Compress Images Online

Compress images locally — free, instant, and private

Image compression is a CPU-bound task, and server-based compressors charge for the cycles or mine your uploads. Modern browsers expose the same encoding routines desktop apps use, and your phone or laptop has plenty of idle CPU. This tool decodes each image to raw pixels, re-encodes it at your chosen quality inside a Web Worker (so the page never freezes), and hands you the result — the photo bytes never touch a server. A side effect worth knowing: re-encoding from raw pixels strips EXIF metadata, including GPS coordinates, which is usually what you want before posting a photo publicly.

Lossy vs. lossless, in one minute

Lossy formats (JPEG, lossy WebP) shrink files by discarding detail the eye barely registers: fine high-frequency texture is quantized away and colour information is stored at lower resolution than brightness, because human vision is sharper at luminance than at colour. Lossless formats (PNG, lossless WebP) only repack the existing data more efficiently, so every pixel survives exactly — and the savings are correspondingly modest. That asymmetry explains the most common surprise here: a photograph can drop 70% as a JPEG, while an already-optimised PNG barely moves.

What the quality slider does, with real numbers

The slider controls how aggressively the encoder quantizes detail. The relationship to file size is steep at the top and flat at the bottom: going from quality 1.0 to 0.8 typically halves a photo or better, while going from 0.5 to 0.3 saves little and looks visibly worse. As a concrete example, a 4 MB 12-megapixel JPEG straight off a phone usually lands around 1–1.5 MB at quality 0.8 with no visible difference at normal viewing size, and around 600–800 KB at 0.6, where flat gradients like skies start to show faint banding. Screenshots and UI graphics behave differently — their hard edges generate exactly the high-frequency detail lossy encoders sacrifice, so keep those as PNG or lossless WebP.

Resize first, then compress

Pixel count dominates file size. A 4,000-pixel-wide photo displayed in an 800-pixel slot carries 25× more pixels than the screen will ever show; capping the width often saves more than any quality setting. The max-width option here does both in one pass — downscale, then encode — which is the right order, since scaling after lossy encoding bakes the artifacts in.

Choosing an output format

• JPEG — best for photographs when compatibility matters everywhere, including old email clients. Quality 0.7–0.85 is the sweet spot for the web.
• WebP — roughly 25–35% smaller than JPEG at equivalent quality, handles gradients better, and supports transparency. Supported by every modern browser.
• PNG— lossless. Use for screenshots, line art, and anything needing pixel-perfect fidelity or transparency. Don't expect dramatic size reductions.

When not to compress

Keep originals untouched if you'll edit them again — every lossy re-encode compounds the loss — and never replace an archival copy with a compressed one. Skip compression for images headed to print, where the artifacts invisible on screen can show at 300 DPI. And if a file is already small for its job, leave it alone: re-encoding a well-optimised 80 KB thumbnail buys nothing.

Related tools

• Image Resizer — change pixel dimensions precisely before compressing.
• Image Converter — switch between JPEG, PNG, and WebP without changing size.
• Bulk Image Resizer — process a whole folder of photos in one go.
• Images to PDF — bundle compressed images into a single document.