Another way to determine equivalent circulating density (ECD) is to use yield point.
This formula below is used to calculate the ECD and it is good for mud weight less than or equal to 13.0 ppg.
Where:
ECD is equivalent circulating density in ppg
MW is mud weight in ppg
Hold ID is inside diameter of hole in inch
Pipe OD is outside diameter of pipe in inch
YP is mud yield point
YP can be calculated by these following equations
YP = Reading at 300 rpm – PV
PV = Reading at 600 rpm – Reading at 300 rpm
Determine the ECD with the following information
MW = 9.2 ppg
Reading at 300 = 25
Reading at 600 = 40
Hole diameter = 6.2
Pipe diameter = 4
PV = 40 – 25 = 15
YP = 25- 15 = 10
ECD = 9.7 ppg
Additional articles about Equivalent Circulating Density (ECD) are as follows:
Equivalent Circulating Density (ECD) in ppg
Equivalent Circulation Density (ECD) with complex engineering equations
Effect of Frictional Pressure on ECD while reverse circulation
Effect of Frictional Pressure on ECD while forward circulation
Ref book: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition