  Using the CHARA Array and the Palomar Testbed Interferometer, the chemically
peculiar star $\lambda$ Bo\"{o}tis has been spatially resolved. We have
measured the limb darkened angular diameter to be $\theta_{LD} = 0.533\pm0.029$
mas, corresponding to a linear radius of $R_{\star} = 1.70 \pm 0.10 R_\odot$.
The measured angular diameter yields an effective temperature for $\lambda$ Boo
of $T_{eff} = 8887 \pm 242$ K. Based upon literature surface gravity estimates
spanning $\log{(g)} = 4.0-4.2$ $[\rm{cm s}^{-\rm{2}}]$, we have derived a
stellar mass range of $M_{\star} = 1.1 - 1.7$ $M_\odot$. For a given surface
gravity, the linear radius uncertainty contributes approximately
$\sigma(M_\star) = 0.1-0.2 M_\odot$ to the total mass uncertainty. The
uncertainty in the mass (i.e., the range of derived masses) is primarily a
result of the uncertainty in the surface gravity. The upper bound of our
derived mass range ($\log(g)=4.2, M_\star = 1.7\pm0.2 M_\odot$) is consistent
with 100-300 MYr solar-metallicity evolutionary models. The mid-range of our
derived masses ($\log(g)=4.1, M_\star = 1.3\pm0.2 M_\odot$) is consistent with
2-3 GYr metal-poor evolutionary models. A more definitive surface gravity
determination is required to determine a more precise mass for $\lambda$ Boo.
