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 581s 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 systems 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.
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