\setKVdefault[ClesConversion]{Longueur,Aire=false,Vol=false,Masses=false,Capas=false,Sans=false,Complete=40pt}%
\defKV[ClesConversion]{Masse=\setKV[ClesConversion]{Longueur=false}\setKV[ClesConversion]{Masses}}%
\defKV[ClesConversion]{Capa=\setKV[ClesConversion]{Longueur=false}\setKV[ClesConversion]{Capas}}%

\NewDocumentCommand\ChoixUniteD{m}{%
  \IfStrEqCase{#1}{%
    {G}{\DeclareSIUnit{\TempoPrefixeD}{\giga}\xdef\ExposantD{9}}%
    {M}{\DeclareSIUnit{\TempoPrefixeD}{\mega}\xdef\ExposantD{6}}%
    {t}{\DeclareSIUnit{\TempoPrefixeD}{\tonne}\xdef\ExposantD{6}}%
    {q}{\DeclareSIUnit{\TempoPrefixeD}{\quintal}\xdef\ExposantD{5}}%
    {k}{\DeclareSIUnit{\TempoPrefixeD}{\kilo}\xdef\ExposantD{3}}%
    {h}{\DeclareSIUnit{\TempoPrefixeD}{\hecto}\xdef\ExposantD{2}}%
    {da}{\DeclareSIUnit{\TempoPrefixeD}{\deca}\xdef\ExposantD{1}}%
    {u}{\DeclareSIUnit{\TempoPrefixeD}{}\xdef\ExposantD{0}}%
    {d}{\DeclareSIUnit{\TempoPrefixeD}{\deci}\xdef\ExposantD{-1}}%
    {c}{\DeclareSIUnit{\TempoPrefixeD}{\centi}\xdef\ExposantD{-2}}%
    {m}{\DeclareSIUnit{\TempoPrefixeD}{\milli}\xdef\ExposantD{-3}}%
    {µ}{\DeclareSIUnit{\TempoPrefixeD}{\micro}\xdef\ExposantD{-6}}%
    {n}{\DeclareSIUnit{\TempoPrefixeD}{\nano}\xdef\ExposantD{-9}}%
  }%
}%

\NewDocumentCommand\ChoixUniteA{m}{%
  \IfStrEqCase{#1}{%
    {G}{\DeclareSIUnit{\TempoPrefixeA}{\giga}\xdef\ExposantA{9}}%
    {M}{\DeclareSIUnit{\TempoPrefixeA}{\mega}\xdef\ExposantA{6}}%
    {t}{\DeclareSIUnit{\TempoPrefixeA}{\tonne}\xdef\ExposantA{6}}%
    {q}{\DeclareSIUnit{\TempoPrefixeA}{\quintal}\xdef\ExposantA{5}}%
    {k}{\DeclareSIUnit{\TempoPrefixeA}{\kilo}\xdef\ExposantA{3}}%
    {h}{\DeclareSIUnit{\TempoPrefixeA}{\hecto}\xdef\ExposantA{2}}%
    {da}{\DeclareSIUnit{\TempoPrefixeA}{\deca}\xdef\ExposantA{1}}%
    {u}{\DeclareSIUnit{\TempoPrefixeA}{}\xdef\ExposantA{0}}%
    {d}{\DeclareSIUnit{\TempoPrefixeA}{\deci}\xdef\ExposantA{-1}}%
    {c}{\DeclareSIUnit{\TempoPrefixeA}{\centi}\xdef\ExposantA{-2}}%
    {m}{\DeclareSIUnit{\TempoPrefixeA}{\milli}\xdef\ExposantA{-3}}%
    {µ}{\DeclareSIUnit{\TempoPrefixeA}{\micro}\xdef\ExposantA{-6}}%
    {n}{\DeclareSIUnit{\TempoPrefixeA}{\nano}\xdef\ExposantA{-9}}%
  }%
}%

