Free JPG Compressor - Reduce Image File Size Online

Compress JPG – Frequently Asked Questions

How JPG Compression Actually Works

Understanding what happens inside a JPG file makes it much easier to choose a quality setting with confidence instead of guessing.

JPG compression works by exploiting the limits of human vision. The encoder first separates brightness information from colour information, because our eyes are far more sensitive to changes in brightness than to changes in colour. It then divides the image into 8×8 pixel blocks and converts each block into a set of frequency components using a discrete cosine transform. Low frequencies describe the broad shapes in the block; high frequencies describe fine detail and sharp edges.

The quality slider controls how aggressively those frequency components are rounded off. At high quality, only the very finest detail is discarded and the result is visually indistinguishable from the original. As quality drops, more of the high-frequency information is thrown away, and eventually the 8×8 block structure becomes visible as blocky squares and as haloing around sharp edges. This is why text, logos and line art degrade so much faster than photographs — they are made almost entirely of the sharp edges that JPG discards first.

Why some images compress better than others

Two photos of identical dimensions can produce very different file sizes at the same quality setting. Smooth, gradual content compresses extremely well, while busy, detailed content does not:

• Compresses very well: clear skies, portraits with blurred backgrounds, fog, calm water, studio shots on plain backdrops
• Compresses poorly: foliage and grass, gravel and sand, crowds, textured fabric, forest scenes, anything with dense fine detail
• Compresses badly and looks worst: screenshots, text, charts, logos and line art — use PNG or WebP for these instead

Choosing a target for common situations

• Website hero and banner images: 1600–2000px wide at 80–85% quality, aiming for under 300KB
• Blog and article images: 1200px wide at 80% quality, typically 100–200KB
• Product photos: 1500px at 85–90% — buyers zoom in, so protect the detail
• Email attachments: 1600px at 75–80% keeps a set of photos under typical 20–25MB limits
• Job applications and online forms: follow the stated limit exactly, and resize before compressing
• Archival and print: 95–100%, or keep the untouched original

Compressing JPGs Without Uploading Them

Most online JPG compressors work by sending your file to a server, compressing it there and sending the result back. For holiday snaps that may not matter. For client photography, medical documents, ID scans, legal paperwork or unpublished work, it means handing a copy of the file to a third party and trusting their retention policy.

This compressor takes a different approach. The compression runs in JavaScript inside your own browser using the standard canvas encoder, so the image is read from your disk, re-encoded in memory and written back out without any network request. You can verify this yourself: load this page, disconnect from the internet, and the compressor will still work normally.

The practical trade-off is that compression uses your own device's processor, so a very large batch takes longer on an old phone than on a desktop. In exchange there are no file size caps, no daily limits, no queue and no account.

A workflow that avoids quality loss

1. Start from the original file, never from an already-compressed copy
2. Resize to the largest dimensions you actually need before touching the quality slider
3. Compress once, at the highest quality that meets your size target
4. Check the result at 100% zoom, paying attention to skies, skin tones and sharp edges — these show artifacts first
5. Keep the original archived so you can re-export later at different settings

If your source images are HEIC files from an iPhone, convert them with our HEIC to JPG converter first, then compress the results here. For PNG screenshots and graphics, the PNG to JPG converter or PNG to WebP converter will usually give you a smaller file than JPG compression alone. There is more background in our guides on image compression best practices and reducing image file size.