CSS Border Radius Generator

Advanced CSS Border-Radius & Smooth Squircle Geometries

The CSS border-radius property rounds the outer corners of HTML element frames. While developers often apply basic uniform pixel radii (e.g., border-radius: 8px;), the CSS Backgrounds and Borders Module Level 3 specification supports independent horizontal and vertical radii, percentage-based pill shapes, and organic asymmetric curves.

8-Value Border Radius Syntax

A slash (/) separates horizontal radii from vertical radii in the expanded shorthand:

border-radius: <horizontal-radii> / <vertical-radii>;
border-radius: 40px 60px 40px 60px / 60px 40px 60px 40px;

This establishes independent elliptical curves for each of the four corners:

• Top-Left horizontal & vertical
• Top-Right horizontal & vertical
• Bottom-Right horizontal & vertical
• Bottom-Left horizontal & vertical

Design Style	CSS Shorthand Value	Visual Appearance
Subtle Card	border-radius: 8px;	Modern clean software card container
Fully Rounded Pill	border-radius: 9999px;	Interactive buttons, badges, tag chips
Circular Avatar	border-radius: 50%;	Perfect circle on square aspect ratio containers
Organic Blob	border-radius: 60% 40% 30% 70% / 60% 30% 70% 40%;	Dynamic animated hero elements

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The Apple "Squircle" (Superellipse) vs Standard Border-Radius

Standard CSS border-radius draws circular or elliptical arcs that connect abruptly to straight edges, creating subtle visual curvature discontinuities where the circle meets the straight line. In contrast, Apple iOS icons use a mathematical superellipse (squircle):

$$\left|\frac{x}{a}\right|^n + \left|\frac{y}{b}\right|^n = 1$$

Where $n \approx 4$ to $5$. To simulate smooth squircles on the web, designers use SVG clip-paths or CSS corner-shape: squircle (where supported by emerging specifications).

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Animating Border-Radius in Modern CSS

@keyframes morphBlob {
  0% {
    border-radius: 60% 40% 30% 70% / 60% 30% 70% 40%;
  }
  50% {
    border-radius: 30% 60% 70% 40% / 50% 60% 30% 60%;
  }
  100% {
    border-radius: 60% 40% 30% 70% / 60% 30% 70% 40%;
  }
}

.morphing-card {
  width: 250px;
  height: 250px;
  background: linear-gradient(135deg, #6366f1, #ec4899);
  animation: morphBlob 8s ease-in-out infinite;
}

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Interactive UI Design Patterns with Rounded Corners

• Nested Border Radius Mathematics: To ensure concentric curves between a parent card and an inner element, calculate the inner radius using: $R_{\text{inner}} = R_{\text{outer}} - \text{padding}$. Failing to adjust the inner radius creates awkward visual pinching.
• Button Grouping Curves: In joined button groups or segmented controls, apply border-radius exclusively to outer corner boundaries (:first-child and :last-child) while zeroing inner corners.
• Micro-Interactions: Smoothly animating border-radius transitions on button press states (:active) provides tactile feedback in mobile touch interfaces.