DPI and print size calculator
Pixels to a print size and back. An image has no physical size until something prints it, and the DPI number inside the file changes no pixels at all. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
An image has no physical size
A 3,000-pixel-wide photograph is not ten inches wide. It is three thousand pixels. It becomes ten inches when something prints it at 300 dots per inch, five inches at 600, and forty inches on a billboard at 75. The pixels do not change.
The DPI number stored inside a JPEG or a TIFF is a note attached to the file saying how big somebody once intended it to be printed. Changing it alters no pixels at all. Most software on the way to a printer ignores it and asks you instead.
DPI and PPI are not the same thing
| PPI, pixels per inch | Belongs to an image or a screen. A pixel is a full colour: any of millions. |
|---|---|
| DPI, dots per inch | Belongs to a printer. A dot is one squirt of one ink, and there are usually four to eight inks. An inkjet at 1,440 dpi is using many dots to reproduce one pixel. |
Which is why “my printer does 1,440 dpi so I need a 1,440 ppi image” is wrong by a factor of about five. The number a print shop asks for is 300 ppi, and it has been 300 for thirty years.
How many pixels is enough
300 is the trade default because past roughly 400 the eye cannot resolve the difference on paper held at arm's length. But viewing distance is the real variable: a banner seen from ten feet is fine at 50 dpi, and billboards are routinely printed at 15. The rule of thumb is that the required resolution halves as the viewing distance doubles.
So “is this photo big enough?” has no answer without knowing how close somebody will stand.
Enlarging invents pixels
If there are not enough pixels, resampling makes more by interpolating between the ones there are. That is not the same as recovering detail; the detail was never recorded. Modern machine-learning upscalers do better than bicubic interpolation at looking plausible, and plausible is not the same as correct — they invent texture that was not in the original, which is fine for a poster and not fine for evidence.
What it will not do
- It will not resize an image. It tells you what you have and what you would need.
- It will not account for bleed. Print work needs 3 mm of image past the trim on every edge, so an A4 flyer is really 216 × 303 mm of artwork.
- It will not know your printer's line screen, which is what actually limits a halftone press, and is quoted in lines per inch rather than dots.