JSFiddle - React, Tailwind, and code Playground

HTML

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

JavaScript

var dpi_x = document.getElementById('dpi').offsetWidth;
var dpi_y = document.getElementById('dpi').offsetHeight;
var width = screen.width / dpi_x;
var height = screen.height / dpi_y;

alert (width + ' x ' + height);