The one relationship to remember
Physical size equals pixels divided by DPI. An image of 1200 pixels across, printed at 300 DPI, comes out 4 inches (about 101.6 mm) wide. The same image at 150 DPI comes out 8 inches wide — same pixels, double the size, half the density. Nothing about the image changed; only the conversion did.
This is why DPI on its own says nothing about quality. “300 DPI” describes how tightly pixels are packed at a given size, and it is only meaningful alongside the pixel count. A small image labelled 300 DPI may be perfectly sharp but physically tiny; a large image labelled 72 DPI may print beautifully at a large size.
The numbers people actually use
- **72–96 DPI** — the historic screen convention. Fine for anything only ever displayed; irrelevant if the file is going to paper.
- **150 DPI** — a working floor for ordinary printed documents. Handouts and internal prints are usually acceptable here.
- **200 DPI** — a middle setting that balances file size and legibility for text-plus-image pages.
- **300 DPI** — the normal quality target for anything a person will look at closely: photographs, certificates, forms.
- **600 DPI** — used for fine art, very small text, or archival scans. For a standard page it quadruples the data versus 300 with no obvious gain.
A4 is 210 × 297 mm, which is about 8.27 × 11.69 inches. At 300 DPI the page needs roughly 2481 × 3507 pixels — about 8.7 megapixels. At 150 DPI it needs about 1241 × 1754, roughly a quarter of the data. Both are exact arithmetic from the definition above; the choice between them is a judgement about how much detail the reader needs.
Checking whether an image is print-ready
- Work out the physical size you intend to print at, in millimetres or inches.
- Open DPI Checker and set the target DPI — 300 is the standard quality target, 150 the common floor.
- Read the print size it computes for your image's actual pixel dimensions, and whether the image has enough pixels for the page size you have in mind.
- If it does not, either print smaller, or obtain a higher-resolution source. Upscaling adds pixels without adding detail, so it does not truly solve the shortfall.
- For an image only going to screen, skip this — pixel dimensions alone are what matter.
Where DPI misleads people
- **Embedded DPI metadata can be wrong.** A photo exported with an arbitrary DPI value still has the same pixels; only the suggested print size changes. Trust the pixel count.
- **Bigger is not automatically better.** Scanning a letter-size page at 1200 DPI produces a file roughly sixteen times larger than at 300 with no visible benefit on a normal reader.
- **Screen and print are different targets.** An image sharp on a phone can look soft on paper, because a phone screen at normal viewing distance shows more detail per inch than a printed page does.
- **Resizing changes DPI's effect.** Halving an image's pixels and keeping the physical size halves the effective DPI — see how dimensions affect file size.
Common questions
Does changing DPI in an editor make my image sharper?
No. It only relabels the print size. Sharpness comes from the pixel data; a DPI change without a pixel change moves the trade-off between size and density rather than improving either.
What DPI should I scan documents at?
150–200 DPI for ordinary text documents you intend to print or read on screen, 300 DPI where fine print, stamps or signatures must be unambiguous, and higher only for archival work on very small text.
Why does my 300 DPI photo print blurry?
Usually because the image was upscaled to reach 300 DPI, or the printer did not receive the physical size you expected. Check the actual pixel count with DPI Checker rather than the label.