How to find pixels per inch (PPI) using ImageMagick

Question

I can get PPI for a JPEG image using the following command:

$ identify -format "%w x %h %x x %y" mypic.jpg 
1600 x 1200 72 PixelsPerInch x 72 PixelsPerInch

However, when I run the same command on PNG format I get Pixels Per Centimeter:

$ identify -format "%w x %h %x x %y" mypic.png 
1600 x 1200 28.35 PixelsPerCentimeter x 28.35 PixelsPerCentimeter

Is there a way to change the command to get Pixels Per Inch (PPI) for PNG format as well or perhaps calculate the PPI based on the pixels per centimeter?

Answer 1

You can simply add the option

-units PixelsPerInch

Answer 2

The resolution and units used are stored in the file, so if the resolution is stored in PixelsPerCentimeter, that's how identify will display it. There isn't a way to do the conversion automatically through identify. But it's just cm to inch conversion math:

PixelsPerInch = PixelsPerCentimeter * 2.54