Calculating image size in inches or dpi from pixels using php

I need t开发者_StackOverflow社区o calculate an approximate image size in inches assuming 300dpi.

I have a target image size in mind, and i want to show the user if the image they have uploaded is adequate, or will need to be stretched, and offer various conditional messages based off of this.

I already have the height and width in pixels, just need to know the math i guess.

DPI = "dots per inch". One dot = one pixel.

300 pixels in 300 dpi: 1 inch. You can work it out :-)

Given that dots == pixels:

size in inches = size in pixels / dots per inch

so if you have an image that's 900 pixels wide, at 300 dpi it will be printed at 3 inches.