Thrust-to-weight ratio (TWR) is the thrust divided by the weight, where weight is mass times gravitational acceleration. Thrust may be in newtons, kilograms-force or pounds-force as long as weight uses the same unit, and the result has no unit. A value of 1 means the thrust exactly balances weight.
For a multirotor, tiltrotor or other vertical-lift aircraft, the relevant figure is the total static thrust of all rotors divided by maximum take-off weight. A ratio above 1 is needed to leave the ground, and the margin above hover decides climb rate, wind and gust handling and the ability to keep control with one propulsion unit lost. Many designers aim at a ratio of about 2 for agile multirotors, though this is a design choice rather than a standard, and heavy-lift craft often accept lower values. For a fixed-wing aircraft the ratio in cruise is simply the inverse of the lift-to-drag ratio.
On a datasheet, motor and propeller makers publish static thrust at stated voltage and throttle; the aircraft maker may list the system ratio at maximum take-off weight.
A frequent mistake is using the thrust of a motor and propeller at maximum throttle, when the real battery sags, temperature rises and thrust is held for only seconds. Another is mixing thrust per motor with total weight or using mass in kilograms against thrust in newtons. It also differs from power-to-weight, which applies to power sources.
