Today’s Question: When I resize an image in Photoshop, the math doesn’t seem to add up. For example, with pixel dimensions of 5472×3648 in the Image Size dialog Photoshop shows “114.2M”, which I assume refers to megapixels. But the actual number of megapixels is about 20. What is the 114 number based on?
Tim’s Quick Answer: The “114.2M” value in this example refers to the estimated file size in megabytes for the image, not megapixels. In addition, the estimated file size will often not be accurate, depending on how you save the image.
More Detail: The Image Size command in Photoshop enables you to change the actual pixel dimensions of an image, so that you can resize an image for specific output. This is helpful, for example, when you want to print an image at a specific size.
The “M” value shown at the top-center of the Image Size dialog stands for “megabytes”, and represents the estimated file size for the image when you save it at the current size. However, there are a variety of factors that affect the file size, which can cause the number shown in the Image Size dialog to be quite inaccurate.
Think of the file size estimate in the Image Size dialog to be based on saving the current image as a TIFF file with no layers, and with no compression applied. If you save an image in this way, the file size shown in the Image Size dialog will be quite accurate.
If you save the image as a JPEG, the file size will be considerably smaller because of the lossy compression applied, with the actual file size depending on the pixel dimensions, the image quality setting used for the JPEG compression, and the complexity of the image itself.
If you save the file as a TIFF with a significant number of layers (especially image layers) and without any compression applied, the file size may be significantly larger than the estimate in the Image Size dialog.
Therefore, in many cases the file size estimate shown in the Image Size dialog is not especially useful. If you are familiar with file size as it relates to the potential output size in the context of a flattened TIFF image with no compression applied, the value may be somewhat useful. But in general I recommend ignoring the value shown, other than as a very rough guide to how much space the file might require when saved to your hard drive.