We report the determination of the QEC value of the mirror transition of C11 by measuring the atomic masses of C11 and B11 using Penning trap mass spectrometry. More than an order of magnitude improvement in precision is achieved as compared to the 2012 Atomic Mass Evaluation (Ame2012) [Chin. Phys. C 36, 1603 (2012)]. This leads to a factor of 3 improvement in the calculated Ft value. Using the new value, QEC=1981.690(61) keV, the uncertainty on Ft is no longer dominated by the uncertainty on the QEC value. Based on this measurement, we provide an updated estimate of the Gamow-Teller to Fermi mixing ratio and standard model values of the correlation coefficients.