How to compress an image to 100 KB

"Please attach a photograph under 100 KB" is one of the most common instructions on the internet, and one of the most annoying to satisfy — because the obvious approach, lowering the JPEG quality, is not the lever that does most of the work.

Two levers, and the right order

Dimensions — the pixel width and height. This is the dominant factor: halving both dimensions cuts the pixel count to a quarter, and file size roughly follows.
Quality — how aggressively the JPEG encoder discards detail. Going from 0.9 to 0.6 typically saves 40–60%, and at print or profile sizes the loss is often invisible.

The correct order is dimensions first, quality second. A 4000×3000 photo dumped to quality 0.4 looks bad and is still 700 KB. The same photo scaled to 1200×900 at quality 0.75 is around 150 KB and looks clean.

Practical targets

20 KB — signature images and some government portals. Maximum width around 400 px, quality 0.5–0.6. Expect visible softness; there is no way around that at this size.
50 KB — ID photo uploads on several online application systems. Width around 600 px, quality 0.6–0.7.
100 KB — the common ceiling for profile pictures and document scans. Width 800–1200 px, quality 0.7–0.8. This is the sweet spot where the result still looks genuinely good.
200 KB — comfortable for most web and portal uploads. Width 1200–1600 px, quality 0.8.
500 KB — plenty for a photograph destined for a messaging app where the receiving client will re-compress anyway.
1 MB — full-page scans and images that need to survive editing or printing.

How to hit the target

Open the image compressor and drop the images in. Set the maximum width to the value from the table above and set the quality slider, then compress. The saving appears beside each file, and the statistics panel shows the total reduction across the batch. If the result is still over the limit, reduce the width by another 20% and try again — chasing the last few kilobytes through the quality slider alone is much less effective.

It runs entirely in your browser, which matters for two practical reasons: your images are never uploaded to a third-party server, and the tool keeps working if your connection is poor. There is no arbitrary file-size ceiling imposed by someone else's disk quota.

When an image will not compress

It is already compressed. Most photos that have passed through WhatsApp, a messaging app or a social platform have already been squeezed. Re-encoding them can even make the file slightly larger. Drop the resolution instead.
It is a PNG with flat colour. Screenshots and graphics with large areas of one colour are best served by PNG, not JPEG — converting them to JPEG blurs the text and can increase the size. For a PNG that must be smaller, reducing the dimensions is the reliable lever.
It contains noise or texture. Film grain, fabric texture and night-sky photographs are expensive to encode. There is a floor below which you cannot go without visible damage.
The target is genuinely impossible. A highly detailed A4 scan cannot be 20 KB. No tool can do it; anything that claims to will have destroyed the image.

Batch work

For a folder of images — a product listing, a set of documents, months of phone photographs — set one quality and one maximum width and run them all at once. Images from the same source usually share the same characteristics, so the settings that suit one will suit the batch. Use "Download all" to save them in sequence rather than one at a time.