**Symbol**: kg / **Base data**: mass. SI International System of Units

Kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015 × 10^{-34}Js.

**The unit of mass is determined by the “Planck constant”**

Matter with the same mass possess the same energy. This concept argues that mass is defined by the energy possessed by a single naturally-occurring photon. The Planck constant is the smallest unit for calculating energy possessed by that photon.

Revised in 2021

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