The Dittus–Boelter equation (for turbulent flow) as introduced by W.H. McAdams is an explicit function for calculating the Nusselt number. It is easy to solve but is less accurate when there is a large temperature difference across the fluid. It is tailored to smooth tubes, so use for rough tubes (most commercial applications) is cautioned. The Dittus–Boelter equation is:
where:
: is the inside diameter of the circular duct
: is the Prandtl number
: for the fluid being heated, and  for the fluid being cooled.
The Dittus–Boelter equation is valid for
The Dittus–Boelter equation is a good approximation where temperature differences between bulk fluid and heat transfer surface are minimal, avoiding equation complexity and iterative solving. Taking water with a bulk fluid average temperature of , viscosity  and a heat transfer surface temperature of  (viscosity , a viscosity correction factor for  can be obtained as 1.45. This increases to 3.57 with a heat transfer surface temperature of  (viscosity ), making a significant difference to the Nusselt number and the heat transfer coefficient.