# Is a 100x100 px image with 100 ppi equal to a 71x71 px with 50ppi?

This should be obvious but the content out there on the web is confusing.

I'm trying to understand the role `ppi` plays when saving an image.

So if I have an image with rows of 1000 pixels, is it the same resolution if there's `100 ppi` as when there's `300 ppi`? Or is the `300 ppi` higher resolution, since the pixels are more packed together?

Or on the other hand, are there more total pixels when there are `300 ppi` or is that like asking if a pound of feathers weighs more than a pound of gold?

I'd really appreciate some help clarifying.

## 1) On a already generated image, like a photo from a camera

The ppi info on a image file is just a tiny little number inside the file. It is only meaningfull when you are thinking on printing the file. It answers just a question. "Of thoose pixels you already have in your photo, how many you need on each phisical inch?"

This produces diferent size in the printed result.

"I want you to use 300 ppi on each inch" or "No I want a bigger print, just use now 150 on each inch" This will make twice bigger print.

In both cases your original photo is exaclty the same.

## 2) Before making a file on a raster image program

If you create a new file on a program like Photoshop and you define it terms of pixels, you just have the previus case.

But if you define a new file in terms of a phisical dimension, for example a 8x10" image. The number you use on ppi affects the real file size.

If you define this 8x10" with 300 ppi you have a file (8x300)x(10x300) = 2400x3000 px.

But if you use 150 ppi you just have a (8x150)x(10x150) = 1200x1500px.

## 3) Wich aproach should you use?

In general terms I recomend defining a file in px, that way you can work on a file that you can handle, for example a digital paint. Some people thinks that if they just use a higher ppi they will get a higher quality, but sometimes they sacrifice the performance making a exagerated file, for example on a billboard.

In this post I posted some graphs regarding this issue:

How to create a very big photo (like for a billboard)

or is that like asking if a pound of feathers weighs more than a pound of gold

Yes. A pixel is a pixel. The PPI setting only applies to the device it's output on (be it a screen or paper).

a thousand pixels is a thousand pixels. Whether you print it at 100ppi or 500ppi, the same amount of data is there.

Is a 100x100 px image with 100 ppi equal to a 71x71 px with 50ppi?

In terms of pixel data, one has 100x100 pixels, one has 71x71 pixels.

In terms of print sizes:

• 100x100px image printed at 100ppi = 1" square image
• 71x71px image printed at 50ppi = 1.42" square image

Given the 100x100 image a) has more pixels and b) is printed smaller, it will look better in most cases.

300ppi is more dense than 100ppi. This is why the file size will increase. Though depending on what you're doing it's usually best to start with your ppi (or dpi for print) set when creating you document.

• In a file of a given pixel size, the only thing that is different for e.g. 100ppi and 300ppi is the ppi value. The file sizes might (actually, should) be equal to the bit. Feb 9 '15 at 20:19
• I must've been thinking about Photoshop and how it automatically scales by default but you're correct in that you are able to change PPI and not have it affect the file size however what's the point then? Feb 9 '15 at 20:24
• The point is changing the ppi value - for example because you know that for your 8 megapixel photo to be sized correctly in a document to be created for print, it should have a ppi value of exactly 300x300 ppi. Otherwise the person who asked for that image will be confused and unable to process your image despite correct pixel dimensions. Feb 9 '15 at 20:34
• What I meant is, if you change the PPI the image is now more dense. If you were to print it, it will now be smaller. So typically when people change PPI they also change the dimension to compensate for the density increase. Usually you don't change the PPI without changing the file dimension. Then again it does depend on what you're doing. Feb 9 '15 at 20:44
• Changing the PPI does change the print size. It doesn't change the amount of pixel data (unless, as you state, you choose to have PhotoShop do that for you).
– DA01
Feb 9 '15 at 21:37