The effect of tube diameter (d) on wall shear stress (τw) measurements using Preston tubes has been investigated in a zero pressure gradient turbulent boundary layer. Five different outside diameter tubes of 1.46, 1.82, 3.23, 4.76 and 5.54 mm, corresponding to (d/δ) of 0.022, 0.027, 0.048, 0.071 and 0.082 were used to measure τw at Reynolds numbers based on momentum thickness (Rθ) of 2800 to 4100. The calibration curves of Patel (1965) and Bechert (1995) are both dependent on the tube diameter. The maximum difference in the τw measurements from the different tubes using Patel’s calibration is about 8%, while Bechert’s calibration gives a maximum difference of approximately 18%. Based on the present measurements, a new Preston tube calibration equation that is less sensitive to the tube diameter is proposed.