# How do I calculate the actual size of an image from pixels and PPI? [duplicate]

If I have drawn a line-art diagram that is 250px x 350px on a screen with 92.56 PPI (Dell U2414H - 1920 x 1080, 23.8 inch diagonal) - I'm confused as to why, when this image is transferred into Photoshop (CS6), its detected dimensions are 3.486" x 4.875".

Should it not be 250/92.56 = 2.7" wide?

Ultimately - I am trying to meet the requirements of a publisher which states the maximum width of an image must be 3.5" - I simply do not know how to calculate whether or not my image falls below this.

• This might be more on-topic on graphicdesign.stackexchange.com Commented Jun 27, 2015 at 3:28

Your image is 250x350 pixels. The dot-pitch of your monitor (92ppi) is irrelevant. The image will have a resolution in dpi (dpi - dots-per-inch - for printing, ppi - pixels-per-inch - for on-screen display) attached to it, and the 'default' is 72 ppi, but that's just a label (It's 72ppi for historical reasons - the dot-pitch of the early Mac monitors which were widely used for DTP, I think). If you change this resolution to 250ppi then Photoshop will tell you it's 1" wide. Change it to 10ppi and PS will tell you it's 25" wide.

You need to know the resolution that that the publisher requires. If it's for a magazine print, then 300dpi is common. If they want a 300dpi image to be printed 3.5" wide, then your image needs to be 1050 pixels wide (and 1470 high to maintain the aspect ratio).

So you're going to have to upscale your image from 250x350 pixels. If you don't you'll have to print at 250/3.5 = 72 dpi (there's that 72ppi again), which probably won't look good. If it's for web use, then it'll be fine.

But any time an image size is mentioned, you need to know both the dimensions of the image in pixels and the required resolution in dpi/ppi. The resolution required for good quality depends upon if the image is for web use (i.e. viewed on-screen) or for printing. Printing requires much high resolution for acceptable quality.

• Good answer. I think the most important point is that the dot-pitch of your monitor is irrelevant. Always think of the size of digital images in terms of pixels. And the digital image's own DPI setting (which is what Photoshop is using to tell you the image's eventual printed size) translates those pixels to printed dimensions. Commented Jun 26, 2015 at 12:15
• Great. That has significantly cleared things up! With the upscaling issue - is this still applicable to text/line-art held within a PDF that can seemingly zoom/upscale without degradation? Commented Jun 26, 2015 at 12:15
• PDFs might be stored as vector images, rather than raster images. That means that they are not stored as a picture, but rather as 'instructions' on how to draw the image. As such, they are redrawn at whatever size requested with no loss of quality. The difference is like having a picture of a circle 1" across and you are asked to produce a circle 10" accross. Raster processing simply magnifies the circle 10x with a loss in quality; vector processing just redraws the circle at the required size, keeping the same quality. Commented Jun 26, 2015 at 12:20