design tool
DPI calculator
Enter the pixel size of a file and the size you want to print it, and the sheet is drawn at its real proportions next to something familiar for scale. The tinted block inside it is the largest area that file covers at full sharpness, so the answer to the only question that matters is a picture rather than a number.
Runs on your device. Nothing is uploaded.
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How to use it
- 1
Enter the pixel size
Type it, or open Advanced options and read it straight from a file on your device. Nothing is uploaded either way.
- 2
Pick the printed size
Business card through to a forty eight sheet billboard, plus a custom size in millimetres. The reading distance follows the size you choose unless you set it yourself.
- 3
Read the verdict and the picture
The words say sharp enough, slightly soft or too soft. The drawing shows the sheet at its real shape with the area your file covers sharply tinted inside it.
- 4
Act on the two numbers
Either start from the pixel size shown, or print no larger than the size shown. Those are the only two ways out, and both are on screen.
Density is a question about distance
Normal eyesight separates detail about one arcminute apart, and reproducing detail that fine takes two pixels across it. Work that through and you get roughly 300 pixels per inch for something held in the hand, about 175 at a metre, and under 40 for a shop window. Those figures match what printers publish rather than contradicting them, which is why the same file can be perfect for a poster and hopeless for a postcard.
Why the print is drawn to scale
Millimetres are hard to picture. The sheet is drawn at its real proportions with a bank card, an A4 sheet or a person beside it, whichever suits the size, so 841 by 1189 stops being an abstraction. The tinted block inside is the largest area the file covers at full sharpness. When it fills the sheet the file is fine; when it covers a third of it, you can see how far short you are without reading a single number.
Two thresholds, not one
The bar has a mark on it at the density needed. Above the mark the file reproduces everything the eye can separate. Between the mark and half of it the print is slightly soft, which is acceptable for most jobs and is where a great many perfectly good posters live. Below half, detail is genuinely gone rather than merely unseen, and no amount of sharpening puts it back. The verdict names which of the three you are in.
DPI, PPI and the tag in the file
In everyday use they mean the same thing: how many pixels of your file land in each inch of the print. Strictly, dots per inch describes the printer's own dot pattern, which is a much larger number and is the printer's business. The DPI value stored in a JPEG is only a hint about intended size and changing it adds no pixels. What decides sharpness is the pixel count and the size you print at, which is what this calculates.
Limits
- It counts pixels. It cannot tell whether a photograph is in focus or was already enlarged once.
- It assumes the whole file is printed at the size you enter, with no cropping.
- Densities are rounded to whole numbers, which is right for print.
- Bleed is not included; ask the printer what they want and add it to the size first.
What it does not do
- It does not resample, sharpen or resize an image.
- It does not change a file's stored DPI tag, which does not affect print size on its own.
- It does not export anything. It answers a question and gets out of the way.
Questions people ask
What density should a flyer be?
About 300 pixels per inch at the finished size, because a flyer is read at arm's length or closer. For A4 that is roughly 2,480 by 3,508 pixels. If it will only ever be read from a metre away, pinned to a noticeboard, 175 is enough and the file is a third of the size.
What about a poster?
Around 175 pixels per inch at full size, which for an A0 board is about 5,800 by 8,200 pixels. Large format printers commonly publish 150 as their minimum, so anything in that region is safe. Designing at 300 means a canvas near 10,000 by 14,000, which is slow to edit and makes no visible difference on a wall.
My printer asked for 300 DPI. Should I argue?
Give them the finished size and the pixel dimensions instead of the density, since those are what they actually need. Three hundred is the correct figure for work held in the hand and many shops quote it for everything out of habit. If they insist for a large poster, ask what visible difference they expect at the distance it is read from.
Can I enter inches instead of millimetres?
The presets cover the common inch sizes, including US Letter, Tabloid and the photographic sizes, and the drawing labels every size in both units. For anything else, multiply inches by 25.4 to get millimetres. Keeping one unit in the input avoids the most common arithmetic mistake in print preparation.
Will changing the DPI in Photoshop help?
Only if you also resample, which invents pixels rather than finding them. The number stored in the file is a note about intended size, and on its own it changes nothing about how much detail there is. Enlarging a small image to hit a target density produces a bigger file that prints exactly as soft as the original.
What resolution should I scan an original at?
It depends on how much you are enlarging it. Advanced options has a box for this: give it the original width, the printed width and the density you want, and it works out the scanning resolution. Above about 2,400 pixels per inch a flatbed adds grain rather than detail, so photograph the original instead.
Does anything leave my device?
No. Reading the pixel size from a file decodes it in this page, and the rest is arithmetic. No network request is made after the page loads, so a client's unpublished artwork is safe to check here. There is nothing stored and no account to create.
Build a board at this size
Take what you made here into posters.
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Background reading: the question page on this.
Sources
Written and checked by the OneCraft team. Last checked .