CubeSats have a 65% success rate. Failures derive from design mistake or components malfunctions. To improve the success rate, Technical University of Dresden and FHWN (University of Applied Sciences Wiener Neustadt) developed GREATCUBE+, a software tool for the conceptual design of CubeSats. Its layered structure comprises three levels: empirical, where successfully flown missions are used for an initial tradeoff; analytical, where a design refinement is performed; and numerical, for the final assessment of the proposed architecture. The tool provides teams with information related to commercial off-the-shelf products which will satisfy the mission requirements. To turn this software into a universally applicable tool, it is possible to perform the design of CubeSat mission with many payload’s typologies such as attitude determination and control subsystem, telemetry telecommunication and command, onboard computer, propulsion unit and technology demonstration or scientific payloads. GREATCUBE+ has been validated using the information of existing CubeSats as baseline for its simulation. The achievable accuracy when comparing the simulated outcomes and the real design is of almost 100% for volumes, 90% for masses, and 80% for power generation. By implementing this tool during the conceptual development phase, it is hoped that teams could benefit in reliability thanks to the usage of flight proven equipment recommended via GREATCUBE+ together with a quicker development time.