Large-scale stationary energy storage demands technologies based on Earth abundant elements.  The scarcity and rising cost of lithium resources necessitate alternative storage devices to Li-ion batteries.  To date, Na-ion batteries receive most attention; however, the challenge associated to the carbon anodes may potentially affect its commercialization.  Here, we demonstrate that all major bulk carbons, including graphite, soft carbon, and hard carbon, can reversibly store K-ions in non-aqueous electrolytes at room temperature.  All carbon electrodes can exhibit a high reversible capacity near 270 mAh/g.  In particular, ex situ XRD studies on graphite electrodes confirm that KC₃₆, KC₂₄, and KC₈ sequentially form upon potassiation, whereas depotassiation recovers graphite through phase transformations in an opposite sequence.  Graphite shows moderate rate capability and relatively fast capacity fading.  In contrast, we discover that both soft carbon and hard carbon exhibit much enhanced rate capability and cyclability compared to graphite.  Cycling of hard carbon is the most stable.  Interestingly, we determined that the hard carbon exhibits an unambiguously better rate capability in K-ion batteries than in Na-ion batteries.  These works may open up a new paradigm toward rechargeable K-ion batteries.