Know Your Photoshop File Sizes

How can one Photoshop image grow and shrink dramatically during production, even without compression? It’s a question that comes up regularly. And the answer isn’t just academic, because understanding why could save you a lot of storage space on your hard drive or in the cloud.

What’s the Document Size in the Image Size Dialog Box?

The first place you might see a document size in Photoshop is at the top of the Image > Image Size dialog box. This number tells you how much disk space the document would use it if it was saved as a single layer. If the Resample option is on, you can watch the Image Size number change as you adjust the Width, Height, or Resolution. (If Resample is off, Image Size doesn’t change because the number of pixels remains constant no matter how you change the numbers.)

The 20.6 megabytes shown above is based on the document using 8 bits per channel (bpc).

Why Does a Photoshop Document Show Two Sizes?

If you configure the Status bar or the Info panel to display the document size, you’ll notice two sizes listed for the same document, separated by a slash. The first number is the same as in Image Size: the document size if saved as a single layer. The second number accounts for layers, channels, and masks, so for layered Photoshop documents the second size would be both larger and more representative of the size on disk.

Why Are Document Sizes in Photoshop Different than on the Desktop?

Having seen the various document sizes reported in Photoshop, you might wonder why those sizes don’t match what your Mac or Windows desktop tells you. For example, the document we’ve looked at so far is a basic 8×10-inch photo at 300 pixels per inch (ppi). But even that same photo can take up very different amounts of disk space depending on the file format and file compression you chose, as in the figure below. The first two examples, Photoshop format and TIFF uncompressed, are close to the document size reported inside Photoshop. The rest are smaller. However, while ZIP compressed TIFF may be the smallest, it can take much longer to save changes and may be compatible with fewer applications.

If you’re handling hundreds or thousands of images for a book or a web site, file size differences can add up quickly, not just affecting how much space you need to store your images but also how much time it takes to upload or download them from project servers, or to back them up.

Images can be compressed further if they use a lossy compression method such as JPEG. Lossy compression doesn’t preserve all of the original quality of the image, which is why in professional workflows JPEG is used only for output where small file size is much more important than perfect quality, such as web publishing.

Remember that this works in reverse, too. The 0.3 MB compressed JPEG file will expand to occupy 21.8 MB of RAM when Photoshop decompresses it for editing. Also, the File > Save for Web command creates smaller files than File > Save As because in the interest of conserving Internet bandwidth Save for Web doesn’t include extras like a preview thumbnail image and various types of metadata (you can choose to preserve some metadata).

Why Are Document Sizes Different Even Without Compression?

If you try comparing file size differences yourself, you may see variations that can’t be explained by the factors seen so far. There are more variables that can affect the size of a basic Photoshop image, including but not limited to the number of:

  • Channels (grayscale is one channel, RGB is 3 channels, and CMYK is 4 channels)
  • Layers
  • Layer masks
  • Embedded Smart Objects

The exact amount that each feature adds to the document size depends on the content in them. You can see examples of document size variations in the figure below.

Other factors affecting file size include embedding a color profile, and turning on the Maximize Compatibility option that may appear when you save (leave it on to see previews of Photoshop files in other applications.)

Why Do Raw Photo Files Get Bigger in Photoshop?

If you’ve ever wondered why a raw file seems to get a lot bigger after it’s opened and saved in Photoshop, there are some clues in the previous figure but there’s also a key piece of information to understand: Camera raw files are typically one gray channel full of numbers, because the raw sensor data hasn’t yet been interpreted into pixels with RGB values. When you open a raw file from Adobe Camera Raw or Lightroom into Photoshop, what was a one-channel grayscale file becomes a three-channel RGB file. Since we know that the number of channels affects file size, naturally a three-channel RGB file will be larger than a one-channel raw file.

Another factor is bit depth. Many digital cameras capture 12 to 14 bits per channel into that raw file. You can set Camera Raw or Lightroom to open that raw file in Photoshop at 8 bpc or 16 bpc, and that too will affect the document size of the resulting Photoshop file. If you convert a 12 bpc raw file to a 16 bpc RGB Photoshop document, you should expect a large increase in file size. You can then expect a large decrease in file size when you save a final copy of the document with much smaller pixel dimensions, as a single flattened layer, in the lossy JPEG format, for posting on your web site. The next figure shows the possible variation in file size for an image that started as a 5184 × 3456 pixel raw file.

The document size of an image is always a compromise between
quality, flexibility, speed of opening and saving, and compressing to save disk space or Internet bandwidth. Knowing the factors that affect the size of a document helps you understand why an image’s size on disk rises or falls.

 

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Posted on: May 13, 2014

Conrad Chavez

Conrad Chavez writes books, articles, and training materials with a focus on digital imaging using Adobe Creative Cloud tools. He is the author of books such as Real World Adobe Photoshop CS5 for Photographers, and is also a fine art photographer. You can find out more about Conrad at his website, conradchavez.com.

