How do you figure out the ppi (pixels per inch)?

[Luba]

I'v read that the 17" has 133 ppi, and the 15" high res has 128 ppi, but how is that figured out? I'd like to do the math for the 24" Apple Cinema Display. Thanks.

 

[sammich]

Diagonal number of pixels: use Pythagoras:

sqrt((width^2)+(height^2))/diagonal length

The ACD 24": (1920^2 + 1200^2)/24 = 94.3 dpi

 

[spinnerlys]

http://members.ping.de/~sven/dpi.html
via http://www.google.com/search?client...8&oe=UTF-8&redir_esc=&ei=Q-3OS4emB8KQOLPCtd4P

 

[Chundles]

Number of Pixels/Number of Inches

Pretty straight forward maths.

 

[striatedglutes]

http://members.ping.de/~sven/dpi.html
via http://www.google.com/search?client...8&oe=UTF-8&redir_esc=&ei=Q-3OS4emB8KQOLPCtd4P

I love that producer price index also comes up. Fewer people probably know about that than pixels per inch :\

 

[ethical]

Diagonal number of pixels: use Pythagoras sqrt((width^2)+(height^2))/diagonal length

This OP ^^

 

[sammich]

fixed that for you.

Damn, I hoped no one would notice that Was too busy adding the ACD calc

 

[ethical]

Damn, I hoped no one would notice that Was too busy adding the ACD calc

haha sorry

i'll edit my post to save you

 

[Luba]

thanks!

 

[jiE]

Number of Pixels/Number of Inches

Pretty straight forward maths.

Well... Not this way. The number of inches is a measure of the diagonal length of the screen, and not (if this formular would be correct) an area of the screen.

 

[wandrer2]

Also, just using the number of inches as specified by Apple is a bit inaccurate.

Measure the width of the screen in inches = w
Count the horizontal number of pixels = p

ppi = p/w

 

[rodrigopolo]

Diagonal number of pixels: use Pythagoras:

sqrt((width^2)+(height^2))/diagonal length

The ACD 24": (1920^2 + 1200^2)/24 = 94.3 dpi

JavaScript:

var width = 1920;
var height = 1080;
var diagonal_inches = 24;
// result:
Math.sqrt((Math.pow(width,2)+Math.pow(height,2)))/diagonal_inches