How does InDesign calculate DPI?

Excellent reading here

http://the-print-guide.blogspot.com/search/label/LPI%2FDPI

I suppose from the image resolution and the print size.

If so, does it use an average printer dot size for the calculation or most printer dots are of the same size?

I am trying to get the DPI based on these 2 values in Photoshop, but it only shows PPI.

By @Chris P. Bacon

Some definitions (some of which are already discussed):

• PPI. Pixels per inch, which you get by dividing the pixel dimensions by the physical dimensions. This is device-independent. 
• Effective resolution. PPI resolution after final scaling on the layout is taken into account. 
• DPI. Dots per inch, which really means device hardware dots per inch. This is not shown in Photoshop Image Size because device dots are not represented there. This is obviously device-specific, so it does not apply until the document reaches the printer. 
• LPI. Lines per inch, used for processes involving halftone dots built from device pixels. LPI is not used as much as in the past because digital printing processes today (such as inkjet printers) tend to use other screening processes such as irrational/stochastic/FM screening. 

Example: You need a 300 ppi print of an image. Just saying 300 ppi actually isn’t enough information, because per the definition above, to get to PPI you need to know both the pixel dimensions and final physical size (inches/cm) of the print. If you say the physical dimension needs to be 10 inches wide, adding that value resolves it: To achieve 10 inches wide at 300 ppi, the image must be 3000 pixels wide (10 inches times 300 ppi). Of course you can use Photoshop Image Size to work that out.

Suppose the printer is a commercial digital printer or platesetter with a hardware device resolution of 2400 dpi. That means you will print a 300 ppi image on a 2400 dpi printer. The image doesn’t have to be 2400 dpi, because those dots are used to build the screening pattern that renders image tones and colors. (Type and vector graphics do render at device dpi.)

If it was a halftone printing process, then you could have three numbers — which are all true for the same image: A 300 ppi image, printed on a 2400 dpi device, using a 150 lpi halftone screen (150 lpi halftone dots built using the smaller 2400 dpi device hardware dots).

But the ppi resolution of an image changes if you resize it on a print layout. If you halve the physical length on the layout (without cropping), the image ppi must double. So a 3000 px wide image prints at 300 ppi at 10 inches, and also at 150 ppi when 20 inches wide, and at 600 ppi when 5 inches wide. The ppi resolution after final scaling is the Effective PPI resolution. Photoshop shows you this in Image Size (if Resample is disabled), but Photoshop doesn’t know if the image will be resized in another application later, changing the Effective PPI again.

Fortunately, the Info panel in InDesign can show you Effective PPI interactively — see the demo below. And of course Effective PPI is not InDesign-specific. This is how image ppi is calculated for print in any print layout application by any company. Because you can’t get around the math: Final image ppi for print is always the pixel dimensions divided by the physical dimensions times the scale percentage (how much it’s resized) on the layout.

In the demo below, a 3000 × 2400 pixel image is placed into InDesign. Because Photoshop embedded a ppi value of 300 in the image metadata, InDesign reads that, divides the pixel dimensions by the metadata ppi, it works out to 10 inches wide, and the Info panel reports 300 ppi as the Effective PPI. But as you resize the image, InDesign recalculates the ppi based on the new physical dimensions, updating the Effective PPI. That’s how InDesign calculates resolution.

And again, dpi does not enter into this until the job hits the printer. At that time, the printer renders the job at the dpi of the hardware.

DPI / 16 = LPI (optimum) 

PPI of the image file = minimum 1.5 * LPI and max 2.0 * LPI.

Also, highend offset and inkjet printing can use stochastic screening—I think of LPI as a measurement of AM halftone screens where the printed dots change size.

Stochastic frequency modulated screening creates the illusion of tonality by changing the spacing of small, same sized dots:

DPI is the print device resolution, which is set by the printer driver not InDesign.

As Peter suggests most print devices convert a continuous tone image into either a halftone screen, which has a resolution referred to as Lines Per Inch (LPI), or a stochastic screen.

The halftone screen dots are drawn with the smaller printer dots, and the illusion of tonality is created by modulating the size of the halftone dot. A platemaker outputting at 2400 dpi can accurately draw a 150 line halftone screen, my 1200 DPI laser printer can draw a 90 LPI halftone screen.

https://en.wikipedia.org/wiki/Halftone

https://en.wikipedia.org/wiki/Stochastic_screening

InDesign can read and scale image resolution, which is referred to as Pixels Per Inch (PPI). The image resolution of  a placed image at 100% is listed as Actual Resolution in Link Info panel, and the scaled image resolution is listed as Effective Resolution. If you scale an image with an Actual Resolution of 300ppi to 50%, the pixels are half the original size so its Effective Resolution is 600ppi.

On a PDF Export you can downsample and lower the Effective Resolution via the Compression tab (you can’t upsample on a PDF export)

There is a differnce between image resolution (pixels per inch, which folks used to refer to as DPI incorrectly) and the number of printer dots per inch, which is better referred to in a converntional haltone as LPI (lines per inch).

What matters to you is the EFFECTIVE resoltion, in PPI, of any image placed in InDesign. This is calculated by dividing the scaled dimensions of the image (in inches) by the number of pixles in the image in each direction.

Resolution in Photoshop is a myth. It doesn't actually exist. What you see listed in the image size dialog is the theoretical resolution of the image if it is reproduced at the listed dimensions. If you change the resolution or dimensions in that dialog without resampling you will see they change together in inverse relationship and the actual number of image pixels remains constant.

Until they are output, images are only pixels and have not phyical dimensions or resolution.