You’ve learned about pressure loss in the drill string from the previous topic (Pressure Losses through Drill String Calculation). The general coefficient for internal diameter of pipe does not account the inside diameter of tool joint.

The general coefficient equation is shown below:

**Coefficient of The Bore of Drill Pipe and Tool Joint**

To get the accurate pressure loss calculation, the coefficient of a section of pipe must be accounted for both the pipe ID and the tool joint ID.

In general, each 100 ft of pipe, there is appoximately 6.7 ft of tool joint length (6.7% of total length) and 93.3 ft of drill pipe (93.3% of total length). Therefore, the general coefficient for internal diameter of pipe can be derived in to this following equation: