The scaling rule of high average power, short-pulse CO2 lasers is described based on the physical fundamentals. Specifications are required for lasers applied to high-volume manufacturing extreme-ultraviolet light sources, simultaneously on the average power over 10 kW, the beam quality less than 2 for M2, and the pulse width in the 10 ns range. The amplifier is operated by a cascade amplifier arrangement, and the ruling principle is approximately described by the Frantz-Nodvik equation, with effective small-signal gain g0 and saturation fluence Es depending on the amplifier medium and amplified pulse width. Thermal distortion is caused in the optical components, leading to beam propagation instability. The general rule of g0L<3, where L is the optical gain length, is the requirement to isolate each amplifier by optical shutters. Multiline amplification is efficient to increase the extraction efficiency in the case of less-efficient rotational relaxation. It is concluded that a well-designed single-beam, short-pulse CO2 laser is operable at 20 kW average output power based on available components, by satisfying all requirements.