\NewDocumentCommand\Convertir{somm}{%
  \useKVdefault[ClesConversion]%
  \setKV[ClesConversion]{#2}%
  \StrBefore{#3}{>}[\PrefixeDepart]
  \StrBehind{#3}{>}[\PrefixeArrivee]
  \ChoixUniteD{\PrefixeDepart}%
  \ChoixUniteA{\PrefixeArrivee}%
  \ifboolKV[ClesConversion]{Aire}{%
    \DeclareSIUnit{\TempoAvant}{\square}%
    \DeclareSIUnit{\TempoBase}{\meter}
    \xdef\PfCConvExposant{2}
  }{\ifboolKV[ClesConversion]{Vol}{%
      \DeclareSIUnit{\TempoAvant}{\cubic}%
      \DeclareSIUnit{\TempoBase}{\meter}
      \xdef\PfCConvExposant{3}
    }{%
      \DeclareSIUnit{\TempoAvant}{}%
      \xdef\PfCConvExposant{1}
    }%
  }%
  \ifboolKV[ClesConversion]{Masses}{%
    \DeclareSIUnit{\TempoBase}{\gram}%
  }{}%
  \ifboolKV[ClesConversion]{Longueur}{%
    \DeclareSIUnit{\TempoBase}{\meter}%
  }{}%
  \ifboolKV[ClesConversion]{Capas}{%
    \DeclareSIUnit{\TempoBase}{\liter}%
  }{}%
  \ensuremath{
    \IfBooleanTF{#1}{%
      \IfStrEq{\PrefixeDepart}{t}{\DeclareSIUnit{\TempoBase}{\relax}}{}\IfStrEq{\PrefixeDepart}{q}{\DeclareSIUnit{\TempoBase}{\relax}}{}
      \SI{#4}{\TempoAvant\TempoPrefixeD\TempoBase}
      \IfStrEq{\PrefixeDepart}{t}{\DeclareSIUnit{\TempoBase}{\gram}}{}\IfStrEq{\PrefixeDepart}{q}{\DeclareSIUnit{\TempoBase}{\gram}}{}
      =
      \IfStrEq{\PrefixeArrivee}{t}{\DeclareSIUnit{\TempoBase}{\relax}}{}\IfStrEq{\PrefixeArrivee}{q}{\DeclareSIUnit{\TempoBase}{\relax}}{}
      \PointsSuspension{\fpeval{#4*(10**((\ExposantD-(\ExposantA))*\PfCConvExposant))}\fpeval{#4*(10**((\ExposantD-(\ExposantA))*\PfCConvExposant))}}~\si{\TempoAvant\TempoPrefixeA\TempoBase}
      \IfStrEq{\PrefixeArrivee}{t}{\DeclareSIUnit{\TempoBase}{\gram}}{}\IfStrEq{\PrefixeArrivee}{q}{\DeclareSIUnit{\TempoBase}{\gram}}{}
    }{%
      \ifboolKV[ClesConversion]{Sans}{%
        \IfStrEq{\PrefixeDepart}{t}{\DeclareSIUnit{\TempoBase}{\relax}}{}\IfStrEq{\PrefixeDepart}{q}{\DeclareSIUnit{\TempoBase}{\relax}}{}
        \SI{#4}{\TempoAvant\TempoPrefixeD\TempoBase}
        \IfStrEq{\PrefixeDepart}{t}{\DeclareSIUnit{\TempoBase}{\gram}}{}\IfStrEq{\PrefixeDepart}{q}{\DeclareSIUnit{\TempoBase}{\gram}}{}
        =
        \IfStrEq{\PrefixeArrivee}{t}{\DeclareSIUnit{\TempoBase}{\relax}}{}\IfStrEq{\PrefixeArrivee}{q}{\DeclareSIUnit{\TempoBase}{\relax}}{}
        \num{\fpeval{#4*(10**((\ExposantD-(\ExposantA))*\PfCConvExposant))}}~\pointilles[25pt]
        \IfStrEq{\PrefixeArrivee}{t}{\DeclareSIUnit{\TempoBase}{\gram}}{}\IfStrEq{\PrefixeArrivee}{q}{\DeclareSIUnit{\TempoBase}{\gram}}{}
      }{%
        \IfStrEq{\PrefixeDepart}{t}{\DeclareSIUnit{\TempoBase}{\relax}}{}\IfStrEq{\PrefixeDepart}{q}{\DeclareSIUnit{\TempoBase}{\relax}}{}
        \SI{#4}{\TempoAvant\TempoPrefixeD\TempoBase}
        \IfStrEq{\PrefixeDepart}{t}{\DeclareSIUnit{\TempoBase}{\gram}}{}\IfStrEq{\PrefixeDepart}{q}{\DeclareSIUnit{\TempoBase}{\gram}}{}
        =
        \IfStrEq{\PrefixeArrivee}{t}{\DeclareSIUnit{\TempoBase}{\relax}}{}\IfStrEq{\PrefixeArrivee}{q}{\DeclareSIUnit{\TempoBase}{\relax}}{}
        \SI{\fpeval{#4*(10**((\ExposantD-(\ExposantA))*\PfCConvExposant))}}{\TempoAvant\TempoPrefixeA\TempoBase}
        \IfStrEq{\PrefixeArrivee}{t}{\DeclareSIUnit{\TempoBase}{\gram}}{}\IfStrEq{\PrefixeArrivee}{q}{\DeclareSIUnit{\TempoBase}{\gram}}{}
      }%
    }%
  }%
}%