Images

DPI, Pixels and Print Size Explained

Why DPI does not exist inside a digital image, how to convert between pixels and physical size, what resolution you need for print, and why upscaling cannot add detail.

7 min read · Updated 2026-07-14

A print shop tells you the file needs to be 300 DPI. A form specifies 200 × 230 pixels. A passport office wants 35 × 45 mm. These sound like three different units of the same thing, and the confusion between them causes a remarkable amount of wasted effort. The resolution is simpler than it looks.

DPI is not a property of your image

This is the crux of it. A digital image is a grid of pixels — say 1200 wide by 1800 tall. That is the whole of its information content. It has no physical size, because it is not a physical object.

DPI (dots per inch) is a ratio between pixels and physical distance. It only becomes meaningful when the image is printed or displayed at a known size. The same 1200 × 1800 image is:

• 4 × 6 inches at 300 DPI — a standard photo print.
• 12 × 18 inches at 100 DPI — a poster, visibly softer up close.
• 2 × 3 inches at 600 DPI — a small, extremely crisp print.

Nothing about the file changed in those three cases. Only the printing instruction did.

The arithmetic

Two formulas cover every case.

• Pixels needed = physical size in inches × DPI
• Physical size = pixels ÷ DPI

For millimetres, divide by 25.4 to get inches first.

Worked example: a 35 × 45 mm passport photograph at 300 DPI. 35 ÷ 25.4 = 1.378 inches; 1.378 × 300 = 413 pixels. 45 ÷ 25.4 = 1.772 inches; 1.772 × 300 = 531 pixels. So 413 × 531. Requiring 600 DPI instead doubles both figures to 827 × 1063.

How much resolution do you actually need?

The honest answer depends on viewing distance, and the standard advice is more conservative than necessary for large formats.

The single most common mistake is preparing a 4-metre banner at 300 DPI, producing a 47,000-pixel-wide file nobody can open, for an object that will be viewed from across a car park.

Upscaling does not create detail

If you have a 400 × 500 image and need 1200 × 1500, resizing up will produce a file with the right pixel count. It will not produce a sharper image. The extra pixels are interpolated — mathematically estimated from their neighbours — and interpolation invents plausible values, it does not recover information that was never captured.

The result is a soft, slightly plasticky image. It may be acceptable at a distance; it will not look like a genuinely high-resolution photograph, and any print professional will spot it immediately.

Downscaling, by contrast, is essentially free. Reducing a 4000-pixel photo to 1200 pixels discards data you were not going to see anyway, and the result is often sharper than a native 1200-pixel capture because averaging reduces noise.

Aspect ratio: the other half of the problem

Resolution tells you how many pixels; aspect ratio tells you their shape. Mismatching them is what produces stretched faces.

A phone photo is typically 4:3 (1.33) or 3:4 portrait. A passport photo at 35 × 45 mm is 1:1.29. IBPS wants 200 × 230, which is 1:1.15. Resizing a 3:4 photo directly to 200 × 230 squeezes it vertically — subtly, but enough that faces look wrong.

Always crop to the target aspect ratio first, then resize. The Image Cropper has ratio presets and a custom option; the Image Resizer then handles the pixel dimensions without distortion.

Screens: forget DPI entirely

For anything displayed on a screen, DPI has no role. What matters is pixel dimensions relative to the display area, and the display's pixel density.

A modern phone or laptop may pack two or three physical pixels into each CSS pixel. An image intended to occupy 400 CSS pixels of width should therefore be supplied at 800 pixels to look sharp on those screens. Supplying 2400 pixels is waste — it costs bandwidth and looks identical.

Frequently asked questions

How do I change my image to 300 DPI?

If the pixel count is already sufficient for the physical size you want, you do not need to change anything — 413 × 531 pixels *is* a 300 DPI 35 × 45 mm photograph. If you have fewer pixels than that, no metadata edit will help; you need a higher-resolution source.

Is 72 DPI bad?

It is meaningless on its own. 72 DPI is a historical convention for screen images and appears in a lot of metadata, but the image is defined by its pixel count. A 3000 × 2000 image tagged 72 DPI has exactly the same content as one tagged 300 DPI.

What size photo do I need for an A4 print?

2480 × 3508 pixels for 300 DPI, which is about 8.7 megapixels. Most phone cameras exceed this comfortably. For a photo hanging on a wall rather than held in hand, 150 DPI (1240 × 1754) is perfectly acceptable.

Can I increase the resolution of an old low-quality photo?

You can increase the pixel count; you cannot recover detail that was never recorded. The result will be larger and softer. For prints, the better approach is to print smaller — the same file at half the physical size has twice the effective DPI.

Why does my image look sharp on screen but blurry when printed?

Screens display at roughly 100–160 pixels per inch, prints at 300. An image that fills a laptop screen may have only a third of the pixels needed to print at that physical size. Check the pixel dimensions against the print size before sending it.

What DPI should I scan documents at?

200–300 DPI for text, which for A4 means roughly 1650–2500 pixels on the long edge. 600 DPI is worth it only for very small print, fine engraving or archival work — it quadruples the file size for detail nobody will look at.