Screen Time Calculator

Not all screen hours are equal, and the difference is distance.

A phone sits 12 to 16 inches from your face. Holding focus that close is real muscular work for the ciliary muscle inside the eye, and it is the same effort whether the screen is bright or dim. A monitor at arm’s length is easier. A television across the room is barely work at all, which is why people can watch for three hours and feel fine, then read a phone for forty minutes and end up with sore eyes.

So this weights the hours rather than adding them:

Weighted hours = Σ (device hours × distance weight)

Worked example: eight hours at a monitor, three on a phone, two of television. 8 × 1.0 = 8.0, plus 3 × 1.5 = 4.5, plus 2 × 0.4 = 0.8, giving a weighted 13.3 on 13 raw hours. Move two of those monitor hours onto the phone and the raw total does not change at all while the weighted figure climbs to 14.3.

What actually helps

The 20-20-20 rule: every 20 minutes, look at something 20 feet away for 20 seconds. It works for two reasons at once. Focusing into the distance releases the muscle that has been held tense at reading distance, and looking up from the screen makes you blink. Blink rate falls by more than half during concentrated screen work, and that is what dries the eye surface out. Grittiness by evening is a tear-film problem far more than a light problem.

On blue light: the evidence that it damages eyes is weak, and blue-light filtering glasses have not performed well in controlled trials. The evidence that bright light close to the eye in the evening delays sleep is much stronger. Treat screens near bedtime as a sleep question rather than an eye-health one, and note that a phone at arm’s length delivers far more light into the eye than a television across the room does.

On the total: the average adult in a desk job now spends most of their waking day looking at something illuminated. That is worth knowing, but the number people can actually act on is not the total. It is the last hour before bed, and how often they look away during the rest.

This page is for adults, and it deliberately has no limit in it. There is no recommended daily figure for a grown-up, which is why the verdict here talks about eye strain rather than a cap. Children are the case where published limits do exist, and they change with age: the kids screen time calculator carries the American Academy of Pediatrics guidance, from none at all under 18 months up to about two hours of recreational use for teenagers.