Torsion Constant for Rectangle Cross-section

Torsion Constant for Rectangle Cross-section formula
$J \;=\; a \cdot b^3 \cdot \left[ \dfrac{1}{3} - 0.21 \cdot \dfrac{b}{a} \cdot \left(1- \dfrac{b^4 }{ 12 \cdot a^4 } \; \right) \right]$
Symbol	English	Metric
$J$ (Greek symbol alpha) = torsion constant	$in^4$	$mm^4$
$a$ = length of long edge of an rectangle	$in$	$mm$
$b$ = length of short edge of an rectangle	$in$	$mm$