What is My Screen Resolution

Screen Resolution Test:

Browser viewport(browser window's size without taskbars/toolbars/address bars):

Screen Resolution

Screen or display resolution of electronic visual display device is the number of individual pixels in each axis that can be displayed on screen.
It is usually noted in a format of width  x  height, for instance: 1920 x 1080 means the width is 1920 pixels and the height is 1080 pixels which in this case would give total of 2,073,600 pixels on screen.
The higher the display resolution is, the sharper and clearer displayed content can be because a higher resolution monitor will be made up of more pixels than a lower resolution monitor. Furthermore, more viewable content can fit onto a higher resolution screen.

Some standard display resolutions and their market names/acronyms:
1280 x 720 (720p, HD, HD Ready, Standard HD)
1920 x 1080 (1080p, Full HD, FHD)
2560 x 1440 (1440p, Wide Quad HD, Quad HD, WQHD, QHD)
3840 x 2160 (4K, Ultra HD, UHD, 4K UHD)
7680 x 4320 (8K, 8K UHD)

Aspect ratio of a display device is a ratio between the width and the height of the screen.
Some common aspect ratios for displays:
4:3 (Fullscreen)
16:9 (Widescreen)
21:9 (Ultrawide)


Pixel(picture element) is a smallest controllable element of a display device and therefore pixels are basic building blocks of any image you see on screen.
Each pixel is divided into three subpixels, each showing one color of varying intensity/brightness: red, green and blue(RGB). Thanks to human perception of colors, combinations of different shades of these three primary colors give single screen pixel way to reproduce all other colors in color gamut.
Contemporary smartphones and tablets may use different number of subpixels per pixel(PenTile matrices) but principle stays the same.

Reference pixel gives physical size to abstract notion of pixel as point of visual information.
As defined by W3C(World Wide Web Consortium), reference pixel is the visual angle of one pixel on a display device with a pixel density of 96dpi(classic desktop monitor) and a distance from the reader of nominal arm’s length of 28 inches.
Visual angle is therefore about 0.0213 degrees and at arm’s length distance of reader from display, one reference pixel corresponds to about 0.26 mm (1/96 inch).
Reference pixel size varies with typical intended user <---> device viewing distance, it becomes larger if the viewing distance increases(TVs) or smaller if viewing distance decreases as in case of handheld devices such as smartphones or tablets.

Pixel density is an attribute of a display device measured as number of device pixels within one inch of screen space, either in horizontal or vertical direction. Value of pixel density is expressed as dpi(dots per inch) or ppi(pixels per inch). Historically term dpi has been used for printers & scanners and even though for electronic displays term dot translates to pixel and therefore ppi would be more correct term, today both terms are used interchangeably.
Pixel density determines how high is the definition of the display. The higher the pixel density of the display, the sharper and crisper are text and images being displayed.

Pixel density is equal to display's native resolution divided by screen size in inches and can be expressed as either of two following formulas:
PPI = total number of pixels horizontally / size of screen in inches horizontally
PPI = total number of pixels vertically / size of screen in inches vertically

Tablets, smartphones and even some modern laptops and computer monitors have pixel densities ranging from 200 ppi all the way up to over 500 ppi, upper part of the range occupied by smartphones.

Color depth represents the number of distinct colors that can be shown by a pixel and it depends on the number of bits assigned per pixel.
24-bit color depth("Truecolor") is used by vast majority of contemporary computer and phone displays and it gives 16,777,216 color variations.

Device Pixel Ratio

If the DPR of your display device is greater than 1, quest for asking yourself what is my screen resolution might have left you wondering why is the info from screen resolution test here different than the resolution listed in the official specifications of the device?
Answer is that applications, including your browser, use CSS pixels while manufacturer of your device gives display resolution info in physical pixels. CSS pixel is a realization of the reference pixel.

Device Pixel Ratio(DPR) is a number given by device manufacturers and it's used for HiDPI(High Dots Per Inch) or Retina(Apple's trademark) displays, which are part of modern smartphones, tablets and even some laptops and monitors.
DPR is in direct correlation with pixel density of the display, the higher the density the greater the DPR value.

DPR is the ratio between physical(device) pixels and logical(CSS) pixels in either horizontal(width) or vertical(height) direction of a screen.
In other words, DPR is a number used for calculating CSS resolution of the screen. From DPR we can directly see how many actual physical hardware pixels make up one CSS pixel.

Apple iPhone 12
Resolution in device(physical) pixels: 1170 x 2532
DPR: 3
Width: 1170/3 = 390, Height: 2532/3 = 844
Therefore, resolution in CSS pixels: 390 x 844

Since DPR is 3, in pixel grid: 3(width) x 3(height) = 9; 9 physical pixels are used to form one CSS pixel.

Brief history and explanation why is DPR necessary:

High pixel density devices were introduced to the consumer market in 2010 when Apple began shipping its products iPhone, iPad and iMac, equipped with Retina displays. Idea behind these high density pixel displays was to provide high-definition display that equals or exceeds the pixel density that can be differentiated by the retina of the human eye.
After the success of Retina display, various other manufacturers introduced their high pixel density displays on the market and they have since spread to the regular users.

Creating modern displays with higher physical pixel densities brought us the benefits of clearer images and sharper text but if browsers kept using one-to-one mapping between CSS pixels and device pixels as they did for classic lower pixel density desktop monitors, everything on screen would be too small to see or read.

Purpose of DPR is to keep consistent size of CSS pixels and therefore consistent size of letters, symbols, images and everything else on screen, across a variety of devices with different physical pixel densities.