Pixels, KB and DPI: What Form Sizes Really Mean
Forms and portals ask for photos in a confusing mix of units: "200 × 230 pixels", "under 50 KB", "300 dpi", "3.5 × 4.5 cm". They sound like different ways of saying the same thing, but they're four separate ideas. Understanding them takes a few minutes, and once you do, you can meet almost any photo requirement without guessing.
Pixels: the size of the picture
A digital image is a grid of tiny coloured squares called pixels. "200 × 230 pixels" means 200 across and 230 down. Pixels describe how much picture information there is, not how big it appears. The same image can be shown tiny on a phone or huge on a billboard.
KB: the size of the file
File size, in KB or MB, is how much storage the file takes, and it depends on three things: the number of pixels, the format (a photo as JPG is far smaller than the same photo as PNG), and how hard it's compressed. A big photo of a plain wall can be a smaller file than a small photo full of detail. File size is unrelated to centimetres or DPI. Quikbench counts 1 KB as 1,024 bytes, though some computers count 1,000, so on tight limits leave a little room.
DPI: how pixels become print
DPI (dots per inch, sometimes written PPI for pixels per inch) only matters when you print. It says how many pixels are packed into each inch of paper. The key formula is simple:
pixels = inches × dpi, or equally printed size = pixels ÷ dpi.
A 1,200-pixel-wide image printed at 300 dpi is 4 inches wide; the same image at 150 dpi is 8 inches wide, but looks softer. Changing the DPI number stored in a file does not change its pixels or how it looks on screen. Most upload forms check pixel dimensions and file size, not the DPI value in the file.
Centimetres and inches: the physical size
Physical sizes, like "3.5 × 4.5 cm", only matter for printed photos. To turn one into pixels, convert to inches (divide centimetres by 2.54) and multiply by the dpi. At 300 dpi:
| Physical size | Pixels at 300 dpi |
|---|---|
| 3.5 × 4.5 cm (35 × 45 mm) | 413 × 531 |
| 2 × 2 in (51 × 51 mm) | 600 × 600 |
| 5 × 7 cm (50 × 70 mm) | 591 × 827 |
| 4 × 6 in | 1,200 × 1,800 |
| A4 (8.27 × 11.69 in) | 2,480 × 3,508 |
For example, a 3.5 × 4.5 cm passport-style photo is 1.38 × 1.77 inches, which at 300 dpi is 413 × 531 pixels.
Putting it together: a typical form
Imagine a form that asks for a photo of 200 × 230 pixels, between 20 and 50 KB, in JPG format. Those are three separate checks:
- Pixels first. Resize the photo to 200 × 230 with Resize Image. If the proportions differ from your photo, crop first so faces aren't squashed.
- Then file size. Compress to the range with the Image Compressor, which can aim for an exact size. Aim near the top of the range if there's also a minimum.
- Then format. Make sure the result is JPG. If it isn't, convert it, for example with the PNG to JPG or HEIC to JPG converters.
For passport, visa and exam photos, the AI Passport Photo Maker sets the size, background and file size together. Our guide to compressing a photo to 20KB, 50KB or 100KB goes through the compression step in detail.
Common confusions
- "My photo is high resolution, but the form says it's too big." Resolution (pixels) and file size (KB) are different. Reduce the pixels, the quality, or both.
- "I changed the DPI and nothing happened." That's expected: it only affects printing, and most forms don't check it.
- "Enlarging a small photo to hit the pixel count." Stretching adds pixels but no detail, so it looks blurry. If you really need a larger image, an AI upscaler can reconstruct detail, as explained in our guide to upscaling a small photo.
- "The size looks right on my computer but the form rejects it." Check the file's size in its properties (not "size on disk"), and check the pixel dimensions, not the on-screen size.
A quick cheat sheet
- Pixels = how much picture. Fix with resizing.
- KB = how big the file is. Fix with compression.
- DPI = print density. Only matters when printing.
- cm or inches = printed size. Multiply by dpi to get pixels.
Set exact dimensions and an exact file size for any form.
Open Resize Image →