OS Details

Apple Watch SE 40mm has been launched in September 2020.
Apple Watch SE 40mm have watchOS 7.0

Screen Dimensions

Apple Watch SE 40mm screen size is 1.57 inch with ~ 57.0% body ratio of actual device size. Physical dimensions for Apple Watch SE 40mm device is 1.57 x 1.34 x 0.41 INCH or ( 40 x 34 x 10.4 MM).

Displaying of Units are in this order "Height x Width x Thickness"

Screen Pixel Density and CSS Pixel Ratio

"Pixel" is smallest unit/element of any display.

The total number of pixels that fit into an inch is referred to as "Screen Density" or "Pixel Density" and it measured as "Pixels Per Inch".
Pixel Desnsity have limits to display Pixels Per Inch depends on different screen sizes. When number of pixels per inch increases than it's screen-size limit, the Display resolution will increase but actual device width/height in pixels remain same. Actual device pixels are known as Device Independent Pixels or CSS Pixel Ratio.

Apple Watch SE 40mm resolution is 394 x 324 ^PX with ~ 326 pixel desnsity.
Apple Watch SE 40mm viewport size is 197 x 162 ^PX with ~ 163 actual pixel density, which means it have 2.0 xhdpi display pixel density.

Screen Resolution and Viewport

The sum of pixels which are displaying on a device is called as "Screen Resolution". and sum of actual pixels of any device is reffered to as "Viewport".

Apple Watch SE 40mm have 1.57" physical screen size and its resolution is about 394 x 324 Pixels with approximately 326 PPI pixel density. Apple Watch SE 40mm has viewport size 197 x 162 Pixels and its pixel ratio is about 2.0.

For better understanding screen actual sizes, viewport sizes, display resolution and about their density click here.

CSS Media Queries

Apple Watch SE 40mm css media queries will help developers to write styles specifically for Apple Watch SE 40mm. Here are details:

Apple Watch SE 40mm Media Queries (In terms of Mobile only)

@media only screen and (min-width: 197px) and (max-width: 767px) { /* Your Styles... */ }

Apple Watch SE 40mm Min-Width Media Queries

@media only screen and (min-width: 197px) { /* Your Styles... */ }

Apple Watch SE 40mm Min-Height Media Queries

@media only screen and (min-height: 162px) { /* Your Styles... */ }

Apple Watch SE 40mm Landscape Media Queries

@media only screen and (min-width: 162px) and (orientation: landscape) { /* Your Styles... */ }

Apple Watch SE 40mm Portrait Media Queries

@media only screen and (min-width: 197px) and (orientation: portrait) { /* Your Styles... */ }

Apple Watch SE 40mm Device Width Media Queries

@media only screen and (min-device-width: 197px) and (orientation: portrait) { /* Your Styles... */ }
@media only screen and (min-device-width: 162px) and (orientation: landscape) { /* Your Styles... */ }

Apple Watch SE 40mm Retina Media Queries

@media
  only screen and (-webkit-min-device-pixel-ratio: 2.0),
  only screen and (   min--moz-device-pixel-ratio: 2.0),
  only screen and (     -o-min-device-pixel-ratio: 2.0/1),
  only screen and (        min-device-pixel-ratio: 2.0),
  only screen and (                min-resolution: 326dpi),
  only screen and (                min-resolution: 2.0dppx) { 
    /* Retina styles here */
}
                                

Retina is actually based upon device pixel ratio. Devices mostly have 2x or 3x display so you can use general retina media queries to display high resolution content on all kind of devices. Retina 2x and Retina 3x media queries given below:

Retina 2x Media Query (for all 2x resolution supported devices)

@media
  only screen and (-webkit-min-device-pixel-ratio: 2),
  only screen and (   min--moz-device-pixel-ratio: 2),
  only screen and (     -o-min-device-pixel-ratio: 2/1),
  only screen and (        min-device-pixel-ratio: 2),
  only screen and (                min-resolution: 192dpi),
  only screen and (                min-resolution: 2dppx) { 
    /* Retina styles here */
}
                                

Retina 3x Media Query (for all 3x resolution supported devices)

@media
  only screen and (-webkit-min-device-pixel-ratio: 3),
  only screen and (   min--moz-device-pixel-ratio: 3),
  only screen and (     -o-min-device-pixel-ratio: 3/1),
  only screen and (        min-device-pixel-ratio: 3),
  only screen and (                min-resolution: 384dpi),
  only screen and (                min-resolution: 3dppx) { 
    /* Retina styles here */
}
                                

Apple Watch SE 40mm Specifications

• Launch Date September 2020
• watchOS 7.0
• Physical Screen Size 1.57 INCH
• Resolution 394 x 324 PX
• Viewport Size 197 x 162 PX
• Pixel Density 326 PPI
• Device-Independent Pixels 163 PPI
• Pixel Ratio 2.0 xhdpi
• Width 394 PX
• Height 324 PX
• Device Width 197 PX
• Device Height 162 PX
• Dimensions 40 x 34 x 10.4 mm (1.57 x 1.34 x 0.41 in)
• Weight 30.7 g - 1.09 oz