Window: devicePixelRatio property

The devicePixelRatio of Window interface returns the ratio of the resolution in physical pixels to the resolution in CSS pixels for the current display device.

This value could also be interpreted as the ratio of pixel sizes: the size of one CSS pixel to the size of one physical pixel. In simpler terms, this tells the browser how many of the screen's actual pixels should be used to draw a single CSS pixel.

This is useful when dealing with the difference between rendering on a standard display versus a HiDPI or Retina display, which use more screen pixels to draw the same objects, resulting in a sharper image.

You can use window.matchMedia() to check if the value of devicePixelRatio changes (which can happen, for example, if the user drags the window to a display with a different pixel density). See the example below.

Value

A double-precision floating-point value indicating the ratio of the display's resolution in physical pixels to the resolution in CSS pixels. A value of 1 indicates a classic 96 DPI display, while a value of 2 is expected for HiDPI/Retina displays.

Other values may be returned in the case of unusually low resolution displays or, more often, when a screen has a higher pixel density than double the standard resolution of 96 DPI. Modern mobile device screens - which offer high display resolutions at small physical sizes - often yield a devicePixelRatio value greater than 2.

Examples

Correcting resolution in a <canvas>

A <canvas> can appear too blurry on retina screens. Use window.devicePixelRatio to determine how much extra pixel density should be added to allow for a sharper image.

HTML

html
<canvas id="canvas"></canvas>

JavaScript

js
const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");

// Set display size (css pixels).
const size = 200;
canvas.style.width = `${size}px`;
canvas.style.height = `${size}px`;

// Set actual size in memory (scaled to account for extra pixel density).
const scale = window.devicePixelRatio; // Change to 1 on retina screens to see blurry canvas.
canvas.width = Math.floor(size * scale);
canvas.height = Math.floor(size * scale);

// Normalize coordinate system to use CSS pixels.
ctx.scale(scale, scale);

ctx.fillStyle = "#bada55";
ctx.fillRect(10, 10, 300, 300);
ctx.fillStyle = "#ffffff";
ctx.font = "18px Arial";
ctx.textAlign = "center";
ctx.textBaseline = "middle";

const x = size / 2;
const y = size / 2;

const textString = "I love MDN";
ctx.fillText(textString, x, y);

Side-by-side comparison of the effect of different devicePixelRatio values on an image shown in a retina display.

Monitoring screen resolution or zoom level changes

In this example, we'll set up a media query and watch it to see when the device resolution changes, logging the new resolution.

HTML

html
<div id="container">
  <p>
    This example demonstrates the effect of zooming the page in and out
    (or moving it to a screen with a different scaling factor) on the
    value of the <code>devicePixelRatio</code> property.</p>
    <p>Try it and watch what happens!</p>
  </p>
</div>
<div id="output"></div>

CSS

css
body {
  font:
    22px arial,
    sans-serif;
}

#container {
  border: 2px solid #22d;
  margin: 1rem auto;
  padding: 1rem;
  background-color: #a9f;
}

JavaScript

The string mqString is set to a media query which checks to see if the current display resolution matches a specific number of device dots per px.

The media variable is a MediaQueryList object that's initialized with the media query string. When the result of running mqString against the document changes, the media object's change event fires, and the code logs the new resolution.

Note that every time the resolution changes, the example has to create a new media query, based on the new resolution, and a new MediaQueryList instance.

js
let remove = null;
const output = document.querySelector("#output");

const updatePixelRatio = () => {
  if (remove != null) {
    remove();
  }
  const mqString = `(resolution: ${window.devicePixelRatio}dppx)`;
  const media = matchMedia(mqString);
  media.addEventListener("change", updatePixelRatio);
  remove = () => {
    media.removeEventListener("change", updatePixelRatio);
  };

  output.textContent = `devicePixelRatio: ${window.devicePixelRatio}`;
};

updatePixelRatio();

Result

To test the example, try zooming the page in and out, and note the difference in the logged value of devicePixelRatio.

Specifications

Specification
CSSOM View Module
# dom-window-devicepixelratio

Browser compatibility

BCD tables only load in the browser

See also