I'm not fond of such calculators. They're often too stringent in insisting on the same number of pixels for a very large and a very small print.

Assuming well corrected close vision, the human eye can resolve around 240 input pixels per inch from an 8x10 print held 13 inches away. But it can only resolve 120 PPI from a 16x20 held 26 inches away, or 60 PPI from a 32x40 at 52 inches... Conversely, we can see MORE than 240 pixels in a 4x5 if we hold it around 8 inches away, assuming we can focus our eyes that closely.

So the formula should have some reference to normal viewing distance... BUT, where this breaks down is *subject-matter dependency*. If you intend to make a print with tons of fine details in it — say, a group of 450 graduating seniors at a high school — then your file should have 240 to 250 pixels per inch in it. The print will be scrutinized at very close distances, as the viewer attempts to recognize individuals in the image.

Ideally, we evaluate the subject matter, the viewing distance, and the print size, and come up with a compromise that works for the application.

Personally, I use 300 PPI for 4x5, 4x6, and 5x7 prints, 250 PPI for 8x10 to 12x18 prints, 200 PPI for 16x20 to 24x36 prints, and 150 PPI for 30x40 to 40x60 prints. This is fine for MOST subject matter, with the caveat mentioned earlier about close viewing.

I generally use Photoshop's crop tool to enter a size and PPI value, and let the software do the calculation for me as I crop the image with fixed proportions... For very large prints, I'll have a lab friend use his dedicated resizing tool (Perfect Resize).