Compress an Image to an Exact Size

Why "compress to 20KB" is often a request that cannot be met

Every tool that offers to compress an image to an exact size will produce a file at that size. Not all of them will tell you the picture is ruined. That is the difference this page is built around.

File size is not a dial you turn independently of everything else. It is the outcome of three things: how many pixels you have, how much detail is in them, and how hard you compress. Fix the first two and the third has a floor. Push past the floor and JPEG starts throwing away information you can see going: edges smear, flat areas break into blocks, and text develops a halo.

What the quality number here actually means

Most compressors report the quality setting they used, which tells you what they asked for rather than what happened. This page reports PSNR, measured by decoding the compressed result and comparing it pixel by pixel against your original. It is a real measurement of the difference between two images, in decibels, on a logarithmic scale.

PSNR is not a perfect model of human vision, and it is worth saying so. It slightly over-punishes noise and under-punishes blur. What it is very good at is telling you the direction and rough scale of the damage, which is exactly what you need in order to decide whether a target is sensible.

The fix when a target really is too small

If the target cannot be met at a decent quality, the answer is almost never "compress harder". It is fewer pixels. Halving both dimensions cuts the pixel count to a quarter, and that buys far more file size than turning the quality dial down ever will, at much lower visible cost.

This is why the tool offers to find the largest dimensions that hit your target while keeping quality in the good range, rather than just squeezing the resolution you started with. A sharp 640 by 360 image at 20KB looks better than a wrecked 1280 by 720 image at 20KB, every time.

Practical targets worth knowing

• YouTube thumbnails: 2MB. Generously large. If you are anywhere near it, something is wrong, most likely a PNG that should be a JPG. A good 1280 by 720 thumbnail lands around 150 to 400KB.
• Forms and uploads asking for 100KB or 50KB. Usually document or ID photos, where the resolution required is far lower than a phone camera produces. Reduce dimensions first.
• Email attachments. The constraint is the total, not the individual image. Several 200KB images beat one 2MB image.
• Web pages. Aim for the smallest size that still looks right at the size it is displayed. Serving a 2000px-wide image into a 600px slot wastes most of the bytes.

If you only want the file smaller without a specific number in mind, the plain image compressor is the simpler tool. To change dimensions directly, use resize and convert.