The resolution is 800 x 1200 pixels. How many dpi is this?

The resolution is 800 x 1200 pixels. How many dpi is this?

You’re confusing different kinds of measurements with each other.

Dots per inch or DPI refers to the number of dots per inch on a printed page. I think you means pixels per inch or PPI. Lots of people mix up the two, though.

So using the correct term of PPI instead of DPI, an image of 800 x 1200 pixels doesn’t have a set PPI. The PPI depends upon the size you plan on printing or displaying the image. For example, if you want to print out the image exactly one inch wide, the PPI would be 800 since there would be 800 pixels in that inch-wide image. If you want to print out the image two inches wide, the PPI would be cut in half to 400 pixels per inch.

How should the 800×1200 pixel picture be set?

Depends on how you want to print it
and at what resolution you want to print it.

You have to do the math.

800x1200 @ 1ppi is 800" x 1200"
800x1200 @ 100ppi is 8" x 12" (sign shop quality)
800x1200 @ 300ppi is 2.67" x 4" (magazine quality)
etc.
You divide the number of pixels by the resolution you want, to get the size of the image in inches.

What is the pi of dpi?

B answered that question for you.

Thank you, or forget it! Although I am not too understanding.

DPI refers to the number of Dots Per Inch on a printed page

When referring to Dots, that is the number of ink dots or line screen of the print itself.
It is often mistakenly used to describe the resolution of a digital image, but digital image resolution should be referred to as PPI or Pixels Per Inch.

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