Does PPI Effect File Size?
72 PPI is a Myth
Every designer has heard it a million times: 300 PPI for print and 72 PPI for digital. In reality, PPI is almost meaningless in digital. No, it does not effect the quality nor the file size.
Let’s build a file
Let’s say I have a photo that is 700 x 500 pixels. How many inches wide do you think 700 pixels is? Well, that depends on the size of the pixel. Our digital devices contain varied sizes and quantities of pixels.
Photo by James Wheeler from Pexels
On a 1080p monitor, it’s about this big.
On a Samsung Galaxy S20+, it will appear this size.
On a 4K TV, it’s this size.
The image isn’t changing sizes, there are just more pixels on a 4K TV than a 1080p monitor, and the pixels on the S20+ are very small. So, the image takes up more/less of that space.
What about ppi?
PPI means Pixels Per Inch, often confused for DPI (Dots Per Inch). PPI defines the size of a pixel for printers. Our computers don’t measure in inches, but printers do. And as we discussed above, pixels can vary from device to device, so printers need a reliable unit of measurement.
Imagine you have to measure the width and height of your bedroom wall in feet. You’d at least need a measuring tape right? Imagine you had two measuring tapes: one that says there are 5 inches in a foot, and the other with 14 inches in a foot. Your measurement of the room would change depending on the tape you chose. The actual size of the room isn’t changing, just the way you measure it is. The only reason we don’t have this problem is because we’ve all decided there are 12 inches in a foot.
Changing the PPI does NOT change the amount of pixels in an image. It only changes the size of the pixels. So when you change the PPI of an image, the physical size of the image changes. This explains why when you print a 72 PPI image it is large and pixelated, but when you print the same image at 300 PPI it is physically smaller and less pixelated. The printer is receiving the same amount of building blocks for the image, but the 72 PPI image just has bigger blocks.
But PPI definitely effects file size and quality, right? No.
This is a very common misconception, but it’s understandable. We’ve all changed the PPI of a file, resampled it, and watched the file size change. The 300 PPI image is definitely bigger and better than the 72 PPI image. So what changed?
Resampling occurs when you add or remove pixels to an image, changing the physical size and data of the image.
Resampling changes the pixel dimensions of the image on screen. Once resampled, our 700 x 500 pixel image is no longer 700 x 500 pixels. At 300 PPI it is now 2917 x 2083, because instead of changing the size of the pixels, we added more of them.
You’ll notice this process also changed the file size, but not because we changed the PPI. The file size increased because we added more pixels. When you add more pixels, you create more data. If you uncheck “Resample”, you’ll notice it automatically changes pixels to inches. This is because the pixel count isn’t changing, but the size of the pixels is. So, the physical size in inches changes. However, the PPI has no impact on the file size. This is because no data is being added or deleted. Only the metadata that tells the printer the pixel size has changed. So you can make a web banner a 900 PPI, and as long as the pixel dimensions are correct, the file will be the same size and quality of a 72 PPI file.
What is the point then?
Well let’s say you’re submitting a 1:1 Instagram post. Instagram's requirements are 1080 x 1080 pixels. So we will build our file to 1080 x 1080. PPI in this case doesn’t really matter, but we'll do 72 just for consistency.
But what if I want a really high resolution post? Should I resample it and make it 300 PPI? No. That would make the file larger than 1080 x 1080. Instagram will size it back down to their recommended size, and that downsampling could actually harm your image. You should follow the recommended guidelines and cram as much cool imagery into that 1080 x 1080 pixel grid as you can.
What if I want to make an HD wallpaper for my 4K TV? Surely I should crank up the PPI for that right? Wrong. You should build your file to 3840 x 2160 pixels and leave PPI alone.
Well what about a web banner? It should be 72 PPI to keep the file size low, right? Nope. Unless you resample it, PPI does not effect the file size.
Unless you plan on printing it, the PPI does not matter.
So what should your PPI be if you do plan on printing? That is a topic for another blog.
Thanks for reading! ❤ Please share! ❤
