How do I calculate the right image resolution?

For your printed matter, DPI (dots per inch, inch = 2,54 cm) is often specified as a quality requirement.
If you want to use DPI as startinpoint for your resolution, you will have to convert it.

For what medium will the image be used and in what size?
For example: an image will be printed in a magzine, siz A5 (21×14,8cm), and a DPI of 150.

We will use the following formula:

Width image:
(21 : 2,54) x 150 DPI = 1240 px

Height image:
(14,8 : 2,54) x 150 DPI = 874 px

Render the image in resolution 1240 x 874 to meet the 150 DPI requirement.

Under output in ViSoft Phototuning you can choose from typical resolutions and paper sizes. Use Custom to set your own pixel resulotion.

Default print DPI resolutions:
Billboard = 15 dpi
Magazine= 150 dpi
Fine art print = 240 DPI

Formula size in cm to pixels

Width image:
(width in cm : 2,54) x 300 DPI = pixel size

Height image:
(height in cm : 2,54) x 300 DPI = pixel size

Formula pixel to width or height in cm

Width image:
(width pixels : 300 DPI) x 2,54 = imagesize in cm

Height image:
(height pixels : 300 DPI) x 2,54 = imagesize in cm

Note: this formula is only a theoretical resolution and does not always apply. It also depends on the quality of the image.