GIMP DPI: check it, change it and set 300 for print

GIMP has two commands that deal with DPI, and they do opposite things. Image > Print Size changes the resolution stored in the image and leaves every pixel alone. Image > Scale Image changes the number of pixels. To make an image say 300 DPI, use Print Size. To make an image large enough to print at 300 DPI at a size it cannot reach now, use Scale Image, and know that the new pixels are calculated, not recovered.

The steps below follow the GIMP 3.0 manual. The same two commands are in the Image menu in GIMP 2.10, whose manual describes them in nearly the same words.

What GIMP calls it

GIMP says resolution, split into X resolution and Y resolution, and usually measures it in pixels/in. That is the figure everyone else calls DPI. The manual is careful about the difference: “Do not confuse the output resolution with the printer’s resolution, which is a printer feature and expressed in dpi (dots per inch); several dots are used to print a pixel.” For an image file the two words point at the same number, as DPI vs PPI explains.

The resolution only decides how large the pixels print:

print size in inches = pixels ÷ resolution

Check the DPI of an image

Choose Image > Image Properties, or press Alt + Return. The Properties tab shows:

Size in pixels: “the physical size of the image”.
Print size: the size it will have on paper, in the current units.
Resolution: “the print resolution of the image”.

One thing catches people out. The manual notes: “PNG images that have a resolution set, are known to be imported with the unit set to metric, meaning you will see pixels/mm here.” So a PNG may show 11.811 where you expected 300. They are the same resolution: 300 ÷ 25.4 = 11.811 pixels per millimetre. Likewise 2.835 pixels/mm is 72 DPI and 3.780 is 96.

Change the DPI without changing the pixels

This is the answer to “how do I make this image 300 DPI” whenever the image already has enough pixels.

Choose Image > Print Size. The dialogue is titled Set Image Print Resolution.
Set the unit beside the resolution boxes to pixels/in if it shows pixels/mm.
Type 300 in X resolution. With the chain symbol closed, Y resolution follows.
Watch Width and Height change to the new print size.
Click OK.

The manual states what this does: “This command does not change the number of pixels in the image and it does not resample the image.” And: “If you increase the output resolution, the printed page will be smaller, since more pixels are used per unit of length.”

You can also work the other way. Type the print width you want, in inches or millimetres, and GIMP sets the resolution the pixels allow: “As soon as you change the Width or the Height, the X and/or Y resolution values automatically change accordingly.” If you type 10 inches for a 3000 pixel wide image, the resolution becomes 300. If it comes out at 120, the image does not have the pixels for that size at print quality.

Increase the DPI at a fixed print size

“Increase the DPI” often means something harder: keep the print the same size and have more pixels in each inch. That takes more pixels, which is a job for Image > Scale Image. The manual: “The Scale Image command enlarges or reduces the physical size of the image by changing the number of pixels it contains.”

Choose Image > Scale Image.
Set X resolution and Y resolution to 300 pixels/in first.
Change the unit beside Width and Height from px to in or mm.
Type the print size you need. Keep the chain closed so the picture is not distorted.
Choose an Interpolation method under Quality.
Click Scale.

Step 2 matters because, as the manual says of physical units, the resolution fields “are used to convert between physical units and image dimensions in pixels”. With 300 in those boxes, a width of 10 inches becomes 3000 pixels.

The interpolation choices, from the manual:

Method	The manual’s description
None	Each pixel is copied from its closest neighbour. Fast and coarse. Elsewhere it is called Nearest Neighbor
Linear	The average of the four closest pixels. It is “a good compromise between speed and quality”
Cubic	The average of the eight closest pixels. “This usually gives a good result”
NoHalo, LoHalo	“These methods perform a high quality interpolation”

For enlarging, the manual quotes the author of NoHalo and LoHalo, who generally prefers NoHalo. For pixel art, where the hard square edges are the point, None copies pixels without blending them.

Be realistic about the result. The manual is blunt: “GIMP calculates the missing pixels by interpolation, but it does not add any new detail. The more you enlarge an image, the more blurred it becomes.” It suggests the Sharpen (Unsharp Mask) filter afterwards, and adds: “Raster images inherently do not scale up well.”

So before scaling up, ask whether you need to. A print seen from a step away is sharp at 150 DPI, which takes a quarter of the pixels. See 150 vs 300 DPI and what DPI for printing.

Which command for which job

You want	Use	Pixels change?
The file to state 300 DPI	Image > Print Size	No
To see or set the print size	Image > Print Size	No
A larger print at the same DPI	Image > Scale Image	Yes, more are calculated
A smaller file for the web	Image > Scale Image, in pixels	Yes, some are removed
To read the current DPI	Image > Image Properties	No

Set the DPI of a new image

In File > New, open Advanced Options. The X resolution and Y resolution fields there “do not affect the size of the image in pixels, but they may determine its physical size when it is printed”. If you give the new image’s size in inches or millimetres, GIMP uses those fields to work out the pixels, so set them before typing the size: A4 at 300 pixels/in comes to 2480 × 3508 pixels.

Why the image looks a different size on screen

GIMP can show an image at its print size or pixel for pixel. View > Dot for Dot switches between them. With it on, at 100% zoom “every pixel in the image is displayed as one pixel on the screen”. With it off, the image is shown at “the size it will have when it is printed”. The manual’s example: a 100 × 100 pixel image at 300 ppi is drawn 100 pixels wide with Dot for Dot on, and a third of an inch wide with it off.

So if the picture seems to shrink on screen after you raise the resolution, nothing is wrong. Dot for Dot is off and GIMP is showing you the new, smaller print size.

Export, then check

Changing the resolution changes the image in GIMP. To get a PNG or JPEG out, choose File > Export As (Shift + Ctrl + E).

Then check the exported file, because that is what you will send. Open it in the image DPI checker: it shows the DPI written into the file, the pixel size and the print size. If the number is not the one you set, the DPI converter writes it into the file directly, without re-saving the picture. For a JPEG that also avoids compressing it a second time.

Common problems

What you see	Cause	What to do
Resolution reads 11.811	The unit is pixels/mm	It is 300 DPI. Switch the unit to pixels/in to see it
The image got smaller on screen	Dot for Dot is off and the resolution went up	Turn on View > Dot for Dot, or ignore it
The image is blurred after scaling	It was enlarged, and the new pixels are interpolated	Scale less, start from a larger original, or print at a lower DPI
Width and height changed when only the DPI should	Scale Image was used	Undo, and use Print Size
The picture is stretched	The chain beside Width and Height was broken	Close the chain and enter the size again

Questions

Does changing the DPI in GIMP reduce quality? Not with Print Size, which changes no pixels. Scale Image does change them.

How do I change the DPI without changing the image size? If you mean the size in pixels, use Print Size. If you mean the size on paper, the pixel count has to change, so use Scale Image.

What DPI should I use? 300 for print held in the hand, 150 for posters. The DPI calculator gives the pixels for any size.

Is there a quicker way? For a finished PNG or JPEG, the DPI converter sets the DPI in the browser without opening an editor.