Use this free radius corner calculator to size rounded corners for a rectangle, check whether your radius is valid, and get perimeter, area, arc length, and CSS output.
Shape Preview
Preview is scaled to fit and keeps the same corner ratio as your dimensions.
A good radius corner value can make designs feel modern, soft, and easier to read. A bad value can make buttons look awkward, cards look over-rounded, or CNC and fabrication specs fail tolerance checks. This calculator helps you choose a practical corner radius quickly.
What is a radius corner calculator?
A radius corner calculator is a rounded rectangle calculator that computes geometry when corners are filleted (rounded). It is useful in UI design (CSS border-radius), product design, CAD work, sheet metal layouts, signage, and machining.
Given width, height, and corner radius, the tool can calculate:
- Maximum allowable radius before corners overlap
- Rounded rectangle perimeter
- Rounded rectangle area
- Arc length for one corner and all four corners
- Straight segment lengths between arcs
- A ready-to-use CSS value
How the math works
Maximum valid radius: rmax = min(width, height) / 2
Area: A = width × height − (4 − π)r²
Perimeter: P = 2(width + height − 4r) + 2πr
These formulas apply when all four corners have the same radius. If you enter a value larger than the max radius, the calculator automatically caps it and lets you know.
Why the maximum radius matters
If your radius is larger than half of the smallest side, the arcs from neighboring corners collide. In CSS, the browser resolves this for you. In manufacturing, you usually need to specify a radius that is physically achievable and unambiguous.
How to use this calculator
- Choose a unit (px, mm, in, or cm).
- Enter rectangle width and height.
- Enter your desired corner radius.
- Click Calculate.
- Review adjusted radius, perimeter, area, and CSS recommendation.
Practical corner radius tips
For web and app interfaces
- Small controls often look balanced between 4px and 10px radius.
- Cards and panels commonly use 8px to 24px depending on visual style.
- Keep radius consistent across component families for clean design systems.
For product and fabrication work
- Match internal corner radius to tool diameter and process limits.
- Include tolerances for laser, router, or milling paths.
- Confirm units before production release to avoid expensive errors.
Common mistakes
- Using a radius larger than half of the shortest side without realizing it.
- Mixing units (for example, inches in one field and millimeters in another).
- Ignoring that perimeter and area both change when corner radius changes.
- Setting inconsistent border radius values across related UI elements.
Final takeaway
Whether you call it a corner radius calculator, fillet radius calculator, or border radius calculator, the goal is the same: choose corners that are visually pleasing, mathematically valid, and production-ready. Use the tool above to get reliable numbers in seconds.