Different equations are used depending on whether the contact angle is greater than or less than 90°:
Only the three required points - the two three-phase points and the apex - are sought in the drop shape analysis. An alternative method is the circle method, in which the whole contour of the drop is evaluated and is therefore more robust.
As the real drop shape deviates from the circular arc for larger contact angles and volumes, other methods should be used for contact angles greater than 20° and volumes of more than 1 µL.
G.L. Mack, D.A. Lee, J. Phys. Chem., 40, 1936, 169-176.