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Radius Match

A free design game: slide the inner radius of a rounded box until the gap to the outer curve is even, then see the outer radius minus padding rule.

What it does

  • Six cards with a rounded box inside a rounded box
  • Slide the inner radius until the gap between the curves looks even
  • An optional hint shows the measurements for a small point cost
  • The subtraction behind each answer, including the square-corner case

FAQ

How do you calculate a nested border radius?

Inner radius equals outer radius minus the padding between them. A 24 px card with 8 px of padding holds a child with a 16 px radius.

What if the padding is bigger than the outer radius?

The subtraction is zero or negative, so the inner corners should be square. There is no curve left to follow.

Do borders count as padding?

Yes. A 1 px border sits between the two curves, so subtract it from the outer radius as well.

📖 Nested border radius: outer radius minus padding

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