Image resizer

Nothing is uploaded

The image is decoded, redrawn at the size you ask for, and handed back as a download, all by JavaScript running in your own tab. There is no upload step, because there is no code on this page that sends anything anywhere. Load it, turn off your network, and resize something: it behaves identically.

Why the ratio is locked to start with

With both fields free it is very easy to type a width, type a height, and end up with a picture that is subtly squashed. So editing one field moves the other, which keeps the proportions exactly. Unlocking is a deliberate act, and the panel warns when the numbers you have entered would change the shape.

If you need an image to fill a specific box of a different shape, resizing is the wrong tool, what you want is cropping, which removes the parts that do not fit rather than distorting the ones that do.

Resizing saves far more than compressing

File size follows the pixel count, and the pixel count falls with the square of the scale. Halving both dimensions leaves a quarter of the pixels, and usually something close to a quarter of the bytes.

That dwarfs what a quality setting can do. If you want a file as small as possible, resize first and compress second, in that order, the compressor on this site handles the second half.

Enlarging

Asking for more pixels than the image has does not recover detail; nothing was recorded to recover. The browser interpolates, inventing intermediate pixels by averaging their neighbours, so the result is bigger and softer.

It is offered anyway, because there are real reasons to need it, a form that insists on a minimum size, a print dimension to match, and refusing would be this tool having an opinion about a legitimate request. It simply says so when it happens.

The limit, and why it is not an error message

Browsers cap the size of the drawing surface this work uses, and the cap is lower than most people expect, some devices refuse anything past about 16 million pixels. The unpleasant part is that going past it does not raise an error: the canvas comes back blank, and a tool that did not check would hand you a white rectangle and call it a resized photograph.

So an oversized request is brought inside the limit with the proportions kept, and the panel says it has been.

Useful sizes

• Full-width web image, 1,920 pixels wide is plenty for almost any screen.
• Article illustration, 800 to 1,200 wide.
• Thumbnail or avatar, 300 to 400 square.
• Email attachment, 1,600 on the long side keeps it readable and the file modest.

Serving a 4,000-pixel photograph into a 600-pixel slot costs whoever loads it most of the download for detail their screen cannot show.