Free Online Image OptimizerChoosing a width, and resizing to it
Almost every picture that is too heavy is too heavy because it is too large. A modern phone camera produces something around four thousand pixels across; a full-width banner needs sixteen hundred and an avatar needs two hundred. Resizing is the single change that moves the number most, and the only hard part is deciding what number to ask for.
Work backwards from where it will be shown
The useful question is not how big the picture is but how big the space is. Measure the slot it has to fill and double it, because plenty of screens pack two device pixels into every layout pixel and a picture supplied at exactly the layout size looks soft on them.
Some rough destinations. A blog illustration spanning the text column: 1,600 across. A card in a grid of three: 800. A thumbnail: 400. A profile picture: 400 as well, since these are usually shown small and cropped square. Anything intended for print is a different calculation entirely and wants the original.
Resize an image starts from the picture you give it and only ever goes down. Asking for more pixels than the file contains cannot invent detail, so the control declines to pretend otherwise.
One picture, one setting
Hand over the photograph and it appears on the stage with its dimensions printed underneath — 1,600 by 1,067, say. Open the settings and the control you want is the first one, Reduce width further.
It offers proportions of what you brought rather than absolute pixel counts, which sounds like an inconvenience and is the safer arrangement: the same setting applied across a batch of differently sized photographs does something sensible to every one of them. Nothing prints the resulting pixel count while you are choosing, so if you need to land on a particular number, take the width shown under the picture and halve or quarter it in your head.
Press the button and the file comes back with its new dimensions and the saving both measured. Halving the width quarters the pixel count, so the drop in bytes is routinely far larger than the number on the control suggests.
The ceilings, and the one that belongs to your phone
Two limits apply and neither is arbitrary. No edge goes beyond 4,096 pixels, and no picture beyond roughly twelve and a half megapixels in total. Which one bites first depends on the shape: a square hits the area limit while a long panorama runs into the edge limit.
They sit where they do because of Safari on iOS, which abandons a canvas somewhere above about sixteen and a half megapixels and hands back a blank one rather than an error. A tool that let you exceed that would produce empty white files on iPhones and no explanation. There is a check for the blank result as well, for the cases where the limits are not enough.
Worth knowing
Resizing a folder at once
Where you have a folder instead of one photograph, the batch page applies a single width to everything dropped on it and returns a zip. Same arithmetic, twenty files at a time.
Do not enlarge, ever
Nothing here will scale a picture up, and no tool that does can add information that was never captured. A small image stretched large is a soft image, whatever the file name says about its dimensions.