# 4.8. Units

By default, Fuego assumes you are operating in centimeter-gram-seconds units (CGS) and Kelvin. If you are using basic properties there is no conversion done, but if you are using Cantera or Tabulated properties, they are converted to your problem units.

Note

It is always recommended you explicitly define your units in the `Fuego Region` block to avoid accidental unit errors.

    Begin Fuego Region Fuego_region

        Set unit system to MKS

When you have a unit system set, Fuego will also use NIST constants for things like the Stefan-Boltzmann constant so you don’t have to supply that yourself.

The available pre-defined unit systems are `MKS` and `CGS`. To use a custom unit system, you can specify conversion factors for length, mass, and time manually. Some examples of this are shown below.

    Begin Fuego Region Fuego_region

        # Does the same thing as "SET UNIT SYSTEM TO MKS"
        SET LENGTH UNIT CONVERSION FACTOR = 100.0
        SET MASS   UNIT CONVERSION FACTOR = 1000.0
        SET TIME   UNIT CONVERSION FACTOR = 1.0
        SET TEMPERATURE UNITS = KELVIN

    Begin Fuego Region Fuego_region

        # Does the same thing as "SET UNIT SYSTEM TO CGS"
        SET LENGTH UNIT CONVERSION FACTOR = 1.0
        SET MASS   UNIT CONVERSION FACTOR = 1.0
        SET TIME   UNIT CONVERSION FACTOR = 1.0
        SET TEMPERATURE UNITS = KELVIN

    Begin Fuego Region Fuego_region

        # use a non-standard mm-g-s unit system
        SET LENGTH UNIT CONVERSION FACTOR = 0.1
        SET MASS   UNIT CONVERSION FACTOR = 1.0
        SET TIME   UNIT CONVERSION FACTOR = 1.0