device.
Can this replace a professional eye exam?
No. This is a rough screen-based estimate for informational purposes only. Screen resolution, viewing distance, ambient lighting, and monitor calibration all affect results. Always consult a licensed optometrist or ophthalmologist for an accurate diagnosis.
What are Snellen fractions like 20/20 or 20/40?
Snellen fractions compare your vision to a standard. 20/20 means you can read at 20 feet what a person with normal vision reads at 20 feet. 20/40 means you need to be at 20 feet to read what a normal eye can read at 40 feet — your vision is below average at that level.
Why do I need to calibrate with a credit card?
Screen pixel density varies widely across devices. A standard credit card (85.6 mm wide) lets the tool calculate exactly how many pixels equal one millimeter on your specific screen. Without calibration, letter sizes would be inaccurate and the test result would be meaningless.
What are Sloan letters and why are they used?
Sloan letters (C, D, H, K, N, O, R, S, V, Z) are the standardized optotype set used in clinical visual acuity testing. They were chosen because they are equally legible and have similar visual complexity, preventing any single letter from being easier to guess than others.
Why test each eye separately?
Most people have slightly different acuity in each eye. Testing separately can reveal amblyopia (lazy eye) or asymmetric refractive errors that would be masked if you tested both eyes together. Cover one eye completely during each phase for accurate results.
What does it mean if I fail the 20/40 level?
In many countries, 20/40 is the minimum driving vision standard. If you struggle below 20/40 corrected (with glasses or contacts), it is strongly recommended to book an appointment with an eye care professional. This online result is not a clinical determination — professional evaluation is always required.