画像サイズ確認

Application portals tend to state three things: format, pixel dimensions and file size. The U.S. Diversity Visa photo, for example, must be a JPEG of 600 × 600 pixels and no more than 240 KB. The checker shows all three on one line, so you know which one to fix before the form rejects the file.

The file size is shown in KB or MB using 1,024 bytes per KB, the way Windows and most upload checks count, together with the exact byte count. A file that macOS Finder lists as 245 KB is 239.3 KB here, which matters when the limit is 240.

The format comes from the first bytes of the file, not from its name. Pictures saved from websites often carry a .jpg name while holding WebP data, because the site sent a different format than the link suggested. A portal that checks the content will reject such a file even though the name looks right. When the extension and the content disagree, the checker says so, and the convert pages turn the file into a genuine JPG or PNG.

Recognised formats are JPEG, PNG, GIF, WebP, BMP, AVIF, HEIC, TIFF, ICO and SVG. Pixel dimensions need the browser to decode the image, so HEIC and TIFF files show their format and size but no dimensions in Chrome, Edge and Firefox. Safari decodes both.

DPI is a note stored in the file that tells printers how large to print it. It does not change the pixels or the file size: a 1800 × 1200 photo is the same image at 72 or at 300 DPI. At 300 DPI it prints 6 × 4 inches; at 72 it would print 25 × 16.7 inches with visibly larger pixels. The checker reads the density stored in a JPEG’s JFIF header or a PNG’s pHYs chunk and shows the print size it implies. Many files store none, which is also shown.

If a print shop asks for 300 DPI, divide the pixel width by 300 to see how many inches you can print sharply.

Only the first 64 KB of each file are read for the format and DPI, and the browser decodes the image for its dimensions. No copy is sent to a server, and the list clears when you close the tab. Each file may be up to 100 MB. If a file is too heavy or too large in pixels, the links under each result lead to the KB calculator with its compressor and to the resizer.