How to change image size from pixels to inches?

I have quite a collection of images in all sorts of different dimensions, and I really need to scale them down to a uniform size. I’ve managed to track down a few programs that handle batch resizing automatically—which would be ideal—but there is a bit of a snag: they only offer resizing by percentage or pixel count, whereas my specific requirement is to set everything in centimeters.
Is there perhaps a reliable formula I could use to calculate, say, how many pixels equate to 12 cm? Or, even better, does anyone know of a dedicated batch resizer that actually supports resizing via centimeter measurements?

Well, you could always just use Adobe Photoshop.
Just open up your file, head over to Image -> Resize/Resample, and under the "Set new size" section, switch the units to inches so you can plug in your desired dimensions. Hit OK, save the file, and you're all set.😉