6/6 = standard visual acuity
The Snellen chart is the most widely recognised tool for testing visual acuity. Developed by Dutch ophthalmologist Herman Snellen in 1862, it remains the global standard for screening distance vision. In the United Kingdom, the chart uses metric notation — distances are measured in metres rather than the feet used in the United States.
Each line is labelled with a fraction such as 6/6, 6/12, or 6/60. The top number (6) is the testing distance in metres. The bottom number is the distance at which a person with standard vision could read that line. 6/6 is considered normal visual acuity and is equivalent to 20/20 in the US system.
This online Snellen chart calculates the physical size each letter must appear on your screen, based on your device and viewing distance. It uses the Sloan letter set (C, D, E, F, H, K, N, O, P, R, S, T, U, V, Z) — the same standardised optotypes used in clinical settings.
The chart supports iPhones and Android smartphones with screen sizes from 5.4 to 6.7 inches. Hold your phone at arm's length (roughly 30 cm) and select the matching screen size. Due to the smaller display, some of the finest acuity lines may be too small to render — this is physically accurate.
Select the laptop distance option (50 cm) for testing from your normal sitting position. For more accurate results, test at 2 metres or more. Always test each eye individually and ensure good, even lighting with no glare on the screen.
An online eye chart is a useful screening tool but is not a substitute for a professional eye examination. If you notice blurred vision, difficulty reading, eye strain, or headaches, book an appointment with a qualified optometrist. The NHS recommends eye tests every two years for most adults, and annually for children and people over 60.