What “4K” means for a picture
4K UHD is 3840 × 2160 pixels, a 16:9 frame. Most photos are 3:2 or 4:3, so a 4K version of a photo is either letterboxed or slightly smaller than the full frame. This tool fits the picture inside 3840 × 2160 (or the portrait 2160 × 3840 when the photo is upright) without stretching. If you need the full frame filled, switch to Pixels and choose Fill & crop, or Pad to add bars.
AI first, then exact
With AI detail on, the picture is enlarged 4× by the network and then resampled to the exact 4K size in linear light. A 1280-pixel-wide photo becomes 5120 wide from the network and 3840 after the final pass, which keeps the reconstructed texture while hitting the frame exactly. If the source is already wider than 3840 px, the job becomes a high-quality reduction instead.
Questions
Can I upscale to 8K?
Yes, choose 8K (7680 × 4320) in the screen presets. On phones the AI memory guard (16 megapixels) means the input is reduced before the AI pass for 8K; desktops allow 48 megapixels.
Will my 4K image look like a real 4K photo?
It will have 4K pixel dimensions and, with AI on, convincingly sharp edges and texture. It cannot contain detail the camera never recorded; fine print in the background, for example, will not become readable.
Why is my 4K result 3840 × 2560 and not 3840 × 2160?
Because your photo is 3:2. Fitting it inside the 16:9 frame keeps every part of the picture; use Fill & crop to cut it to exactly 16:9.
Is my picture uploaded anywhere?
No. Decoding, scaling and encoding all run in this browser tab, and the scaling engine runs in a Web Worker with no network code in it. The only download the site can make for you is an AI model file, and only after you press the button that names its size — that request contains no picture.