File Size and PPI Resolution


Today’s Question: If pixel per inch (ppi) resolution doesn’t matter for the digital display of photos, wouldn’t saving at 300 ppi create a larger file than with 72 ppi?

Tim’s Quick Answer: No. The pixel per inch resolution setting for an image has absolutely no impact on the file size.

More Detail: The total number of pixels in the image combined with the file type and related settings determine the file size for an image. The ppi resolution is simply a metadata setting used for determining the output size of an image when it is printed.

As may have been made abundantly clear by the number of questions I’ve addressed recently related to resolution in general and ppi resolution in particular, clearly this is a topic that leads to confusion.

The pixel per inch resolution is simply a way to explain the overall pixel dimensions in an image in a way that is intended to be more clear. For example, I think most people would agree it is more meaningful to say that an image is 10-inches wide by 10-inches tall rather than that the image consists of 3,000 by 3,000 pixels. But in order to say that an image is 10-inches across, we would need to specify the pixel density, which is what the ppi resolution provides. So instead of saying the image is 3,000 by 3,000 pixels, we can say the image is 10-inches by 10-inches at 300 pixels per inch. Both are saying the exact same thing in terms of pixel dimensions, just using different terminology.

Changing the pixel per inch resolution for an image will not affect the file size, all other things being equal. For example, let’s assume an image that is 1,000 by 1,000 pixels, in the 8-bit per channel bit depth, saved as a TIFF file without compression. The file size will be about 3MB. If you change the ppi resolution to 300 ppi, the file size is still 3MB because it still contains the same number of pixels. If you change the ppi resolution to 1,000,000 ppi the fie size is still 3MB because it still contains the same number of pixels.

Just remember that the ppi resolution simply provides a way for you to translate a set number of pixels to a set number of inches on the printed page. The ppi number is just a metadata value, intended to provide helpful translation information but unfortunately leading to confusion for many photographers.