How to read the multiplier table
An offset moves a pipe or conduit run sideways to a parallel line. Measure the offset centre to centre between the two lines, then multiply it by the travel multiplier for the fittings you are using. The answer is the centre-to-centre length of the angled piece. Multiply the same offset by the run multiplier to find how far the offset advances along the line, which tells you where the first fitting has to go.
Worked example: a 14 in offset with two 45° fittings needs 14 × 1.414 = 19.799 in of travel, or 1' 7-13/16" to the nearest sixteenth, and advances 14 in along the line.
Where the multipliers come from
The offset, run and travel form a right triangle with the travel as the hypotenuse. With the fitting angle θ, the offset is the side opposite the angle, so travel = offset ÷ sin θ and run = offset ÷ tan θ. That makes the multipliers exact trigonometric values: 1 ÷ sin 45° is the square root of 2, and 1 ÷ sin 30° is exactly 2. This page computes them from those formulas and rounds only for display.
Choosing a fitting angle
Plumbers mostly use 45° (1/8 bend) fittings for drainage offsets because they keep flow smooth and travel reasonable. 22-1/2° (1/16 bend) fittings give a gentler offset over a longer run. 60° (1/6 bend) fittings make a short, tight offset. Electricians bending EMT use 30° often because its travel multiplier of 2.000 is easy to work with in the head, and 10°, 22-1/2° and 45° for other jobs.
Common pipe offset mistakes
- Measuring the offset to the edge of the pipe instead of the centre line.
- Cutting the pipe to the travel length without subtracting fitting make-up.
- Using the run multiplier in place of the travel multiplier at angles other than 45°.
- For conduit, forgetting shrink: an offset shortens the overall length of the stick slightly, so add the shrink to your mark when bending toward an obstruction.
Drainage offsets must still follow your plumbing code's rules for fitting types and changes of direction.