How to compress images without uploading them
Compression is a trade, not a free lunch. Here is exactly what is being traded, and how to read the numbers so you can decide for yourself.
Last updated: 2026-10-11
The short answer
Pick the output format first, then pick a quality level by looking at the size readout rather than guessing. If you need a specific file size, use a target-size setting instead of a quality slider — it searches for the highest quality that still fits. If your file is a PNG, no quality setting will help you, and the next section explains why.
What compression actually does
"Compressing" a photo means decoding it to raw pixels and re-encoding it with fewer bits per pixel. For JPEG and WebP that means quantising colour detail: the encoder throws away information your eye is unlikely to miss and keeps what it will notice. There is no trick and no clever optimisation involved — the file gets smaller because it stores less.
In a browser this runs on the same codecs your phone and camera use, because the browser calls the platform's encoder. That is why results here match what you get from a desktop image editor at equivalent settings.
How quality maps to size
Quality is a 0–1 number, and its effect on file size is not linear. In practice, for a typical photograph:
| Quality | Typical effect | Good for |
|---|---|---|
| 0.90–1.00 | Large files, near-original detail | Archival copies, further editing |
| 0.80–0.89 | Large savings with few visible artefacts | Photos that will be viewed closely |
| 0.65–0.79 | Substantially smaller, mild softening | Web pages, email, social posts |
| 0.40–0.64 | Clearly visible artefacts in gradients and fine texture | Thumbnails and previews |
| below 0.40 | Blocky, banded, often with colour shifts | Rarely worth it |
The gap between 0.85 and 0.95 is often a doubling of file size for a difference most people cannot see on a screen. The gap between 0.75 and 0.60 is usually still acceptable and much smaller. Move the slider, read the size readout, and stop when the saving flattens out.
Why PNG ignores the quality setting
PNG is lossless: every pixel is stored exactly, and the format has no notion of "how much detail to discard". A quality value passed to a PNG encoder is simply ignored by the browser.
This matters because it is easy to build a tool that pretends otherwise — the slider moves, the status says success, and the file is exactly the same size as before. The compressor here says so plainly: when you choose PNG, the quality control is disabled and the item is marked as lossless, not quietly "compressed".
If you need a PNG to shrink, you have three real options: reduce its pixel dimensions, convert it to WebP (which supports transparency and lossy compression), or accept it. That third option is sometimes correct — a small logo or icon is often fine as a PNG.
Hitting a target file size
A quality slider makes you guess. A target size reverses the problem: you say "under 200 KB" and the tool searches for the highest quality that still fits, by encoding, measuring and adjusting. The result reports the quality it settled on, so the number is never a mystery.
Two honest caveats. First, the search can fail: if even the lowest quality is still too large, the file cannot be made to fit without resizing, and the tool says so instead of returning a file that misses the target. Second, this only works for lossy formats. PNG cannot be sized this way.
Lossy means lossy
Every time you save a JPEG, you lose a little more. Compressing an already-compressed photo at a low quality compounds the damage rather than starting fresh, because the encoder is preserving artefacts from the previous round along with the actual image. If you plan to edit a photo later, keep the original and compress a copy.
A recipe that works
- Convert PNG photos to WebP if you want transparency and small files.
- Set quality to 0.80 and process the batch.
- Look at the before/after sizes. If savings are under about 20%, drop to 0.70 and compare.
- If the largest images are still too big, cap the longest edge (1920 px is a common ceiling for full-width web images) and re-run.
- Download the batch as a ZIP and spot-check two or three images at full size.
Open the Bulk Image Compressor and try it on your own files. Nothing is uploaded; you can verify that by disconnecting from the network before you start.
Keep reading
- Resizing images for the web: sizes, ratios and when to bother — How to choose real pixel dimensions, why the DPI field in your editor does not matter online, and the order in which to crop and resize.
- JPEG, PNG, WebP and AVIF: choosing the right format — When each format is the right answer, what transparency costs you, and how EXIF orientation quietly affects every rotated photo.