The torque applied to thin-walled tubes is expressed as

$T = 2Aq$

where T is the torque in N·mm, A is the area enclosed by the center line of the tube (as shown in the stripe-filled portion) in mm^{2}, and q is the shear flow in N/mm.

The average shearing stress across any thickness t is

$\tau = \dfrac{q}{t} = \dfrac{T}{2At}$

Thus, torque T ca also be expressed as

$T = 2At\tau$