The solid-electrolyte interphase (SEI) formed on graphite anodes affords lithium-ion batteries their remarkable stability and capacity retention, yet a full quantitative description of the SEI remains elusive. In this work, we demonstrate a novel method to quantify components comprising the graphite SEI (including solid carbonates, inactive Li metal, and lithium acetylide) with a variety of electrolyte compositions, and we determine the contribution of each SEI species to the observed capacity fade during fast charge.