PPI and DPI: Two Numbers, One Question
Every raster image has two sizes. The first is how many pixels it contains, fixed the moment the photo was taken or the scan was made. The second is how much paper those pixels will cover, and that one is yours to choose. Resolution is the relationship between the two: pixels per inch, or PPI. Spread 3,000 pixels across 10 inches and the image is 300 ppi. Spread the same 3,000 pixels across 20 inches and it is 150 ppi. Nothing was added or lost; each pixel is simply printed twice as wide.
DPI, dots per inch, properly describes the output device. Our HP Indigo presses lay down 812 dots per inch, and the press builds every tone out of clusters of those dots, a halftone. A halftone cell is coarser than a 300 ppi pixel, so the screen, not the pixel grid, is what you see on paper. That is why 300 prints cleanly, and why you never need to match image ppi to press dpi.
In everyday use the two terms have blurred together. Our submission guidelines say "300 dpi," Photoshop's Image Size dialog says "Pixels/Inch," InDesign's PDF export says "pixels per inch," and this site uses both. When anyone in print asks what resolution an image is, they mean pixels per inch at the size it will print. The number is what matters; the letters in front of it are habit.
PPI describes the image
How many pixels land in each printed inch. It changes every time the print size changes, and it is the number you control.
Read it for: photographs, scans, anything built in Photoshop.
DPI describes the press
How many dots the machine can make in an inch. It is fixed by the hardware, 812 on our HP Indigo, and you never need to match it.
Read it for: equipment specs, and everyday shorthand for image resolution.
Why 300 Is the Number
At 300 pixels per inch a photograph reads as continuous tone at arm's length; the pixels are too small to pick out. Push the count lower and the grid starts to show. The press will print every pixel you give it, faithfully, which is the problem when there are too few: the result is ragged edges, worst on a foil separation cut from a soft mask, and a print that looks worse than it did on screen. A monitor shows every image at the screen's own pixel density, so a 72 ppi file looks fine there; only the paper tells the truth. A common expression in this industry is "garbage in, garbage out," and resolution is where it applies most literally.
Going far above 300 buys very little. The press images at 812 dots per inch whatever the file carries, so the extra pixels are hard to see on paper and easy to feel in the upload. A little over 300 is harmless. A lot over is a bigger upload for the same print.
The Rule
Every raster image in the file prints at 300 ppi or better at its final printed size. Type, logos and shapes drawn in Illustrator or InDesign are vector and have no resolution of their own.
One exception: drop shadows, glows and blurs applied in Illustrator are rasterized on export at the value in Effect > Document Raster Effects Settings. Set it to 300 ppi for print; a document started from a web profile defaults to 72. Where the raster/vector line falls, and what to do with a logo that arrived as pixels, is Module 6.
How to Check and Change Image Resolution
The number you are looking for is effective resolution: the image's pixels divided by the inches it occupies in the layout. Each application reports it in a different place.
Photoshop: Image > Image Size
The dialog shows pixel dimensions, print width and height, and resolution together. With
Resample unchecked, type 300 into Resolution and the width
and height update to show exactly how large the image can print. The pixel count does
not move; you have changed the ruler, not the picture. That is the only way to change
resolution without inventing pixels.
InDesign: Window > Links
Select a placed image and the Link Info section lists Actual PPI (the file's own resolution) and Effective PPI (after any scaling in the layout). Effective is the one to read. Window > Output > Preflight will also flag low-resolution images before you export, as Module 2's InDesign tutorial notes, provided the profile you pick checks image resolution; the default [Basic] profile does not, so define one under Define Profiles > Images and Objects.
Illustrator: Window > Document Info
Select the placed image, open Document Info and choose Linked Images or Embedded Images from the panel menu; it reports the resolution at the size the image sits on the artboard. The PPI readout in the Control bar (or Properties panel) shows the same figure.
Scaling changes the number, so check again
Enlarging a placed image to 200 percent halves its effective ppi; a 300 ppi photo becomes 150. Read every scaled image at its final size, not the size it arrived at.
If an image reaches 300 ppi only at 4 inches wide and the layout needs 8, the next section covers your options.
Upscaling: What It Can and Cannot Do
Resampling upward, which is what happens when Resample is checked and you type a larger size, asks the software to invent pixels between the ones you have. The new pixels are interpolated from their neighbors: that smooths the grid away, but it cannot recover detail that was never captured. A 72 ppi web image resampled to 300 ppi has about four times the pixels in each direction and no more information. It prints soft instead of jagged, which is a different failure rather than a fix.
Treat upscaling as a last resort, keep the ratio small, and look for a better source first. The original camera file, a higher-resolution scan or the designer's vector logo usually exists.
The sources that most often fall short:
Images saved from a website
Web images are typically 72 dpi and, more to the point, small in pixels. A 1,200 by 800 pixel image prints only 4 by 2.67 inches at 300 ppi. Ask for the original.
Screenshots
A screenshot has only as many pixels as the screen it was taken on, and the logo or chart you wanted usually occupies a few hundred of them. That is enough for a thumbnail, rarely for print. Ask for the source file.
A small file relabeled 300 ppi
The tag says 300, but the pixel count is what it always was; relabeling only shrinks the size the image can print at. Read the pixel dimensions, not the number next to them.
Type drawn as pixels
On The Kiss the background text came out of Procreate as raster, and when the piece needed to go smaller the background had to be rebuilt as vector; a foil edge is only as clean as the outline it is cut from. Keep type vector from the start.
Resolution Requirements for Different Print Sizes
The arithmetic is inches times 300. The table gives the pixel count a few common finished sizes need, and what they need once bleed is added, because an image that runs to the edge has to cover the bleed too: .0625" per edge for most jobs, more for booklets, as Module 1 explains. Figures are rounded up.
| Finished size | Pixels at 300 ppi | With .0625" bleed per edge |
|---|---|---|
| 4 × 6 in | 1,200 × 1,800 | 1,238 × 1,838 |
| 5 × 7 in | 1,500 × 2,100 | 1,538 × 2,138 |
| 8.5 × 11 in | 2,550 × 3,300 | 2,588 × 3,338 |
| 11 × 17 in | 3,300 × 5,100 | 3,338 × 5,138 |
Those numbers assume the piece is read in the hand: cards, brochures, books. A poster seen from across a room can get away with less. The 150 dpi in the sidebar is a rule of thumb for large format seen at a distance, not a hard spec, because the right figure depends on how close people will stand. Working on something big? Tell us the size and where it will hang and we will tell you what the image needs.
High-resolution images make large files; that is normal. Do not downsample them to shrink the upload. Module 2's export tutorials cover the Compression panel (300 ppi, Maximum quality). For anything over 30 MB, email the order details to order@indigoinkprint.com, use the Upload Files link at the top of indigoinkprint.com, and let the project management team know a file is waiting.
Before the file leaves your desk:
- Every raster image reads 300 ppi or better at final size, in Image Size or the Links panel.
- Images that bleed are sized to the bleed dimension, not the trim.
- Type, logos and line art are vector, not pixels.
- Nothing in the layout is linked to a file we will not receive. A missing link prints as its low-resolution preview; package the native file, or export a PDF the way Module 2 describes.
If an image is borderline and you are not sure it will hold up, send it over with the size it prints at. We will read the Links panel and tell you straight, and if it comes up short we can talk through where a better source might live.