We propose and characterize graphene quantum capacitors, tunable with voltage by the control of their charge density, for tunable LC tanks as essential building blocks for Radio-Frequency (RF) functions in densely integrated circuits. We fabricate and investigate their performance in RF, and we demonstrate quantum capacitances, C-q, in the range of pF with a tuning range of >1.3:1 within 1.25 V, with Q-factors up to 14.5 at 0.4 GHz. Our capacitors have a high capacitance density, up to 2.65 fF/mu m(2), which is 100x higher than the one of RF MEMS capacitors. Based on calibrated models and simulations, we demonstrate the potential to replace their semiconductor counterparts and RF MEMS capacitors in LC tanks, for key RF analog application.