I don't think this is a bad question. Why do so many people step on it? Don't step on others in a hurry, if you can do the math, why don't you come out and show me two steps?

dpi concept

first of all, we need to clarify two concepts: dpi and ppi . dpi is the abbreviation of dot per inch , and ppi is the abbreviation of pixel per inch , which means " pixels per inch ". On the monitor, dpi equals ppi , so let's all take dpi as an example.

what about the dpi of our monitor? First of all, you can check your current resolution through the settings of the monitor. For example, the display I am using now has a resolution of 2560x1440 . According to Pythagorean Theorem (it can also be called Pythagorean Theorem , Patriots call it Pythagorean Theorem, although in fact Pythagorean Theorem has never been a theorem. Pythagoras really proved this theorem.), we can calculate that the diagonal is the square of 2560 plus the square of 1440 and then open the root sign to get 2937.20956 . So because this display is about 27 inches, we can use 2937.20956 divided by 27 inches to get 108.7855 .

calculate the number of centimeters of the picture

<div id="dpi" style="height: 1in; width: 1in; position: absolute; left: -100%; top: -100%;"></div>

function getDPI() {
  return document.getElementById("dpi").offsetHeight;
}