PPI vs DPI: What's the Difference?
One counts the pixels in a digital image; the other counts the ink dots a printer lays down. They get used interchangeably, and here is when that is fine and when it will bite you.
PPI (pixels per inch) measures how densely pixels are packed in a digital image; DPI (dots per inch) measures how many dots of ink a printer lays down on paper. One is about the file, the other about the print. They are closely related and people say DPI for both all the time, which is usually harmless - but they are not the same thing, and the difference matters the moment you move between a screen and a printed page.
The difference in one line
PPI describes pixels; DPI describes ink dots. A pixel is a single coloured square in your image file. A dot is a spot of ink a printer sprays. To reproduce the colour of one pixel, a printer usually lays down a small cluster of dots in different inks, so the two numbers count different things and are almost never equal.
What PPI means
PPI is a density: how many pixels fall within one inch when the image is displayed or printed at a given physical size. It is the honest term for a digital image, because a file is made of pixels, not dots. Take the pixel width, divide by the physical width in inches, and you have the PPI. The same file can be any PPI you like - it depends entirely on how big you choose to show it. Shrink a 3000-pixel-wide photo into a two-inch space and you are viewing it at 1500 PPI; stretch it across twenty inches and you are down to 150. The pixels never changed; only their density on the surface did. For the full picture of pixels and how much detail a file really holds, our guide on image resolution covers it.
What DPI means
DPI belongs to the printer. It is how finely the hardware can place ink, and it is usually a much bigger number than PPI because of how printing works. A printer cannot make a pixel's exact colour from a single dot, so it builds that colour from many tiny dots of a few base inks, a technique called halftoning. That is why an inkjet advertised at 1200 or 4800 DPI can still be feeding it an image at 300 PPI: the DPI is the dot grid the machine paints with, and the PPI is the pixel grid you sent it. They sit on different axes.
Why people mix them up - and when it is fine
Software is the main culprit. Photoshop's dialog says "resolution" in "pixels/inch" but the field is labelled to match print habits, scanners report their sampling as DPI when it is really PPI, and export menus offer a "DPI" box for a web image where it does nothing. So the words got tangled decades ago. For most everyday work the slip is harmless: if you are only ever looking at an image on a screen, neither number changes a single pixel, and you can ignore both. It is when a file crosses onto paper, or a spec demands an exact size, that the distinction earns its keep.
Where the difference bites
On screens, DPI does nothing. A browser reads the pixel grid and draws it; the DPI tag in the file's metadata is ignored. Saving a web image at "300 DPI" changes nothing you can see and nothing about the file size - if you want a bigger or sharper web image you need more pixels, which is a matter of resolution, not a metadata label.
In print, PPI decides sharpness. The convention is roughly 300 PPI at reading distance - enough pixels per inch that individual ones disappear. Work backwards from the size you want: multiply the inches by 300 to get the pixels the file needs. A sharp 4x6 inch print wants 1200x1800 pixels. If your file has fewer, the printer stretches what it has and the result looks soft, no matter how high the printer's DPI is. DPI cannot rescue a photo that does not have the pixels. For the print maths on its own, our guide to DPI walks through it.
ID photos are where both worlds collide. A passport spec is written in millimetres because the photo was historically printed, but you upload a digital file, so the requirement gets converted into an exact pixel count at an assumed print density. That is why a spec quotes both a size and a pixel figure. You do not have to do that arithmetic - the country pages under the Passport Photo Maker hub output the right pixels for each spec.
How this plays out on babapic
Every tool here works in pixels, because that is what a digital image is - none of them touch a DPI tag, since on screen it would change nothing. If a photo is bigger than the space it is shown in, Resize Image sets exact pixel dimensions and Resize by Percentage scales it by a proportion. If it needs tighter framing, Crop Image trims pixels away. And if the file is simply too heavy, Compress Image cuts the weight without changing the format. All of it runs in your browser - nothing is uploaded. More explainers like this live in the guides.
FAQ
Are PPI and DPI the same thing?
No. PPI counts pixels in a digital image; DPI counts ink dots a printer lays down. They are related and often used interchangeably, but a printer typically uses many dots to reproduce a single pixel, so the numbers differ.
Which one should I care about for a web image?
Neither, really - browsers ignore both and simply draw the pixel grid. What matters on screen is the pixel dimensions. Think in pixels and size the image to where it is displayed.
Why is a printer's DPI so much higher than my image's PPI?
Because a printer can only place small dots of a few base inks, so it builds each pixel's colour from a cluster of dots. It needs many dots per pixel, which is why a 300 PPI image prints fine on a 1200 DPI printer.
Is PPI just DPI for screens?
Roughly, that is how the words get used, but it is worth keeping straight: PPI is the honest term for pixels, whether on a screen or destined for print, while DPI properly belongs to the ink dots a printer lays down. Screens have no dots and no DPI - they display the pixel grid directly.