We have used the Sydney University Stellar Interferometer ( SUSI ) to measure the angular diameter of \beta Hydri .
This star is a nearby G2 subgiant whose mean density was recently measured with high precision using asteroseismology .
We determine the radius and effective temperature of the star to be 1.814 \pm 0.017 { R } _ { \odot } ( 0.9 % ) and 5872 \pm 44 K ( 0.7 % ) respectively .
By combining this value with the mean density , as estimated from asteroseismology , we make a direct estimate of the stellar mass .
We find a value of 1.07 \pm 0.03 { M } _ { \odot } ( 2.8 % ) , which agrees with published estimates based on fitting in the H-R diagram , but has much higher precision .
These results place valuable constraints on theoretical models of \beta Hyi and its oscillation frequencies .