Today’s Question: I am limited to an image size of 500 KB when uploading for my blog. I am finding images that appear to be under the limit but are huge. One image shows as 465 KB on my disk but when I open it in Photoshop it is over 2000 pixels wide and around 8.7 MB in size. My online form sees the 465 KB, accepts it and then stalls.
Tim’s Quick Answer: In this case the confusion is simply a matter of one way that Photoshop reports the size for an image. The actual file size you’re seeing outside of Photoshop is accurate. The size reported inside Photoshop doesn’t relate to the actual size of the file on the hard drive.
More Detail: When you are limited to a particular file size for an upload, it is the actual size of the file on the hard drive that is the issue. It is important to keep in mind, however, that sometimes that file size is reported in different ways. In other words, there are situations where the file size is below the limit, but the website you are trying to upload to calculates the size differently. I do realize that this seems odd (and it is odd), but this is an issue that we run into at times. (For those interested in the more specific details, as an example one megabyte is sometimes considered to be 1,000 kilobytes and sometimes considered to be 1,024 kilobytes, depending on context.)
When you open an image in Photoshop, the status bar will by default show you the document sizes. This relates not to the actual size of a file stored on your hard drive but rather to the amount of data in the image file itself. This is presented as two numbers. The first number can be thought of as the size the image file would be if you saved the image as a TIFF image without any compression applied, and with all layers flattened into a single layer. The second number represents the image if it is saved as a TIFF image with no compression, but with all layers preserved.
In other words, when it comes to the size of a file on the hard drive, you can largely ignore the document sizes presented in Photoshop, especially if the image will be saved as a JPEG. But it is also important to keep in mind that if the size of that saved JPEG is close to the limit for an upload, it is still possible the upload will fail. And, of course, there are other reasons beyond file size that an upload might fail.