  GJ 581 is an M dwarf host of a multiplanet system. We use long-baseline
interferometric measurements from the CHARA Array, coupled with trigonometric
parallax information, to directly determine its physical radius to be $0.299
\pm 0.010 R_{\odot}$. Literature photometry data are used to perform spectral
energy distribution fitting in order to determine GJ 581's effective surface
temperature $T_{\rm EFF}=3498 \pm 56$ K and its luminosity $L=0.01205 \pm
0.00024 L_{\odot}$. From these measurements, we recompute the location and
extent of the system's habitable zone and conclude that two of the planets
orbiting GJ 581, planets d and g, spend all or part of their orbit within or
just on the edge of the habitable zone.
