**Units of measure defined exclusively in terms of 4 universal physical constants.** These were originally suggested in 1899 by Max Planck, hence the name. They depend on: - the speed of light - the Gravitational Constant - the Reduced Planck Constant - the Boltzmann Constant Some people add in the electromagnetic dimension by including the Electric Constant These include: | Name | Expression | Value in terms of SI | | --- | --- | --- | | Planck length | l_p = root(h_bar_G/c^3) | 1.616e-35 m | | Planck mass | m_p = root(h_bar_c/G) | 2.176e-8 kg | | Planck time | t_p = root(h_bar*G/c^5) | 5.391e-44 s | Using these, you can define derived units for many other types of units (area, volume, momentum, temperature, energy, force, density, acceleration, frequency, etc) These are essentially useless in day-to-day life. They are useful when working at the “Planck Scale”, which only happens in university-or-greater-level physics classes. **** # More ## Source - [Planck units - Wikipedia](https://en.wikipedia.org/wiki/Planck_units) ## Related - [[International System of Units]]