Resize an image

Two things get called "too big" and they need opposite fixes. Too many kilobytes for an upload limit is a compression problem — the picture stays the same size on screen and is stored more tightly. Too many pixels is a dimensions problem: the image is 4000 across and the box it goes in is 400.

Resizing fixes the second and, as a side effect, fixes the first far more effectively than compression does. Halving the width quarters the pixel count, and the file follows. If your goal is simply a smaller file, resizing first and compressing afterwards will beat compressing alone at any quality setting.

Ask for 4000 pixels from a 900-pixel photograph and every tool that agrees is selling you an idea. The detail was never captured; enlarging produces a larger file containing the same information, blurred. Some tools now dress this up with machine learning, which invents plausible detail rather than recovering real detail — occasionally useful, never faithful, and not what somebody resizing a photo for a form is asking for.

So a request larger than the source is capped at the source, and the page tells you that is what happened rather than handing back an upscaled file and letting you discover it.

The height follows the width, always. Distorting an image by setting both edges independently is something people do by accident far more often than on purpose, and a tool that allows it without comment produces squashed faces that nobody notices until later.

If you genuinely need a fixed rectangle of a different shape — a square avatar from a landscape photo — that is a crop rather than a resize, and it needs a rectangle dragged over a preview to be usable at all. Typing four numbers to choose a crop is worse than not offering it, so it is not here.

There is no height box on this page, and that is the decision rather than an omission. Give a width in pixels, or pick 75, 50 or 25 per cent, and the height is calculated from the picture's own proportions.

A tool that takes both numbers has to answer what happens when they disagree, and every answer is a bad one: stretch the picture, crop it, or quietly meet one number and miss the other. Removing the second box removes the question. If you genuinely need a fixed rectangle, that is a crop and it belongs in something with a preview.

Reducing an image by half does not mean keeping every second pixel. Each output pixel is calculated from the several input pixels it covers, which is what stops fine detail from turning into a jagged mess and is why a proper resize looks smooth where a naive one does not.

It also means detail is genuinely gone rather than hidden. A 4000-pixel photo taken down to 800 has had its information averaged away, and enlarging the result later recovers none of it. Keep the original if there is any chance you will need another size from it.

A form that says “400 by 400” is describing the space the picture will occupy, not the file it wants. Phone and laptop screens draw two or three physical pixels for every one of those, so an image supplied at exactly 400 looks soft on most devices people actually use.

Supplying twice the stated figure is the usual answer and costs little, because the extra pixels compress well. The exception is a hard upper limit — an avatar capped at 200 by 200, a form that rejects anything larger — where the number is a ceiling and exceeding it means the upload fails rather than looks better.