Any math wizzes wanna verify these formulas??

[MDesigner]

I've always wondered.. say I have an LCD monitor that's X inches wide.. and AxB native resolution.. and I ditch the monitor for one that's Y inches wide, and CxD native resolution. How big/small would the images on the screen look compared to my old monitor?

I came up with an Excel doc.. rather than post it somewhere I'll just post the formulas used.

I'll give some variable names here:

oldSize (old monitor size, inches diagonal)
oldWidth (old resolution, width)
oldHeight (duh)
newSize (new monitor's size, inches diagonal)
newWidth
newHeight

ratio = (sqrt(oldWidth^2 + oldHeight^2) / oldSize) / (sqrt(newWidth^2 + newHeight^2) / newSize)

Basically, I'm using Pythag. theorem to get the # of pixels running diagonally, and dividing by the diagonal size in inches, to figure out ppi of the display.

Now.. here's where I'm wondering if this is correct: say I have an image 400x300 on my old monitor. If I multiply that by the ratio, I SHOULD get the dimensions of what that image would look like on my new monitor.

So let's say I'm going from a 20" widescreen, 1680x1050, to a 24" widescreen, 1920x1200. Ratio = 1.05. I take a jpeg or whatever into Photoshop.. it's 400x300.. I resize it to 420x315. What I am now looking at is how this image will look size-wise on my new LCD.

Right? Or wrong?

 

[MDesigner]

Nevermind, I tested it out by changing my resolution from 1680x1050 to 1280x800.. and sure enough, a certain window measured exactly as it did when I resized it up in Photoshop when I was at 1680x1050.

Sweet.