The Inner Corner Radius Should Follow the Outer One

Give a card and its inner panel the same border-radius and the result often looks wrong. The CSS values match, but the curves do not: the inner corner sits closer to the center and needs a smaller radius.

The gap belongs in the calculation

Tiny design detail: nested border radii look really funky if they're the same. To maintain the same curvature, the outer radius = inner radius + padding. pic.twitter.com/Oe8ZqFyr4J

— Lily Konings (@lilykonings) September 7, 2022

For concentric circular corners, subtract the space between the elements from the outer radius:

--outer-radius: 1em;
--padding: 0.5em;
--inner-radius: calc(var(--outer-radius) - var(--padding));

The same relationship appears in a concrete example:

.inner {
    width: 5em;
    height: 5em;
    background: black;
    border-radius: 1em;
}
 
.outer {
    display: inline-block;
    background: lightblue;
    padding: 3em;
    border-radius: 4em;
}

The outer radius is 4em, the gap is 3em, and the inner radius is therefore 1em.

Store two tokens or calculate one

Separate inner and outer radius tokens give a designer explicit control. They also create more values that must stay synchronized.

With consistent padding, a calculated property is usually easier to maintain. Change the outer radius or gap and the inner curve follows automatically.

Where I use this

• an image inside a padded card;
• an input inside a rounded form panel;
• a button inside an alert;
• nested surfaces in a component.

It is a geometric starting point, not a law. Non-uniform padding, elliptical radii, borders, and deliberately exaggerated shapes may need visual adjustment.

A reusable custom property

:root {
  --base-radius: 1em;
  --nested-radius: calc(var(--base-radius) - var(--padding));
}

Make sure the calculation cannot produce an unwanted negative result when the gap exceeds the radius. For ordinary nested cards, the rule I keep in mind is simple: the difference between the two radii should match the gap between the two edges.