Enter a couple of numbers for your display, and it will tell you the width and height, and the DPI. Simple.

	Pixels wide	Pixels high
			Guess

Diagonal (" or cm)

Display name

Name	Pixels	DPI	Diagonal	Size	Pixel Count
Thinkpad t60	1400x1050	124	14.1" (35.8 cm)	11.3" x 8.5" (28.7cm x 21.5cm)	1470000	Remove
ibook	1024x768	106	12.1" (30.7 cm)	9.7" x 7.3" (24.6cm x 18.4cm)	786432	Remove