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Module 6: Plumbing Math

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Measuring Pipe
A plumber must be able to measure pipe accurately and quickly. Measuring is basic to the plumbing profession and lies at the heart of many other trade skills as well.

Pipe like other construction materials, such as 2 × 4 lumber, comes in nominal sizes.

Nominal sizes are used for the purpose of general identification. The actual size of the piece will be approximately the same as the nominal size, but won’t be exactly the same.

In the fabrication and installation of piping systems, a plumber often uses center-to-center (C-C) measurements between two fittings. The length of the pipe and fitting are measured along centerlines. The extensions of the centerline of the pipe and the centerline of the fittings meet inside the fitting to create a center point.

Parts of Fittings
The terms face, center, and back are used to describe the parts of fittings.

These elements are important when you measure pipe length because they define the beginning and the ending points of the measurement.

Measuring Pipe
Click on each term associated with pipe fittings to know more:

Makeup
Thread makeup, also known as thread-in or thread engagements, is the distance that the pipe screws into the threaded fittings.

Takeoff
Fitting allowance, or takeoff, is the distance from the end of the pipe that goes into the fittings (face of fittings) to the center of the fittings.

Some joints, such as plastic glued or threaded joint, have takeoffs. Others such as cast iron, no-hub, and face-to-face flanged joints, do not have takeoffs.

To calculate fittings allowance measure the fittings from its center to its face. Then subtract the thread-in measurements.

Calculating Pipe Length
The measuring technique you use determines how you calculate the length of pipe that is to be cut. Fittings dimensions are used as part of the calculation in most pipe measuring techniques.

Center-to-Center Method:
To determine the length of pipe between two fittings using the center-to-center method. Subtract the fittings allowance from the center-to-center measurements.