Why does image size shown not the same as image size saved

Question

After I camplete cropping of my image, I look under adjustments for file size.  I see 8 MB.  Is this the actual size of the image when I save it to submit it to a website?

D Fosse · Accepted Answer

Those two sizes are in fact unrelated. Image size is not the same as file size.

The number in Image Size works like this: One byte per color channel for every pixel (two bytes for 16 bit files). Note that this is the flat size of a single layer. It's image size; not file size. So you get total megapixels x3 or x6. This is the reported image size in bytes.

To get KB you divide by 1024 (not 1000). And then to get MB you divide by 1024 again.

The size of the stored file on disk is a whole other story. It may have layers, alpha channels or other additional properties. It may have metadata. And most importantly - it may be compressed. The number in the Image Size dialog doesn't account for any of this. So you normally end up with a whole other number.

The file on disk is packaged in a storage container - the file format. An open file doesn't have a file format. Jpeg compression in particular reduces file size dramatically (at a cost, but that's a different subject). A jpeg can be shrunk to 2-10 % of its native "natural" size, at the same total pixel count.

EDIT cross post

Conrad_C · Answer

File size depends on the image file’s characteristics (width/height in pixels, bit depth…), its file format, number of layers, its compression settings, and more. Change any of those, and the file size becomes different.

The file size shown in Image Size is based on the current dimensions in pixels, at the current bit depth, uncompressed.

If you save that image in TIFF (uncompressed) format or Photoshop format, the file might be about the same file size as what it says in Image Size.

If you are exporting a copy to upload to a web site, as JPEG for example, now you are changing lots of things about the file:

If the original file had lots of layers, now it is flattened to just one (JPEG does not support layers). File size goes down.
If the original file had lots of pixels (like 6000x4000 px from a 24 megapixel camera), it probably has fewer pixels now if you are exporting to the typical dimensions of a web graphic, such as 900x600 px or 1200x800 px. File size goes down some more.
If the original file was 32 or 16 bits per channel, now it has only 8 bpc because that is all JPEG can handle. File size goes down by about half.
JPEG uses lossy compression, so file size goes down some more. The exact amount will depend on how much JPEG compression you asked for; did you set it to a higher quality level (larger file size) or lower quality (smaller file size)?

If you put all these together, a 100MB Photoshop file could become a 1MB (or larger or smaller) JPEG file depending on the export choices you made.

If you are saving to JPEG using the Photoshop commands Save a Copy, Save for Web (Legacy), or Export As, the export preview shows you an estimated file size, which changes depending on what options you choose. You can use that to preview the final export size and help you decide which settings you want to use.

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