DPI vs Pixels: What Actually Changes a Photo's File Size

Pixels are the photo

A digital photo is a grid of coloured squares called pixels. A 12-megapixel phone photo is about 4000 × 3000 pixels. On screen, the number of pixels decides how much detail there is; nothing else does.

DPI is a printing instruction

DPI (dots per inch, also called PPI for pixels per inch) is a number stored in the file that tells a printer how many pixels to place in each inch of paper. A 600 × 600 pixel photo prints at 2 × 2 inches at 300 DPI, or at 6 × 6 inches at 100 DPI. The file and its pixels are identical in both cases.

Changing the DPI value alone never changes the file size or how the photo looks on screen.

When a form asks for a specific DPI

When a form says something like 3.5 × 4.5 cm at 300 DPI, what it really needs is enough pixels: 413 × 531. Create the photo at those pixel dimensions with the 35 × 45 mm preset. The conversion is explained in the passport photo size guide.

A small number of systems read the stored DPI value itself. If a form rejects a correctly sized photo for its DPI, open it in a desktop image editor, set the resolution to the required value without resampling, and save it again.

What actually changes file size

• Pixel count: fewer pixels means a smaller file.
• Compression quality: lower JPEG quality means a smaller file.
• Content: plain backgrounds compress better than detailed scenes or noisy low-light photos.
• Format: JPG and WebP are far smaller than PNG for photos.

Pixels needed for common print sizes

Use this table when a form or printer asks for a print size at 300 DPI. At lower DPI values the pixel counts shrink proportionally; at 150 DPI, halve them.

Why screens ignore DPI

A screen shows an image using its own pixels, not inches. A phone with a sharp display may pack 400 or more pixels into an inch, while an older monitor may have about 100. The same 600 × 600 photo therefore looks small on one and large on another, regardless of the DPI value stored in the file. Only printing turns pixels into physical inches, which is the one moment DPI matters.