How to Figure Conduit Offset and Shrink
Why this matters
An offset is two equal bends that move a conduit sideways or up by a set distance and send it on parallel to where it started. It is the most common bend on any job that runs pipe along a wall or ceiling, and two numbers decide whether it lands: the distance between the two bend marks, and the shrink. Get the first wrong and the pipe jogs too far or not far enough. Forget the second and the offset finishes past the obstacle it was meant to clear, so the stick hits the beam, the box or the duct an inch or two before the bend starts.
Neither number takes more than one multiplication. This card covers where both come from, how to place the marks so the offset finishes exactly at the obstacle, and how to check the bend before the stick goes up. Who may install conduit is set by your state and local licensing rules; this card is the geometry, not a license.
The two constants, and where they come from
Bend a stick at angle A, run it straight on the slant, then bend it back by the same angle. The slanted piece between the two bend marks is the hypotenuse of a right triangle whose short side is the offset depth. So:
- Distance between marks = offset depth x multiplier, where the multiplier is 1 / sin A (the cosecant).
- Shrink = offset depth x shrink per inch, where shrink per inch is tan(A / 2). It is the difference between the slanted length you used and the ground the slant actually covers along the run.
Bending tables print rounded versions of both. Here is the set most hand-bender tables carry, with the exact values beside them so you can see what the rounding costs:
ANGLE MULTIPLIER SHRINK PER INCH OF OFFSET
table exact table exact (decimal in)
10 deg 6.0 5.76 1/16 0.087
22.5 2.6 2.61 3/16 0.199
30 deg 2.0 2.00 1/4 0.268
45 deg 1.4 1.41 3/8 0.414
60 deg 1.2 1.15 1/2 0.577
Marks apart = offset x multiplier
Shrink = offset x shrink per inch
Example: 6 in offset at 30 deg
6 in x 2.0 = 12 in between marks
6 in x 1/4 = 1-1/2 in of shrink
Two of those rounded constants move the result enough to matter. At 10 degrees the table multiplier of 6.0 runs 4 percent long, so a 6 in offset figured on 6.0 puts the marks 36 in apart and actually jogs 36 x sin 10 = 36 x 0.174 = 6.25 in, a quarter inch deep. At 60 degrees, 1.2 against an exact 1.155 does almost the same: 6 x 1.2 = 7.2 in apart, 7.2 x 0.866 = 6.24 in of jog. On a parallel run beside another pipe, a quarter inch shows. Where it matters, use the exact multiplier and round only the final mark distance, per Write the Calculation Down: The Field Math Habit.
The shrink column runs the other way at the shallow end: 1/16 per inch at 10 degrees is under the exact 0.087, so at 10 degrees the table understates shrink. At 30 degrees the table's 1/4 is 0.018 in per inch light; on a 6 in offset that is about 1/8 in, which you will see in the worked example below.
Step 1: Measure the offset depth and the distance to the obstacle
Two measurements, both taken from fixed things:
- Offset depth: how far the conduit has to move, measured square to the run, from the centerline where the pipe is now to the centerline where it has to be. Measuring from the back of the pipe on both ends gives the same number as long as the pipe size does not change.
- Distance to the obstacle: along the run, from a fixed reference on the stick (its end, a coupling, the box connector) to the face of the obstacle where the offset must already be finished.
Why: both later numbers multiply off the depth, so an eighth of an inch of error in the depth comes out as a quarter inch of error in the mark spacing at 30 degrees and three quarters of an inch at 10 degrees.
What breaks if you skip it: measuring the depth to the surface of the obstacle instead of to where the pipe's centerline must sit leaves the pipe touching the obstacle when it arrives. Add the clearance you want before you multiply.
Step 2: Pick the angle from the space you have
The angle is a trade-off between length and sharpness:
- Shallow angles (10 and 22.5 degrees) need a long run between marks and shrink very little. They pull wire easily and look clean, but a 6 in offset at 10 degrees needs 34-1/2 in between marks using the exact multiplier, which many runs do not have before the obstacle.
- 30 degrees is the default because both constants are easy to carry in your head: twice the depth between marks, a quarter inch of shrink per inch.
- 45 and 60 degrees fit tight spaces and cost the most shrink, and every degree counts against the bend limit between pull points.
That limit matters more than people expect. NEC 358.26 for EMT, as adopted by your jurisdiction, allows no more than the equivalent of four quarter bends, 360 degrees total, between pull points; the other raceway articles carry matching sections. A 30 degree offset spends 60 degrees, a 45 degree offset spends 90. Two 45 degree offsets plus two 90s is already at the limit.
What breaks if you skip it: a run that passes on paper and then needs a pull point added, or a wire pull that strips insulation because the run carries more bend than anyone counted.
Step 3: Multiply for the distance between marks
Distance between marks = depth x multiplier. Write the units on the line: 6 in x 2.0 = 12 in.
Why: this is the length of the slanted section measured along the stick. It has nothing to do with how far along the run the offset reaches.
