r

The r attribute defines the radius of a circle.

Two elements are using this attribute: <circle>, and <radialGradient>

<svg viewBox="0 0 300 200" xmlns="http://www.w3.org/2000/svg">
<stop offset="0"    stop-color="white" />
<stop offset="100%" stop-color="black" />
<stop offset="0"    stop-color="white" />
<stop offset="100%" stop-color="black" />
<stop offset="0"    stop-color="white" />
<stop offset="100%" stop-color="black" />

<circle cx="50"  cy="50" r="0"/>
<circle cx="150" cy="50" r="25"/>
<circle cx="250" cy="50" r="50"/>

<rect x="20"  y="120" width="60" height="60" fill="url(#myGradient000)" />
<rect x="120" y="120" width="60" height="60" fill="url(#myGradient050)" />
<rect x="220" y="120" width="60" height="60" fill="url(#myGradient100)" />
</svg>

circle

For <circle>, r defines the radius of the circle and therefor its size. With a value lower or equal to zero the circle won't be drawn at all.

Value | 0 Yes

Note: Starting with SVG2, r is a Geometry Property meaning this attribute can also be used as a CSS property for circles.

The gradient will be drawn such that the 100% gradient stop is mapped to the perimeter of this end circle. A value of lower or equal to zero will cause the area to be painted as a single color using the color and opacity of the last gradient <stop>.

Value | 50% Yes

Specifications

Specification Status Comment
Scalable Vector Graphics (SVG) 2
The definition of 'r' in that specification.
Candidate Recommendation Definition as a geometry property
Scalable Vector Graphics (SVG) 2
The definition of 'r' in that specification.
Candidate Recommendation Definition for SVG2 paint servers <radialGradient>
Scalable Vector Graphics (SVG) 1.1 (Second Edition)
The definition of 'r' in that specification.