What rounded corners mean in an image file
A rectangular image has four sharp corners. Rounding them replaces each corner with a smooth curve, and the area outside the curve is left empty. For that empty area to look right on a web page or in a slide, it has to be transparent, which is why the result needs a format that supports an alpha channel. PNG does.
The size of the curve is the corner radius, measured in pixels. A small radius gives a gentle softening, while a very large radius produces a pill shape or, on a square picture, a full circle. The tool limits the radius so it can never be larger than half of the image's shorter side, because a bigger value would not make geometric sense.
When you would want this
Rounded images are common in app icons, profile pictures, cards, thumbnails, and presentation graphics. Many designs call for soft corners to match the rest of an interface, and baking the shape into the file is useful when the place you are posting does not let you apply rounded styling itself.
It is also a simple way to make a screenshot look more polished before dropping it into a document, or to turn a square portrait into a circular avatar by choosing a large enough radius.
How to round corners with this tool
- Drop an image onto the upload area, or click to select one.
- Move the Corner radius slider. It goes from 0 to 200 pixels and starts at 40.
- Watch the preview, which sits on a checkerboard so you can see where the transparent corners will be.
- Click Apply to create the finished image.
- Click Download to save it, or Start over to choose another picture.
The file is saved as a PNG with -rounded added to the original name, for example portrait-rounded.png. The processing happens in your browser, and your image is not uploaded.
Tips, limits, and common mistakes
The radius is in pixels of the original image, not of the preview. The preview on screen is scaled down to fit, so a radius that looks right in the preview can look smaller on a very large photo. A 600 by 600 picture with a radius of 100 shows a clear curve, while the same radius on a 6000 pixel image is barely noticeable. If your photo is large, expect to use the higher end of the slider or a smaller image.
For a circle, start with a square image and set the radius to at least half its width. A 600 by 600 picture needs a radius of 300, but the slider stops at 200, so a circle from larger squares requires resizing the image first with the Image Resizer or cropping it square with Crop Image.
Remember that the transparent corners only show as transparent where the image is displayed on top of something. Some apps place PNGs on a white or black background, which will fill the corners. If you need to add a colored frame as well, the Image Border Tool draws a square border, so apply rounding to a finished image rather than the other way around.