A pipe offset is calculated when a pipe is altered in both the vertical and horizontal planes of a piping system. This type of occurrence is known as a "rolling offset." Following Pythagoras' theorem, the first number, or true offset, is calculated by taking the offset squared and adding the rise squared to obtain the true offset squared. Take the square root of the answer to obtain the offset.
Once the true offset is known, the pipe fitter can utilize a table to find out the setback and diagonal center. Most fitting constants are 45 degrees in the plumbing trade. If the fitting angle is 60 degrees, the diagonal for the pipe is equal to the true offset times 1.155. If the fitting angle is 45 degrees, the true offset is multiplied by 1.414 to obtain the diagonal. For any fitting angle that is 22.5 degrees, the true offset is multiplied by 2.613 to get the answer for the diagonal.
The setback for fitting a pipe equals the true offset multiplied by 0.577 for a fitting angle of 60 degrees. The true offset multiplied by 1.000 equals the setback for a 45-degree fitting angle. Multiply the true offset by 2.414 for the setback for a 22.5-degree fitting angle. Using the above calculations, the true offset for a 45-degree fitting is 14.42 inches and the same number for the setback.