What is your screen time actually costing you?
Move the slider to your daily average, which your phone already knows. Nothing is sent anywhere; this runs entirely in your browser.
A caveat worth stating: not all screen time is equal. Video calls with people you love, maps, music and reading are not the same as a feed you did not choose. The number is only useful as a prompt to ask which kind yours mostly is.
Which number to put in
Use the daily average your phone already reports, not your estimate of it. On iPhone that is Settings, then Screen Time. On Android it is Settings, then Digital Wellbeing and parental controls. Both have been recording whether or not you have looked.
Your guess and the logged figure will not agree, and you cannot assume you know which way you are wrong. The largest analysis of this question, a meta-analysis of 106 comparisons between self-reported and device-logged media use, covering 52,007 people, found the two track each other only loosely, at a correlation of about 0.38. It also found no reliable direction to the error: people over-report and under-report in roughly equal measure. So a number you typed from memory is not a weaker version of the logged one. It is a different measurement.
Why there are two versions of “days a year”
The first figure divides by 24, which is the arithmetic most calculators stop at. The second divides by 16, and it is the one worth looking at, because you were never going to spend the other eight hours doing anything. Counting sleep as time your phone competed for makes the total look smaller than it is. At four hours a day the difference between the two framings is about 25 extra days a year, which is the gap between an interesting statistic and a fortnight of waking life.
The books figure assumes roughly eight hours for an average-length book. It is a unit of comparison rather than a target, and it is there because hours are hard to picture and a shelf is not.
What the total does not tell you
A daily average is a poor description of most people’s phone use, because the damage is usually in the shape rather than the size. Four hours in two deliberate sittings is a different problem from four hours in ninety scattered pickups, and only the second one fragments the day around it. The total cannot tell them apart, which is why the useful next step is a week of watching when the pickups cluster rather than trying to shrink the number directly.
The category split matters as much. Video calls, maps, music, reading and messages all land in the same total as an infinite feed, and a total that treats them identically will overstate a problem for some people and understate it for others. If yours is mostly the first kind, the number is not telling you much. If you cannot say which kind it is, that is itself the finding.
How to track a baseline week covers the method, and what the research actually shows covers which of the widely quoted numbers survive a check.