How to Tell an Image's Size
And How to Determine a Graphic's Filesize
Many websites have image size requirements: 250x250, 100x100, no more than 1MB...but how can you tell? How do you know the size of a graphic, and how can you resize it if it's too big? And what IS "image size", anyway?
Above is a cropping of a drawing of my cat in Photoshop. (The "Info" palette is found under the "Window" menu: W and H are dimensions in pixels, Doc is its file size, that is, how much computer memory it uses to load and store the graphic.)
What's Meant by "Size"?
When talking about image size, people can mean:
X by Y: the dimensions of the graphic IN PIXELS. (e.g. 250x250).
K, M, G: the amount of memory the image uses, its filesize.
We don't usually talk about inches (display size); it varies depending on monitor or printer resolution.
How to Tell the Pixel Size (Dimensions) of a Graphic - And How to Resize It
When websites tell you an image has to be a certain size and lists two dimensions, such as 250x250, they mean the size in pixels, the number of dots that make up the picture.
So how do you tell the pixel size of a graphic?
- An image saved on your computer: Many graphics programs show you the pixel size on one edge of a graphic, and most graphics programs have an "Adjust Size" or "Image Size" tool under the Edit menu. This tool also tells you the pixel dimensions of the graphic right now. You can then change the size as needed. Be sure to save a NEW file with a different name, rather than overwrite the original.
EXAMPLE: "Image Size" under "Edit" menu in Photoshop
The Image Size tool gives the graphic's current dimensions. Hit cancel if you don't want to resize. Notice the dimensions are also given at lower left.
- An image found on the web: Say you've found a free web graphic that's legal to use, such as a Creative Commons graphic. What size is it?
Right-click the image and choose "Open in a new tab." Some web browsers will give the pixel dimensions of a graphic if you open it in its own window or tab! (Chrome doesn't. Boo.)
If that doesn't work, right-click the image and choose "Copy Image Location" or "Copy Image URL." There's an online tool that will do the job for us.
If the graphic is in a common file format -- jpg, png, or gif -- and the url ends with .jpg, .png, or .gif, then you can use this free online image editor (choose your graphic, then click the "Upload" button) or many other online image resizers to discover the image's pixel size, resize it, and save it on your computer. Then you can upload it.
The pixel dimensions (and file size) are given at the bottom left. Click the "resize" tool and drag left or right to shrink or scale.
Tip1: In that image editor, you can click the "Save" button and the folder icon to save the image to your default download location on your computer. But I prefer more control, so I right-click and choose "Save as..." so I can pick the filename.
Tip2: Be careful about scaling pictures up. The graphics program has to guesstimate pixels that aren't there, and if you blow up the image too much, it'll be fuzzy.
Flickr Image Size
Tip: Click the "Actions" menu and choose "View All Sizes" to get an image's dimensions on Flickr.com.
How to Tell the File Size of a Graphic - This means storage space
Many online websites allow you to upload images, but only have so much memory -- storage space -- so they set limits on file sizes. This has nothing to do with how the graphic looks, but how much room it uses on their server to store it.
How do you tell the file size of a graphic, and how can you adjust it?
Terms to know:
byte = basic unit of computer memory.
K = 1000 bytes (a kilobyte)
M, MB = 1000 K (a megabyte)
G, GB = 1000 MB (a gigabyte)
- When viewing a file in your computer's directory, right-click and choose "Properties" on Windows, or Command-I or "Get Info" on Mac, to see the file's size.
- In the online image editor I showed you above, it lists the image filesize at the bottom of the screen next to its pixel dimensions. Once you resize an image, click "Apply" and it should recalculate the size.
- In Photoshop, click the right-arrow at the bottom of the image pane to toggle between image dimensions and filesize:
Ways to Reduce an Image's Filesize - Also called "Optimizing" or "Compressing" a Graphic
Rule of thumb: Images with FEWER pixels, FEWER colors, or MORE image compression take up less memory, but may also look fuzzy, like an old YouTube video, because they're doing cheats to store as little data as possible to make the image.
On the plus side, smaller filesizes make webpages load faster, and speedy webpages get rated better by Google and tend to hold reader attention better! Generally, the amount of quality and pixels you need for webpages is far less than you need for digital printing, so you can get away with a lot of file compression for webpages.
There are many different ways to shrink an image's filesize, all of which have tradeoffs.
WARNING: If you save a compressed, optimized or shrunk version of a photo, rename it; don't overwrite the original unless you're absolutely sure you'll never want it! Digital cameras snap very large-file size, high-quality jpgs that take lots of memory. That's detail you don't want to lose, for two reasons! First, digital printing services and print-on-demand websites use the full-quality, full-sized images and will look terrible with the smaller-filesize, fewer-pixels versions of the images blown up to supersize. Second, in five years, what seems like a lot of memory now will be nothing for the computers of tomorrow.
Here's some ways to save memory and reduce filesize.
- Images with fewer pixels take up less memory, so if you can shrink the pixel dimensions or crop an image, it will also use less memory to store it. (A Squidoo column width is a maximum of 590 pixels wide, so you never need more than that here.)
- PNG format preserves all the colors and pixels, but it also is more of a memory hog than jpg or gif. Try saving a png as a jpg or gif: that will often do it!
- Many graphics programs let you fine-tune the compression of a jpg more or less, letting you balance the trade-off between image quality and filesize. Very High and Maximum quality jpgs are NOT very compressed, so they have larger filesizes and take more room. High and Low quality jpgs are more compressed, so their filesizes are smaller, but you may begin to see quality problems with large-pixel-dimension jpgs or bigscreen monitors.
- PNG-8 and gif formats solve the filesize issue differently: they limit the image to 256 colors. For screencaps of computer dialog boxes and simple diagrams and charts with only a few colors, this can sometimes be the best way to get a small filesize.
- Programs like Photoshop let you fine-tune how much an image is optimized or compressed. Choose "Save for Web" under the file menu, and you'll be presented with options to try out gif, png-8 (fewer colors), regular png or jpg at different compression levels (higher quality = less file size compression, lower jpg quality = more memory savings). Some graphics programs have an "optimize" or "optimization" tool instead.
© 2012 Ellen Brundige
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