Image pixels, file size and DPI: which setting should you change?
Understand width and height, KB limits and print density, with concrete examples for resizing an image without confusing DPI with detail.
Pixel dimensions, file size and DPI describe different things. A form asking for 600 × 800 pixels needs a resize or crop. A 200 KB limit needs compression. A 300 DPI requirement concerns the intended print density stored with the image. Changing one does not automatically satisfy the other two.
Three measurements, three decisions
| Measurement | Example | What it controls |
|---|---|---|
| Pixel dimensions | 600 × 800 px | The image's raster width and height |
| File size | 180,000 bytes | Encoded data that must be uploaded |
| Print density | 300 DPI | How pixels map to a physical print size |
A 1200 × 1800 image printed at 300 pixels per inch is 4 × 6 inches. Changing the density metadata to 600 changes that default print size to 2 × 3 inches; it does not create more pixels. “DPI” is often used in software for this image-density setting even though PPI is the more precise term.
Resize versus crop
Resize changes dimensions. With the aspect ratio preserved, fitting a 1200 × 800 image within 600 × 600 gives 600 × 400. It cannot fill a square without either blank space or cropping.
Crop removes part of the image. If the destination requires a square or a portrait ratio, crop deliberately so that important content is not removed, then resize. Do not stretch a face just to fill the required dimensions.
When a DPI tag matters
If the destination explicitly validates image density, use the JPEG or PNG DPI option and inspect the saved result. Density metadata support varies by format and reader. AttachReady writes explicit DPI for JPEG/PNG; this is not a promise that every upload portal will read it in the same way.
Do not upscale a low-resolution photo expecting new genuine detail. Increasing dimensions creates interpolated pixels. Keep the original and compare the output before accepting it.
A useful order of operations
Read the receiving site's rules, crop only if necessary, resize to the required pixels, set density only if requested, and finally compress to the byte limit. Inspect the output's actual width, height, format and bytes with image inspection.
For background on pixel dimensions and resolution, see Adobe's image-size documentation. Specific application or passport rules must come from the receiving organization, not a generic DPI recommendation.