There is no single right way to change an aspect ratio
A landscape photograph does not contain the information needed to be a portrait. Something has to give — either you lose part of the picture, or you add something that was not there. Every tool that does this silently has quietly picked one of those for you, and it is often the wrong one.
So all four options are here, each previewed against your actual image, because which one wins depends entirely on the picture. A landscape with the subject dead centre crops beautifully. A wide group shot does not, and needs a fill.
The four, and when each is right
Smart fill puts the whole image in the middle and fills the gaps with a scaled-up, heavily blurred copy of itself, darkened slightly. This is the look social platforms made familiar, and it is what most people actually want: nothing is lost, and the result does not read as a mistake. The blur and darkening are both adjustable.
Crop to fit fills the frame properly and cuts the overflow. It gives the best-looking result when it works, and decapitates people when it does not — so the focal point is a draggable dot rather than a fixed centre. It starts positioned by an edge-density guess at where the subject is, which is a rough heuristic, not face detection, and it is meant to be adjusted.
Solid colour is the clean, deliberate choice for product shots and graphics. The colour picker starts from the image’s own dominant colour, so the default already belongs with the picture.
Stretch distorts the image to fill the frame. It is included because leaving it out would be pretending it is never useful — occasionally, for an abstract texture, it is. For anything with a face, a building or a straight line in it, it is not.
Sizes that come out sensibly
Rather than upscaling to some fixed dimension, the output frame is sized to keep roughly the pixel count of your original. A 1200×800 photo becomes about 876×1095 as a 4:5 portrait — no invented pixels, no needless enlargement, and small enough to post without further compression.
If you need exact dimensions for a particular platform, the platform sizes tool has them all with the correct pixel counts and safe zones. Use this one when the shape is what matters and the exact numbers are not.
Everything here happens in this browser tab. The image is decoded, redrawn and re-encoded by the Canvas API on your own device, and the result is assembled in memory and handed to you as a download. Nothing is uploaded, which you can confirm in about fifteen seconds with the network panel open.