What does 300 DPI mean?
300 DPI means 300 dots — pixels — for every inch of paper. A 5 × 7 in card at 300 DPI is 1500 × 2100 pixels, because 5 × 300 is 1500 and 7 × 300 is 2100. That is the whole calculation.
A picture does not have a DPI on its own
This is the part that causes most of the trouble. A file has a number of pixels. DPI only appears when you decide how big to print it.
The same 1500 × 2100 px file is 300 DPI printed at 5 × 7 in, 150 DPI printed at 10 × 14 in, and 72 DPI printed at about 21 × 29 in. Nothing about the file changed. What changed is how far you spread the pixels it already had.
So "is this 300 DPI?" is not a question about a file. It is a question about a file and a size.
The common sizes
| 4 × 6 in card | 1200 × 1800 px |
|---|---|
| 5 × 7 in invitation | 1500 × 2100 px |
| 8.5 × 11 in (Letter) | 2550 × 3300 px |
| A4 (210 × 297 mm) | 2480 × 3508 px |
| 2 × 2 in sticker | 600 × 600 px |
| 11 × 8.5 in chip bag | 3300 × 2550 px |
Why 300 and not more
300 is roughly where a normal eye stops picking out individual dots at reading distance, and it is what commercial printers have asked for long enough that it is simply the default. Below it, edges soften and small text goes muddy.
Above it there is usually nothing to gain. Most presses cannot reproduce more detail than that, so a 600 DPI file of a 5 × 7 card is four times the pixels and the same printed result. Large-format work printed to be viewed from several feet away often goes lower on purpose — a banner at 150 DPI is normal, because nobody reads a banner with their nose against it.
You cannot add detail that was never there
Two different things get called "making it 300 DPI", and only one of them works.
| Changing the DPI field | Tells software what size to print the existing pixels at. Adds nothing. A 600 × 900 px picture relabelled 300 DPI is still 600 × 900 px, and will print sharply at 2 × 3 in and no larger. |
|---|---|
| Resampling upward | Invents pixels between the real ones by averaging. The file gets bigger. The detail does not come back, because it was never recorded. |
The only real fix for a picture that is too small is a bigger original, or printing it smaller.
Vector is the exception
Shapes and text drawn as vectors have no pixels and therefore no DPI. They are instructions — this line, this curve, this letter in this font — and the printer works them out at whatever resolution it prints at. A PDF of vector artwork is sharp at any size.
DPI only matters for the parts of a design that are pictures: photographs, scans, and anything imported as a PNG or JPG. That is why a print-ready PDF can be small and still perfect, right up until somebody drops a photograph into it.
Where it goes wrong
- Judging sharpness on screen. A monitor shows about 100 pixels to the inch. Print is three times denser, so everything looks fine zoomed to fit and disappointing on paper.
- Using a picture saved for the web. Images on web pages are sized for screens. One that looks large in a browser is usually nowhere near enough pixels to print at the size it appears.
- Enlarging to hit a pixel count. Scaling 600 px up to 1500 px satisfies a checklist and not an eye.
- Assuming the whole design needs it. Text and shapes do not. Only the pictures do.
Questions
Is 300 DPI the same as 300 PPI?
In everyday use, yes. Strictly, PPI is pixels in an image and DPI is dots a printer puts down, and a printer lays several dots per pixel. Nobody selling a printable needs the distinction; when a print shop asks for 300 DPI they mean 300 pixels per inch of finished size.
My design is 72 DPI. Is it ruined?
Not necessarily — check the pixels, not the label. If the file is 1500 × 2100 px it will print beautifully at 5 × 7 in whatever its DPI field says. If it is 360 × 504 px, no setting will save it at that size.
What DPI should I sell printables at?
300, at the finished size, is what buyers expect and what print shops ask for. It is also what BoboCreate's print presets use by default.
Does a bigger DPI make a better PDF?
Only for the pictures inside it. Text and shapes are vector and already as sharp as the printer can manage.
BoboCreate exports PNG and JPG at whatever resolution you ask for, and a design that has a physical size works the pixels out for you: pick 300 on a 5 × 7 in card and you get 1500 × 2100 px without doing the arithmetic.