How do you convert between energy units: 231 cal to kJ?

Well, $1 \cdot J \equiv 0.239 \cdot c a l \ldots \ldots .$
And thus $231 \cdot c a l = \frac{231 \cdot c a l}{0.239 \cdot c a l \cdot {J}^{-} 1} = 966.5 \cdot \frac{1}{J} ^ - 1$
$= 966.5 \cdot \frac{1}{\frac{1}{J}} = 966.5 \cdot J \equiv 966.5 \cdot J \times {10}^{-} 3 \cdot k J \cdot {J}^{-} 1$
$\cong 1 \cdot k J$