One of the more confusing concepts in digital design is "dots per inch", or DPI for short. I've received many support emails from Repper users over the years asking about it. In this article I explain what DPI is, what it isn't, and how to use it while preparing your designs for print.
DPI in print
Essentially, DPI refers to the number of ink dots that a printer puts along every inch of paper (or any other printable material). The higher the DPI, the clearer and more detailed the printed image. Typical values you'll see are 150 DPI for textile and poster prints, 300 DPI for regular paper print and 600 DPI for high quality photo prints.
I won't go into the nitty gritty about how printers exactly lay out those dots in a few colours to create a full color image. Let's assume that each "ink dot" on paper is a "pixel dot" in our image.
In the digital world, DPI is mostly used to describe images, but lets first briefly go into computer screens.
DPI of computer screens
This refers to the amount of pixels in an inch of screen, sometimes also called pixels per inch or PPI. This used to be 72 or 96 DPI on Mac and Windows respectively—you may have seen those numbers around. These days, screens have much higher pixel densities: many laptop screens are now around 300 DPI and the latest iPhone 12 has a whopping 460 DPI.
While it's good to know that DPI in digital design may refer to your screen, you'll mostly deal with it in the context of image files, so let's dive into that.
DPI of digital images
If you think about it, it's a little strange to talk about the physical properties of a digital file. Does an image file even have a DPI? The answer is no... and yes.
A bitmap image is made up of pixels. Those pixels don't have a physical size: you can display an image on your screen or print it at any size you want. In this sense, the image has no inherent DPI.
However, it can be handy for a printer to know at what size an image is intended to be printed. That's why a bitmap image has an optional DPI value in its metadata. This DPI value is nothing more than a number and it doesn't change the image itself. All it says is "Hi printer! Unless told otherwise, please print this image at this DPI."
Important note about resolution
The word "resolution" is used in different ways, which can be seriously confusing.
The resolution of a print typically refers to its DPI. A high resolution print has a high DPI, a low one a low DPI. I recommend simply calling this DPI and not resolution.
The resolution of a digital image typically refers to its pixel size. For example, an image may have a resolution of 4,000×3,000 pixels (12 megapixels). This is the more common use of the word these days, so that's how I would use it.
The key takeaway is that DPI doesn't tell you something about the quality or resolution of the digital file. You can set a 300×300 pixel image to 30 DPI or 300 DPI. It won't change the image itself, just the size it intends to be printed at.
Calculating with DPI
So how can you calculate how large a printed image will be, or what resolution image you'll need to make the print you wish?
Calculate print size
Given the image size in pixels and DPI, here is how to calculate how large it will be printed:
pixel size (px) / DPI = print size (inch)
Printing a 600×600 pixel image at 300 DPI means it will be 2×2" (600 / 300 = 2) or approximately 5×5 cm.
Calculate pixel size
If you want to print an image at 20×20" at 150 DPI, you're going to need 3,000×3,000 pixels, because:
print size (inch) / DPI = pixel size (px)
Don't worry (too much) about DPI while designing
DPI matter mostly during print, but there are two things you want to keep in mind:
Begin with high quality images
Processor power isn't really a bottle neck these days, so try to find or create the highest quality source images possible. Don't scale them down, instead use them at their original size in your graphics software.
Make sure output resolution will meet your printing needs
Before getting to work with your imagery, make sure it will meet your requirements. For example, if your source image is 500×500 pixels, you will never be able to make large and detailed patterns with it: the source material just doesn't have the resolution for that. However, if you plan to make a small-scale pattern with a high repeat, the small image may suffice.
A large image will give you more flexibility to work with. This is especially true in Repper, where you tend you use only a small part of the source image for your pattern.
Getting ready for print
Let's say you've made a beautiful pattern and you are ready to print it on a textile swatch. These are typically 6×6".
In Repper, this is pretty straightforward. In the surface export settings, set the dimensions to 6 by 6, the unit to inches and the DPI to 150 (typical for fabric print). Repper will automatically calculate the export size in pixels: 6 inch * 150 DPI = 900 pixels wide and tall.
The set DPI is a recommendation, not a rule
When you set your image 900×900 pixel image to 150 DPI, you indicate it should be printed at 6×6". However, you probably know that can adjust the size of a print in the printer software you use. This is important to keep in mind: the size can always be overruled and the DPI in your image file is just a recommendation.
This is also good to keep in mind when using online print services like Spoonflower. They print at 150 DPI on fabric, so uploading a file with a different DPI may lead to unexpected sizing of the pattern on the fabric.
Therefore, there is no difference between these two images (for a computer):
- 1) a 1″x1″ image at 300 DPI (= 300×300 pixels on hard-drive)
- 2) a 10″x10″ image at 30 DPI (=300×300 pixels on hard-drive)