How to set 300 dpi in photoshop

'Standard for lots of web to share is 300 DPI'...'it's also minimal standard to such page like 500px and more'

 

PPI (note it is PPI not DPI, literally pixels per inch) is irrelevant for web use.

It is ignored by browsers and whether it is 1ppi, 600ppi, or no ppi value at all it makes no difference to the displayed image on screen. All that matters is the pixel size of the image i.e. x pixels wide by y pixels wide.That is why it is removed on Exporting and in Save for Web (unless the all metadata option is chosen).

Change dpi and size of image online

Both are designed for on screen images where PPI is ignored. The reason you see 96ppi reported in an image viewing application is that with no ppi present a value is inserted in case the image size for printing needs to be calculated.

Photoshop inserts 72ppi in that event.

 

As Kevin states above you can use Save As , or Save for Web with all data checked, to include the ppi value in the metadata with your image but browsers will just ignore it and

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