Resizing images in a specific way

We need to resize images to a target size.
Usually these are scans, we remove blemishes and adjust colours in Apple Photos. Often need is a print on A4 but the pic needs to a specific size, e.g. 23 cm (9 inches) wide, with height proportionally. The full A4 size print eventually gets framed.
Apple Photos can only export in pixels, not centimetres.
Does anybody know a tool for this task, preferably free? Photoshop can prob do it but is total overkill for this.

GraphicConverter is certainly much less expensive than Photoshop. It offers extensive Services support so that chances are you can get it to do whatever you need it to right from a right-click on the doc in Finder. I’m by no means a heavy user, but I’ve always been a big fan of Lemke and GraphicConverter.


Unless you specify resolution, it makes no sense to talk of exporting at a particular “size” measured in inches. Your exported image is composed of pixels. If you print it at 150 pixels per inch, the print will be twice the size of the same image printed at 300ppi. Which is “correct”?

@Simon GraphicConverter is a good tool, but I’m afraid for this particular purpose, just to solve this one sizing problem still very much overkill.

@jbr My desired correct result is achieved when the resulting picture has a width of, for example, 9 inches. I’m not talking about file size in MB. Printing at either 150 or 300 ppi should still lead to the same 9 inch width on the given a4 paper, albeit at a better quality for the higher ppi value. No?

Apple Photos only knows px as a unit, but I now found that Preview has a resize function that can indeed change the unit of measurement. This surprises me as I think that both the Photos app and Preview should use the same API from macOS.

@jbr is correct. The image is in pixels and the print resolution will determine the size. If you’re genuinely interested, take the time to watch this video which explains it very well.

If you only want the basics, it covers it in the first couple of minutes - well worth watching for anyone unsure of how resolution works. It also nicely explains the difference between dpi and ppi.

For the OP, if you want to deal in inches, place the image into a layout app and stipulate inches. Using InDesign (or Affinity Publisher, Quark or even Pages) you could simply place the image onto an A4 page and scale it to 9 inches. This will give you the size you want and the resolution of the image and printer will determine the quality.

Many graphic formats (including JPEG, as far as I know) have metadata specifying the image’s “preferred” resolution. That is in order to allow devices to render it at the size recommended by whatever device/software created the image. An especially important feature for things like web pages where you may have displays of many different resolutions from old 72-ppi up to retina displays that rival some printers.

When software displays these images and an explicit destination size isn’t provided, that software should use the image’s resolution and the output device’s resolution to determine a default scale factor and not just assume every image pixel corresponds to a single device pixel.

But that’s not the question here. The question here is how to get an image to print at a particular size.

Some photo editors (including the not-too-expensive Photoshop Elements) will let you specify a resolution and/or physical size (the two parameters are, of course tied to each other) when resizing an image. Maybe this will be sufficient for @mHm’s purpose, depending on what the printing software expects to receive.

Otherwise, you’re right, it should be possible to just tell the printing software the size you want to print (letting it preserve the aspect ratio and crop the edges if the image aspect doesn’t match the paper’s aspect). But that will, of course, depend on the software doing the printing.

I appreciate all answers, I really do, but I think @Shamino understood best what I needed here.

A little bit more background: I’m helping a not-so-young-anymore gentleman who is a picture framer. Some of his customers bring in their historic photos to get them framed, with all sorts of blemishes. He scans, removes spots and similar problems, eventually prints to the size he decided for the frame that he builds (never prefabricated frames). Up to recently he was working on a really old DELL on Photoshop v3. The DELL had started to smell (literally), PSv3 (from 1994 !) would not have run on any new PC, so I got him a dirt cheap used iMac running HighSierra of which he immediately said: “It’s so much easier to use”.

Apple Photos now for the photo “editing”.
Apple Preview to print the result in the size required.
A nice and very cheap solution that works perfectly well.

Thanks again for all contributions.

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Maybe an Automator or Shortcut would work? As long as they scan at a consistent level of detail.

Interesting idea, unfortunately only px or percent are available to set the size this way.

We all understood what you wanted, it’s just that images don’t work the way you think they should work. See the link I posted to understand why.

Refer to my previous suggestion to use the free Pages app. Create an A4 page, drop on the image and size to 9 inches, use the guides to centre on page then print.

Simple, easy and free.

Edit to add, you could even set up a template with an image placeholder set in the correct position and at the right size. Then he could simply drag the photo in and print.

If he’s more comfortable with Photoshop, I would suggest buying a copy of Photoshop Elements. It has an MSRP of $100 for a perpetual license. Which may be more acceptable than a Creative Cloud subscription.

On the other hand, if he’s good working with Apple’s bundled software, there’s nothing wrong with that.

IIRC one of the things I hated about Elements was that it only did measurements in inches. So if II entered pixel dimensions, it converted them to really awkward decimal inches values, Maybe someone who has used it more recently can verify that.

I don’t think that is the case any more. Here are two screen shots from Photoshop Elements 2021’s image-resize dialog:

Screen Shot 2023-09-22 at 00.34.41

Note that you can change either the pixel dimensions (to specific sizes or to a percentage of the current size) with or without preserving the aspect.

Or you can set the document size in real-world units. And as the second image shows, you can choose the units you prefer.

Yes, if you change the document size, it also changes the pixel dimensions. But that’s what I’d expect. If you change the real-world size and/or the resolution, the pixel-size is going to be computed from them. And vice-versa.

I typically set the resolution to what I feel will be appropriate (usually 72 dpi for on-screen images or 300 dpi for printable images) and then enter pixel dimensions, letting it compute the real-world dimensions.

But if you’re trying to make it a specific real-world size (maybe for inclusion in a larger document), then you would want something else. Maybe select the size and resolution and let it compute the pixel dimensions.