Electrified aircraft are a promising solution to reduce aviation’s carbon footprint across general and commercial aviation sectors. However, existing computational tools for aircraft design require information about the aircraft’s configuration, forcing the designer to down-select before optimizing their design. As a result of the limited flexibility, sub-optimal system architectures may be selected during the early phases of conceptual design. To address this, the Future Aircraft Sizing Tool (FAST), an open-source, MATLAB-based tool, was developed as a propulsion system-agnostic tool for early-phase conceptual design. Using limited design parameters, FAST facilitates rapid and comprehensive aircraft sizing and performance evaluation, utilizing an extensive database of over 450 historical aircraft. Both data-driven and physics-based models are combined to seamlessly integrate new electrification technologies into a design while rapidly predicting its performance. This capability enables early-stage design space exploration to rigorously assess a wide range of propulsion architectures, energy sources, and operational strategies for novel aircraft configurations. This paper presents the key features of FAST, including workflows for aircraft sizing and analysis. The paper also presents a case study involving a commercial freighter, demonstrating FAST’s ability to perform design space exploration and early-phase trade studies.