Screen Eye Strain Calculator

Computer Vision Syndrome (also called Digital Eye Strain) affects an estimated 50–90% of people who work at computer screens. Symptoms include eye fatigue, dryness, blurred vision, headaches, and neck pain.

Key risk factors:

Screen distance: The American Optometric Association (AOA) puts a computer screen 20 to 28 inches (50 to 70 cm) from your eyes, with its centre 15 to 20 degrees below eye level. Focusing effort is one over the distance in metres: 2 diopters at 50 cm, 2.5 at 40 cm, 3.3 at 30 cm. So each inch costs more the closer you already are, and under 16 inches (40 cm) the eye is working hard all day.

Screen time duration: The AOA names two or more continuous hours a day as the point of greatest risk. The average office worker spends 6–8 hours looking at screens daily. Adding phone and TV use, many people exceed 10 hours total.

The 20-20-20 Rule: Every 20 minutes, look at something 20 feet (6 meters) away for at least 20 seconds. This relaxes the ciliary muscles that control focus.
The evidence is thinner than the rule’s fame suggests. A 2023 study from the University of Valencia and Aston University (Talens-Estarelles and colleagues) gave 29 computer users break reminders for two weeks, and their eye strain and dry eye symptoms fell. A week after the reminders stopped, the gain was gone. It works while you actually do it.

Blink rate: Normal blink rate is 15–20 times per minute. When staring at screens, blink rate drops to 5–7 times per minute, causing dry eyes. Consciously blinking more often helps, as does using artificial tears.

Lighting: Glare from overhead lights or windows reflecting off your screen is a major contributor. Your screen brightness should match the ambient light level - not brighter, not dimmer. A too-bright screen in a dark room is particularly tiring.

Blue light: Blue light (400–490 nm wavelength) from screens can disrupt sleep patterns when viewed in the evening. Blue light blocking glasses are another matter: a 2023 Cochrane review of 17 trials found they may make no difference to eye strain from computer work, at least over the short follow-ups studied. The bigger factor is overall screen habits.

Tip: Position your screen so the top of the display is at or slightly below eye level. Looking slightly downward reduces the exposed surface area of your eye, which slows tear evaporation.

How the score is built

Four factors, 100 points between them. The weights are this page’s own judgement, not a published clinical score, so read the total as a ranking of what to fix first rather than a measurement. Hours carry the most because duration is the one factor every other habit gets multiplied by.

Screen hours score at 4 points per hour and stop climbing at 40, because past ten hours a day the other three factors decide whether you get symptoms, not the total.
Sitting further than 28 inches scores 5, slightly worse than the ideal band. Text that small pulls most people forward or into a squint unless they enlarge it.

Under 20 is low risk, 20 to 39 mild, 40 to 59 moderate, 60 to 79 high, and 80 or more very high. The bands are a rough guide to how likely daily symptoms are, not a diagnosis. Distance and breaks are the two you can usually change today; hours are usually set by your job.

One point the score cannot capture: an uncorrected prescription. If you already sit at a sensible distance, take breaks, and still get headaches by mid-afternoon, an eye exam is worth more than any setup change. Presbyopia starting in the early forties shows up at a computer first, because the working distance is exactly where the loss of accommodation bites.