If you need to reduce the size of an image (or images) because they are too large to email, to use online, or for another reason, you may be thinking that you need an expensive app such as Photoshop, but you can easily resize images on a Mac using Preview, which comes free with macOS.
Here's how to resize images on Mac, including how to resize images without losing quality, resize images without stretching or distorting, how to resize images to same height and width and more.
First we'll look at the basics...
How to resize an image using Preview
Here's what you need to do to change the dimensions of an image using Preview.
- Locate the image (either using the Finder, or on your Desktop).
- Double click on the image - this should open the image in Preview, if it doesn’t, close it and right click to choose Open With and select Preview.
- Click on Tools.
- Choose Adjust Size (you can also open this menu by clicking on the square with arrows icon in the menu bar).
- Now, assuming want your image to look the same after - rather than stretched or squat, you need to make sure that Scale proportionally is selected. This means you can only change one dimension, length or width, but it means everything will be adjusted equally.
- Choose whether you want to resize according to pixels, percent, inches, cm, mm or points.
- Adjust either the Width number or the Height number - the other will automatically adjust proportionally.
That will change the dimensions of the image, perhaps you need the width to be the same as the width of a webpage you are adding the image to, for example. But what if it's the size of the image - in terms of MB - you want to change?
How to make an image smaller
If you are trying to make the size of the image smaller (perhaps so you can send it via email) there are a couple of ways you can do so.
If you open the Adjust Size menu as above you will notice that when you adjust the dimensions of the image you will see a preview of the size (in MB or KB) of the image before and after you have make your changes. (It may take a while to calculate the size.)
If you are trying to shrink the image to 500KB for example, you can keep adjusting until you are happy with the result here.
Another way to adjust the size of the image is via the quality settings in the Export menu.
- Click on File > Export.
- You will see various file formats to choose from. HEIC (the new file format), JPEG, JPEG-2000, OpenEXR, PDR, PNG or TIFF. Below the list is a measurement of file size. If you click through the different file options you can see the file size change. JPEG is smaller than PNG, TIFF is bigger than PNG, for example. You could just select JPEG to get a smaller file size.
- Even having selected JPEG you can go a step further to reduce the file size by sliding the Quality bar from Best to Least. Just adjust it until you get to a size that suits you. But remember that some of the quality will be lost.
- When you are happy click on Save.
How to adjust an image, but maintain 300dpi
Speaking of quality… You may also want to get the best result possible at 300dpi (usually recommended for print), or another resolution. In that case, here's what to do:
- Deselect Resample image (scale proportionally will remain selected but greyed out.)
- In the section where it says resolution pixels/inch change the number to 300 (or whatever dpi is requested).
- You will notice that the width and height measurements will adjust, click OK.
- This won’t adjust the actual size of the file, but it means that if the image was printed it would be smaller, but clearer, thanks to more compact pixels.
Resizing to particular dimensions
What if you want to resize both the height and width of your image to particular dimensions, such as 1,600 x 900?
In Preview the best way to do this is to use the crop tool. We’ll look at how to do that next.
How to crop an image in Preview
If you want to crop an image it’s not immediately obvious how to do so because there isn’t a traditional crop tool in the menu, but it’s actually very easy.
- Click on the image in Preview and drag so that the blue box surrounds the part of the image you want to crop.
- If you want the image to be a particular size, like 1,600 x 900 in the example above, you can drag the box to the dimensions you need. You will see a little counter in the bottom right corner that shows how many pixels are selected.
- Once you have the correct sized box un-click the mouse, and you’ll be able to move the box to prime position.
- Once you are happy click command + K and that will crop the image.
Resize without losing quality
You can’t add pixels to an image so the only way to make an image larger without losing quality (which is determined by the number of pixels in a square inch) is to crop the image following the instructions above.
However you could choose HEIC, which is a space saving file type (it stands for High Efficiency Image Format) that should create an image that is smaller than a JPEG, but with the same quality.
HEIC files are half the size of a JPEG but offer the same quality.
You can only open a HEIC file on a Mac with High Sierra or Mojave installed, or on iPhones and iPads running iOS 11 or iOS 12.
We have an article about opening HEIC files on our sister site Tech Advisor. The advice is to use a third party app, such as such as iMazing HEIC Converter or Apowersoft's HEIC to JPG converter, or an online converter such as HEIC to JPG.
How to batch resize
You can also batch resize if there are a number of images you want to adjust in one go. To do so:
- Select all the images you want to resize by clicking on each one in the Finder, or on your Desktop, while pressing the Command key.
- Right click and choose Open with Preview.
- All the images will open in Preview. Select all by pressing Command + A (or choose Edit > Select All).
- Now go to Tools > Adjust Size (as we did earlier).
- If you are just adjusting width or height don’t forget to select Scale Proportionally.
- You will see Multiple Values appear as the measurement in the columns (assuming all the images are different sizes). If you want them all to be 1000 pixels wide, type 1000 in the Width box.
- Click OK to adjust the size of the selected images.
- Now press Command + S to save (or File > Save) and all your images will be saved with the new dimensions.
How to change the file format
Speaking of saving what if you want to change the file format of the image. We touched on this earlier when we discussed reducing the file size as some file formats are smaller than others.
- To change the file format click on File in the menu.
- Click Export…
- From the Format dropdown many choose either HEIC (the new file format), JPEG, JPEG-2000, OpenEXR, PDR, PNG or TIFF.
- With some of these options you can also adjust quality (and therefore size). e.g. if you select JPEG you will see a quality slider and it will show you the file size so you can adjust it down to a size that suits you.
- When you are happy click on Save.