11 Comments on Know Your Photoshop File Sizes

  1. Campbell David

    May 14, 2014 at 5:17 pm

    Conrad – thank you so much; this article is truly a revelation. I have been trying to save a set of images for a stock photo site. The requirements are for file sizes of at least 4 MB and every time I thought I had it right, the size on the desktop told me otherwise. I will revisit this abandoned project and adjust my images by taking into account some of the factors I had not known about. So helpful!

  2. ConradChavez99

    May 14, 2014 at 11:42 pm

    Hi David, that’s certainly a practical side to this that could be expanded on. When someone asks for photos at a particular file size, that’s a very ambiguous request because of all the different ways that file size could be reached. Without more specific requirements there’s the potential for misunderstandings.

    In the case of a stock photo agency, when they ask for at least 4MB, what you really need to know is, 4MB in what file format, color mode, bit depth, and pixel width and height? For example, they might mean at least 4MB as a flattened, uncompressed TIFF in RGB color with 8 bits per channel and enough pixels wide and tall to print a full magazine page, but it’s those details that you should look into and confirm.

  3. Sandhya Hariharan

    May 20, 2015 at 6:13 am

    Hi Conrad, your article has definitely helped in tiny bits.
    Still I am a little unclear with file size.
    Some stock agencies are asking for 34MB-50MB uncompressedfile size. Raw image size from my cam is 21-25Mb at the max with my 20.2 MP Canon 6D camera. So how do I still achieve that size?

  4. Conrad Chavez

    May 21, 2015 at 1:15 pm

    Hi Sandhya, the answer is similar to the previous one, so let’s look more closely at what the stock agency is asking for. Your camera shoots 21-25MB raw, but because stock agencies generally don’t take raw files, we know you have to change the file format (which you will do after editing anyway). If the stock agency wants files in the range of 34-50MB uncompressed, first you need to know what file format they are asking for. Is it TIFF? If so, try saving as TIFF with the Uncompressed option selected.

    I downloaded a sample Canon 6D file from the web. In JPEG format it’s 5.2MB. Without making any changes, if I save it as TIFF Uncompressed, it’s 59.9MB. If I save it as Photoshop (PSD), it’s 45.5MB. So it is definitely possible to get your Canon 6D files into the right file size range, but first you must clarify which file format the stock agency needs so that you can save the image in the correct format for them.

  5. I think this is one of the best articles we’ve published this year. Conrad, you’ve cleared up a LOT of confusion for me (and others). Thank you!

  6. Hi Conrad, I have a file size issue that your article doesn’t address or maybe I just missed it. I am rendering out png sequences from After Effects. When Photoshop loads up the images they’re reading much larger than what the file size reads in Windows and After Effects. For example, 1 png at 158 kb is reading in Photoshop at 3.78M/4.00M. The image is rgb 8bit/channel with no compression.

    Any ideas?

    Thanks,
    Justin

  7. Conrad Chavez

    May 28, 2015 at 3:25 pm

    Hi Justin, I’m going to guess that difference in displayed file sizes is because of PNG compression: The PNG is 158KB compressed, and when opened in Photoshop it expands to around 4MB uncompressed. You did say “with no compression,” but I am under the impression that most implementations of PNG are compressed (though I’m not 100% positive). It’s lossless compression, so no image quality is lost.

    What are the pixel dimensions of one frame of the PNG sequence? If the math works out to a lot more than 158KB, then the frame is compressed. The math is ((pixels tall * pixels wide * bit depth)/8).

    If the content in the image is synthetic (not photographic), especially involving solid colors, it’s plausible that 4MB uncompressed could turn into 158KB PNG compressed.

    In After Effects, I looked at the Output Settings and while there is a Compression option for PNG that has settings of “None” and “Interlaced”, it looks to me like those control whether interlacing is off or on, not whether compression is off or on.

  8. Conrad Chavez

    May 28, 2015 at 3:31 pm

    Hi Anne-Marie, you’re welcome! And thank you as well, since I’m inspired in part by how well you and David B. have demystified so much about InDesign!

  9. Thank you for this! I recently purchased an adphoto online. When I received it, it was only 300kb in size! I thought I got ripped off! Then I found out it is ACTUALLY 3MB when opened with Photoshop (so I wasn’t ripped off after all) :)

  10. Why does a 948 Mb tiff (Flat) when placed into INDD at 100%, does the size grows to 2.4 gig?

    • Conrad Chavez

      July 22, 2015 at 6:53 pm

      Hi Amy, which file is growing to 2.4GB, the TIFF file or the INDD file? Placing a TIFF file in InDesign should not change the size of the TIFF file, so I’ll guess that you’re talking about the INDD file.

      What was the size of the INDD document before the TIFF was placed into it? When a graphic is placed in InDesign, InDesign generates a preview and stores it inside the document, and that can increase the INDD file size. (To see the file size of the same InDesign document with all the previews and other extra data removed, save an IDML version of it.)

      The other thing that can increase INDD file size is if you embedded the link (choosing “Embed Link” in the Links panel menu). That would store the entire image in the InDesign document, instead of linking to it externally.

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