**The schedule of a steel pipe refers to the thickness of the wall of the pipe, with schedule 40 being a medium thickness considered standard for many applications.** The wall thickness of the pipe varies based on diameter of the pipe itself.

The wall thickness of steel pipes, as well as pipes made from other materials, is organized into what are referred to as Schedules. Schedules range in number from five to 160, with an additional XXH or Double Extra Heavy wall thickness for the thickest pipe walls. As the numbers grow, the wall thickness increases. Some schedules have similar numbers, but are followed by a lowercase "s" which designates the schedule for stainless steel.

Schedule 40 and Standard Weight, which is sometimes abbreviated to "Std," "STD" or "ST," have identical wall thickness until the nominal size of the pipe reaches 12 inches. Above this size, Standard Weight wall thickness remains at .375 inches even as the nominal size continues to increase. At the nominal size of 36 inches, while Schedule 40 pipe has a wall thickness of 0.750, Standard Weight wall thickness remains at 0.375 inches.

Velocity of oil flow for Schedule 40 pipes can be calculated by multiplying the flow of oil in gallons per minute by .3208 and dividing that product by the interior area of the pipe in square inches. The weight of a given length of Schedule 40 steel pipe can be calculated by multiplying the difference of the outside diameter of the pipe and the wall thickness by 10.68. Multiply that product by the wall thickness, which returns the weight of the pipe in pounds per foot.