What breaks if you skip it: eyeballing the second bend from the first produces an offset that is right in angle and wrong in depth, and an unequal pair of bends sends the stick off at a slight angle instead of parallel.
Step 4: Add the shrink to the distance to the obstacle, then mark back
This is the step that lands the offset on the obstacle instead of past it. With the reference end fixed and the offset bent toward the obstacle:
- Far mark = distance to obstacle + shrink, measured from the reference end.
- Near mark = far mark minus the distance between marks.
The reason is geometric. The first bend stays where it was marked. The slanted section covers less ground along the run than its own length, and the shortfall is exactly the shrink. So the second bend lands one shrink closer to the reference end than its mark. Adding the shrink up front moves it back out to the obstacle face.
What breaks if you skip it: the offset finishes one shrink short of where you wanted it. On the 6 in, 30 degree example that is 1-1/2 in, enough for the straight part of the stick to hit a beam flange square on.
When you do not add it: if you are working from the obstacle back toward a free end that will be cut to length afterward, measure from the obstacle side and let the shrink come off the waste end. The shrink still exists; it just lands on the piece you were going to cut anyway.
Step 5: Bend the pair
Line the first mark up on the bender's reference for an offset as the bender's own instructions describe (hand benders differ in which mark they use for offsets), bend to the angle, then rotate the stick exactly 180 degrees and bend the second mark to the same angle. A level or a pencil line run down the top of the stick before the first bend keeps the rotation honest.
Why: the 180 degree roll is what puts both bends in one plane. A stick rolled 170 degrees produces a dog leg, a slight twist that shows as the far end swinging off line.
What breaks if you skip it: a dog-legged offset that will not sit flat against the wall, which usually gets "fixed" by kicking it, adding bend the run did not have.
Worked example: dropping under a duct
An EMT run along a ceiling has to pass under a rectangular duct. The bottom of the duct sits 5-1/4 in below the pipe's current centerline, and the installer wants 3/4 in of clearance below it, so the offset depth is 5-1/4 + 3/4 = 6 in. The duct's near face is 48 in from the end of the stick, which goes into a coupling. Angle: 30 degrees, plenty of room.
Depth 5-1/4 in + 3/4 in clearance = 6 in
Marks apart 6 in x 2.0 = 12 in
Shrink 6 in x 1/4 = 1-1/2 in
Far mark 48 in + 1-1/2 in = 49-1/2 in
Near mark 49-1/2 in - 12 in = 37-1/2 in
Check where the second bend actually lands. The first bend sits at 37-1/2 in along the run. The slanted section is 12 in long and covers 12 x cos 30 = 12 x 0.866 = 10.39 in of run. 37.5 + 10.39 = 47.89 in. The offset finishes 48 - 47.89 = 0.11 in before the duct face, about 1/8 in early, which is the table's 1/4 in shrink running short of the exact 0.268 x 6 = 1.61 in. Early is the safe side here: the pipe is already down when it reaches the duct.
Check the depth. The slant drops 12 x sin 30 = 12 x 0.5 = 6.00 in. At 30 degrees the multiplier is exact, so the depth is exact.
Check the stick is long enough. The stick lost 1-1/2 in of reach along the run to shrink. If the far end has to reach a box at a fixed distance, cut it after the bend, not before.
What would change if the space were tight: say a sprinkler branch sits 10 in before the duct face, so the whole offset has to fit in the last 10 in of run. At 30 degrees the slant covers 10.39 in, which does not fit. At 45 degrees with the exact multiplier the marks are 6 x 1.414 = 8.49 in apart, the slant covers 8.49 x 0.707 = 6.00 in of run, and shrink is 6 x 0.414 = 2.48 in, so the far mark goes at 48 + 2.48 = 50.48 in and the near mark at 50.48 - 8.49 = 41.99 in. It fits, and the run now carries 90 degrees for the offset against 60, which counts toward the 360 degree limit.
How to check the bend before it goes up
- Lay it on the floor and measure the depth from a straightedge along the first leg to the centerline of the second leg, at the far end of the second leg rather than at the bend. A depth correct at the bend and wrong a foot later means the legs are not parallel.
- Measure from the reference end to where the second bend ends. It should read the distance to the obstacle, within the 1/8 in the shrink rounding allows at 30 degrees.
- Sight down the stick. Both legs and the slant should sit in one plane. A twist means the second bend was not rolled 180 degrees, and the fix is a new stick rather than a kick.
The usual slip is not the multiplication. It is adding the shrink to the near mark instead of the far one, or forgetting it, and both come out as an offset that finishes 1-1/2 in out of place. Measuring the finished bend against the obstacle distance catches both before the strap goes on.
References
- NEC 358.26 (EMT, bends in one run), in the edition your jurisdiction has adopted; other raceway articles carry matching bend-limit sections
- The bender manufacturer's instructions for offset alignment marks
- See related: Pipe Offset Constants for Every Fitting Angle
- See related: Write the Calculation Down: The Field Math Habit
- See related: Units Carried Through Every Line of a Calculation