Hi Edward,
Because lists are being passed to HB
and many operations thread across lists.
Pi * {10, Quantity[1, "Millimeters"]}
(* {10 \[Pi], Quantity[\[Pi], "Millimeters"]} *)
Pass the right quantities instead
HB[Quantity[500, "Kilograms"], Quantity[10, "Millimeters"], Quantity[1.62, "Millimeters"]]
(* Quantity[240.975, ("Kilograms")/("Millimeters")^2] *)