People often use DPI as shorthand for image resolution. For the calculations on Swetter, PPI is the more precise term: pixels per inch of the printed image. Printer DPI describes output dots. Those dots and image pixels are not interchangeable units.
Count pixels first
Suppose an image is 1,200 × 800 pixels. At 6 × 4 inches it supplies 200 PPI. At 12 × 8 inches it supplies 100 PPI. Neither result depends on whether the image's metadata originally says 72, 96 or 300. The result follows from the actual pixels and the physical size you request.
A tag is not extra detail
Changing a resolution value while keeping the same pixels can alter the default physical size shown by an application. It does not recover texture, sharpen an out-of-focus subject or add captured detail. Adobe explains how image dimensions, resolution and resampling interact.
Resampling actually changes the pixel count. Enlargement software estimates new pixels. That can improve appearance in some cases, but a larger number in the file properties is not proof that the image has the same detail as a higher-resolution original. Swetter's print checker does not resample or enhance your file.
Choose the useful question
Instead of asking whether a file is “300 DPI,” ask: how many pixels remain after cropping, and how many inches will they cover? If a print provider asks for 300 PPI at 8 × 10 inches, plan for at least 2,400 × 3,000 pixels in the final crop. If the original has a different shape, some pixels may be discarded.
Use the result with context
Swetter compares the calculated density with the target you select. Meeting that target is a geometric result. It does not evaluate image noise, lens sharpness, compression, color profiles, printer settings or the paper surface. A lower density may be acceptable for a distant-viewed poster; fine text at close range may need more care.
Start with the best original file available, use the crop preview, check the calculated density, and ask the print provider about their requirements. If color or sharpness is critical, a physical proof is more informative than a resolution label alone.