There is a lot of confusion over this topic even today. The calculations are not difficult; the confusion lays within the two acronyms and using them properly. Somewhere down the line DPI and PPI have crossed paths to be a universal term and meaning.
I’ll give an example of a naughty word. Close your eyes little ones. The F-word. This word can be traced back to medieval times. However, it was never used to define rogering, in fact, it was a word used to describe the act of striking or hitting. No, not like I am going to hit or tap that. Focus, now! Later it became a word to describe the act of love making -most of like that! That was around the 16th century But, then around the 19th century it became a word to insult a person and yet at the same time used to talk dirty while beneath the sheets of love. For further reading on the origins of the F-word:
Here is the simplest difference between the two.
DPI refers to the number of printed dots contained within one inch of an image printed by a printer.
PPI refers to the number of pixels contained within one inch of an image displayed on a computer monitor.”
DPI: = printers
PPI: LCD monitor and of course, our other devices that display an image.
Calculation: To obtain the PPI of your monitor, take the square root of the horizontal square and verticle square of the monitor, then divide by the diagonal measurement of the monitor in inches. For example,
√ (〖1920〗^2 +〖1080〗^2 )=2202.90717
2203 / 13.3in = 165.6390977 rounds to 166 PPI
Note that the PPI of a monitor is a fixed value.
An easier method to calculate the size of an image and determine the print size is to use an online calculator found here: