How to Batch Resize and Convert Images for Free With ImageMagick
Resize dozens of photos, convert formats and reduce file size using free ImageMagick commands without risking original pictures.
Before you begin
Resizing one photo is easy; resizing two hundred product photographs or assignment images one by one wastes time. ImageMagick is free, open-source software with command-line tools for resizing, converting and processing images in batches. It works especially well when you need consistent dimensions for a website or online catalog. The most important skill is protecting the originals because one commonly used command can overwrite them.
1Install ImageMagick from its official instructions
Start at imagemagick.org, choose the instructions appropriate to Windows, macOS or Linux, and verify the command-line tool works. Modern examples use the 'magick' command; older installations and operating systems may have different entry points. Avoid unsolicited download links for 'premium unlock' editions, since ordinary resize and conversion features are part of the free software. Keep a few sample images for testing.
2Decide the output goal before choosing commands
For a website, you might want photographs no larger than 1200 pixels wide, a consistent format and reasonable download sizes. For printing, preserving sufficient pixel dimensions and aspect ratio is usually more important than choosing the smallest file. Make a separate output folder and write down target dimensions, image format and whether transparency needs to survive. Converting a transparent PNG to JPEG can replace transparency with a background.
3Resize a single photo first
From a terminal in your working folder, try 'magick input.jpg -resize 1200x1200 output.jpg'. This creates an output file fitted within the specified dimensions while preserving aspect ratio, rather than stretching it. Inspect the output in a normal image viewer. Images already below the requested dimensions might be enlarged depending on the command; ImageMagick offers geometry modifiers to control that. Test the command on ordinary copies before processing an entire folder.
4Use mogrify carefully for a batch
The official documentation shows 'magick mogrify -resize 256x256 *.jpg' as a batch-resize example, but it also warns that mogrify overwrites the original files unless options change the destination or format. Run it only inside a folder that holds copies, or configure a separate destination using documented options such as '-path'. For format conversion, the examples show 'magick mogrify -format jpg *.png', producing converted JPEG files while leaving the source PNGs. File-format differences still need visual inspection.
5Balance quality, size and readability
Resize reduces pixel dimensions; compression quality changes encoded file size and image artifacts. For photographic JPEG or WebP output, test one moderate quality level and compare the final image with the original at normal viewing size. Avoid excessively compressing text-heavy screenshots because small letters become unreadable. Retain enough detail for the page's purpose and include descriptive alt text when placing images on a website. A smaller image is not useful if readers cannot understand it.
6Finish with a safe quality-control routine
Compare the number of output files with source files, inspect randomly selected images, and check that filenames are still associated with the correct products or documents. Confirm portrait photos remain portrait and transparent assets retain transparency where needed. Keep original metadata or remove it deliberately according to your privacy requirements, not by accident. Automate only after a successful small test batch, then document the command for repeatable future work.
Frequently asked questions
Do I need a paid image editor?
No. ImageMagick can perform many common resizing and conversion tasks without a paid subscription.
Why did my originals change after running mogrify?
By default mogrify edits matching source files in place. Restore from backups or use a separate working folder for future commands.
Official sources and further reading
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