How to compress images without losing quality

The phrase "without losing quality" is technically impossible: every compression step throws something away. What is very possible is losing quality nobody can see, which is what everyone actually means and what modern encoders are extremely good at.

The gap between a 4 MB photo and a 300 KB version of the same photo is almost entirely invisible on a screen. Getting there reliably is a matter of doing three things in the right order and knowing which setting to stop at.

Where the weight comes from

An image’s file size is driven by three things, and most people only ever adjust the third.

That first row is why so much compression advice disappoints. Dragging a quality slider on a 6000-pixel photo is polishing the wrong thing. Scale it to 1200 pixels first and the file is already a tenth of the size before compression touches it.

The four-step method

Step 1: Decide the display size

Measure how wide the image will actually appear. On most websites, the content column is 700–900 pixels. On a full-width banner it might be 1920. Whatever the number, double it for high-density screens and stop there. A 1600 px image in an 800 px slot is already as sharp as any display can show.

Step 2: Resize to that number

Scale down, never up. Use your operating system’s built-in resizer, Squoosh, or the export dialog of whatever made the image. Keep the aspect ratio locked so nothing stretches. This one step usually saves more than any quality setting.

Step 3: Export at quality 80

Set the quality slider to 80 and export. In Squoosh this is the number in the right-hand panel, and in Photoshop’s Export As it is the Quality field. Phone apps often use a high, medium or low picker instead. Watch the projected file size update as you move it.

Step 4: Inspect before you commit

View the result at 100% zoom and compare it against the original side by side. Look at three places specifically: large flat areas for blocking, sharp edges for halos, and any text for smearing. If it looks clean, you are finished. If not, raise the quality by five and re-export. Do not edit the compressed file.

Never compress over the top of your originals. A batch tool pointed at your photo library with “overwrite” ticked will happily destroy a decade of images in thirty seconds, and there is no undo. Always output to a new folder, check a few files, and only then delete anything.

Tools worth using

Squoosh (squoosh.app) is the best single-file tool there is. It runs entirely in your browser, nothing uploads, and the split-screen preview with live file sizes makes the quality decision obvious rather than a guess. It handles JPG, PNG, WebP and AVIF.

Your operating system handles batches without any download. Windows PowerToys adds an Image Resizer to the right-click menu that processes a whole folder. macOS Preview can open dozens of files, select all, and resize them in one action; the Finder’s Quick Actions do format conversion the same way.

Command-line tools are worth ten minutes of setup if you do this weekly. ImageMagick and cwebp will process thousands of files with one line and exactly repeatable settings. They are also the only realistic option once a folder passes a few hundred images.

Whichever you use, the settings matter more than the brand. A famous paid tool at quality 95 produces a bigger, worse-value file than a free one at quality 80.

What quality setting to use for what

Two mistakes that cost the most

Compressing something that was already compressed. A JPG downloaded from a website has already been through an encoder. Running it through another one re-analyses the artefacts as if they were real detail, so you pay in quality twice. If you can possibly get the original, get the original.

Leaving metadata in by accident. Camera files carry EXIF data: model, settings, often GPS coordinates. On a 40 KB thumbnail, 80 KB of metadata is the majority of the file, and the location data is a privacy problem on top. Most export tools have a “strip metadata” checkbox; tick it unless you specifically need copyright fields preserved.

In short

Resize, then compress, then check at full size. Do those three in that order and a 4 MB photo becomes a 180 KB file that nobody can tell apart from the original.

If you need to hit a specific number rather than just “smaller”, the technique is slightly different. See how to reduce an image to an exact file size. For the resizing half of the job, resizing without distortion covers the arithmetic, and batch resizing for free handles the case where you have four hundred of them.