Resize Image

How do I resize an image?

1. Drop JPG, PNG or WebP files onto the drop zone.
2. Pick a Resize by mode: Width (keep aspect ratio), Height (keep aspect ratio), Percentage, or Exact width and height.
3. Enter the Width, Height or Percentage you want.
4. Choose an Output format: Same as input, JPG, PNG or WebP, and set Quality for JPG or WebP.
5. Click Resize.
6. Download the image, or Download all as ZIP when resizing several.

Resize Image decodes each file with createImageBitmap in a Web Worker, draws it onto an OffscreenCanvas at the new dimensions and encodes the result. The browser's canvas scaler uses high quality filtering, which gives smooth results when shrinking photos and avoids the jagged edges you get from nearest-neighbour scaling. Because the whole pipeline is in a worker, a batch of large camera files will not freeze the page.

The four modes cover the common cases. Width is the usual choice for web images: enter 1200 and the height follows automatically. Height suits fixed-height layouts such as a row of thumbnails. Percentage is quickest when you know you want, say, half size across a mixed batch of different dimensions. Exact width and height ignores the aspect ratio and stretches the image to fit, so use it only when you deliberately need a specific canvas, for example a 1200 by 630 social preview where slight distortion is acceptable. For a distortion-free fixed size, crop first with Crop Image and then resize.

Output format lets you convert at the same time. Same as input keeps PNG as PNG and JPG as JPG. Switching to JPG or WebP for a photo that arrived as a PNG usually cuts the file size by 5 to 10 times, and WebP is a further 25 to 35% smaller than JPG at the same visual quality. The Quality slider applies to JPG and WebP output; 85 is a safe default. PNG output is lossless and ignores the slider. Converting a transparent PNG to JPG flattens the transparency onto white.

Upscaling is allowed, up to 400% in Percentage mode or any width you type, but it cannot add detail that was not in the original. A 640 pixel image enlarged to 1920 will look soft. It is fine for making a small logo fill a slide, less so for print.

Files are processed on your device and never uploaded. There is no cap on the number of files or their size beyond your browser's memory, and the tool works offline once loaded.

How it works: this tool uses your browser's Canvas and WebCodecs APIs, plus libheif for HEIC. No server is involved, so Kilnfile never sees your file. Last reviewed 20 September 2026.

Frequently asked questions