What "compressing a JPG" actually does
Compressing a JPG means rewriting the file so it stores the same picture using fewer bytes. A smart encoder looks at the image and discards the details your eye is least likely to notice — subtle colour shifts, high-frequency texture — while keeping the parts you actually see. The result is a much smaller file that, at sensible settings, looks identical to the original.
Crucially, compression and resolution are separate things. Compressing does not shrink the pixel dimensions of your image; it simplifies how those pixels are stored. You can compress a photo and it stays the same width and height — just lighter.
The quality slider, visualised
Every JPG has a quality level from 1 to 100. Lower means smaller and rougher; higher means larger and crisper. The sweet spot for photos is usually 70–80, where the file shrinks dramatically but the eye can't tell.
Where JPGs waste space
- Oversized dimensions — a 6000px photo shown at 1200px is storing 25× more pixels than it shows.
- Maxed-out quality — quality 100 is far larger than 80 with no visible gain.
- Embedded metadata — EXIF, GPS and camera thumbnails add weight (and leak privacy).
- Baseline encoding — progressive JPGs are often smaller and feel faster to load.
Start here
To understand the engine behind all this, read how JPG compression works. Ready for practical wins? Our 9 ways to reduce JPG file size is a step-by-step checklist.
FAQ
How much can I compress a JPG without it looking bad?
Most photos can be reduced by 50–80% at a quality setting of around 70–80 with no visible difference. Beyond that, blocky artefacts start to appear around edges and in flat areas like skies.
Does compressing a JPG lower its resolution?
Not by itself. Compression reduces file size by simplifying the stored data; the pixel dimensions stay the same unless you also resize the image.
Why should I compress JPGs for my website?
Smaller images load faster, improve Core Web Vitals and search rankings, and cut bandwidth costs — all while looking identical to visitors.