# Quantity and unit conversion in ccx

Hello all,

I am using CalculiX for conducting thermal simulations with radiation as only heat sink. Since my geometric model is given in mm I have to adjust the units of all other quantities accordingly. I did some reading in the ccx manual and the tips by Prof Kraska but still it is confusing which units I have to convert. From my understanding I have to convert Stefan-Boltzmann constant (W/mm^2/K^4), conductivity (W/mm/K), density (kg/mm), contact conductance (W/mm^2/K), volume and area heat flow (W/mm^3 and W/mm²) but energy (J) and Power (W) stay the same as if I would use meters? Or do I have to convert them to mm aswell (so for example: Watt as kgmm^2/s^3 instead of kgm^2/s^3)?
I would be glad if an experienced user could tell me which units I have to use for the above quantities.

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What you need is called a consistent unit system. One of the possibilities:

Or you can Google it to find all the quantities that you need.

Besides looking up a list as Matej says, To avoid confusion and overlooking things, convert every unit to SI base units, then swap what’s needed (m->mm) then convert back. Eg:

J = kg m^2 / s^2
Change m to mm: kg mm^2 / s^2
= micro J

I think some of the examples you gave like W/mm/K for conductivity aren’t correct because you have to convert the W as well as the m.

below is another examples using consistent units, depending on units of geometry you have been chosen. if in ‘mm’ units of geometries, so the mass and energy could be in ‘tonne’ and ‘mJ’ respectively.

Thanks for the numerous answers and for clearing my confusion.