Free-lift is the amount of lift that exceeds the total weight of a high altitude balloon system, including the balloon itself, the payload, and any supporting structures. It represents the extra upward force available to initiate and sustain the balloon's ascent.

In high altitude ballooning, free-lift is carefully calculated and controlled to achieve the desired ascent rate. A higher free-lift results in a faster ascent, while a lower free-lift produces a slower climb. The balance of free-lift is crucial for mission planning, as it affects the altitude profile, balloon expansion dynamics, and overall flight stability. Proper free-lift ensures that the balloon can overcome inertia and atmospheric resistance to reach target altitudes efficiently.