The multiplier is one over the sine
An offset is a right triangle drawn in steel. The depth you need is the opposite side, the distance between the two bends is the hypotenuse, and the bend angle is the angle between the hypotenuse and the run. So the distance between marks is the depth divided by the sine of the angle, and dividing by a sine is the same as multiplying by its reciprocal. That reciprocal is the multiplier every chart in the trade prints.
| Angle | 1 / sin | Chart multiplier | Shrink per inch of offset |
|---|---|---|---|
| 10° | 5.759 | 6 | 0.087" (about 1/16") |
| 22.5° | 2.613 | 2.6 | 0.199" (about 3/16") |
| 30° | 2.000 | 2 | 0.268" (about 1/4") |
| 45° | 1.414 | 1.4 | 0.414" (about 3/8") |
| 60° | 1.155 | 1.2 | 0.577" (about 1/2") |
The chart figures are the exact ones rounded, always upward for the shallow angles, which produces an offset a hair deeper than asked for. That is the forgiving direction: a conduit that clears the obstruction by an extra sixteenth is fine, and one that clears it by minus a sixteenth is scrap.
Shrink, and why the run comes up short
Bending consumes length. Before the bends, a straight conduit covers the full distance between the two marks along the axis of the run. After the bends, that same section is tilted, so its horizontal projection is shorter by the difference between the hypotenuse and the adjacent side. Written out, the shrink is the distance between bends times one minus the cosine of the angle, which is the same as the depth times the multiplier times one minus the cosine.
At 30 degrees that comes to about a quarter inch of shrink for every inch of offset depth. A 4 inch offset at 30 degrees eats a little over an inch of run. That is why a piece cut to the measured distance between two boxes ends up short after the offset is put in, and why the fix is to cut long by the shrink and trim afterward rather than to hope.
Shrink is the number most people never learn, because on a short run inside a panel it hides inside the slack. On a wall run between two fixed points it is the difference between a coupling and a wasted stick.
Saddles
A three-point saddle crosses something and returns to the original line — a pipe, a beam flange, another conduit. The standard field version uses a 45 degree bend at the centre and 22.5 degree bends either side, with the outer marks placed at 2.5 times the obstruction depth from the centre mark, and a shrink of roughly 3/16 inch per inch of depth. The 2.5 figure is a taught constant rather than a derived one, and it works because the geometry of the two outer bends is not a simple triangle.
A four-point saddle is two offsets back to back, and it is what you want when the obstruction is wide rather than tall — a duct, a beam, a group of pipes. Both offsets use the same angle and the same multiplier, the inner bends sit at the two edges of the obstruction, and the shrink is simply twice the shrink of a single offset. It is easier to lay out than a three-point saddle and easier to get right, at the cost of using more material.
Bending is a skill the chart does not contain
The numbers above are geometry and they are exact. What they do not include is the bender. Every hand bender has a mark to line up against, and which mark — arrow, star, tear-drop — depends on which bend you are making. Springback varies with the material and the wall thickness, so aluminium behaves differently from steel and thin-wall differently from rigid. The shoe deforms the tube slightly as it grips, and the amount depends on how hard you push and how fresh the shoe is.
The consequence is that everyone develops a personal correction, and it is worth a foot of scrap tubing to find yours. Bend a 4 inch offset at 30 degrees, measure what you actually got, and note the difference. That number will be more useful than any chart, including this one, for as long as you own that bender.
Electrical work on a building generally needs a permit and an inspection, and many jurisdictions restrict who is allowed to do it at all. An error in this part of a house burns houses down and kills people. This page sizes and estimates. It does not replace the judgement of a licensed electrician, and it gives no instruction for opening equipment or making connections.
Once the run is laid out, check what will go in it with the conduit fill calculator, and check the length against the voltage drop calculator before committing the route.
Questions people ask
What is the multiplier for a 30 degree offset?
Two. The distance between the two bend marks is twice the depth of the offset, because one divided by the sine of 30 degrees is exactly 2. That exactness is why 30 degrees is the everyday offset angle — the arithmetic is done in your head, a 3 inch offset gets marks 6 inches apart, and there is no rounding to argue about. The shrink at 30 degrees is about a quarter inch per inch of offset depth, so that same 3 inch offset shortens the run by roughly three quarters of an inch.
What is conduit shrink and do I have to account for it?
Shrink is the length the run loses because the bent section is tilted, so it covers less horizontal distance than the same length of straight tube did. It equals the distance between bends times one minus the cosine of the angle. On a short piece with slack at both ends you can ignore it. Between two fixed points — box to box, coupling to fitting — you cannot, and the symptom is a piece that measured right, bent right, and now will not reach. Cut long by the shrink amount and trim after bending.
Which angle should I use for an offset?
Thirty degrees is the default for good reasons: the multiplier is exactly 2, the shrink is a manageable quarter inch per inch, and the resulting bends are shallow enough that conductors pull through easily. Go shallower — 10 or 22.5 degrees — when there is plenty of straight run available and you want the gentlest possible pull, which matters on long runs with several bends. Go steeper — 45 or 60 — when the offset has to happen in a short distance, accepting that the pull gets harder and the shrink gets large. A 60 degree offset of 6 inches shrinks the run by three and a half inches.
How do I lay out a three-point saddle?
The standard field method uses a 45 degree bend at the centre of the obstruction and 22.5 degree bends either side, bent the opposite way, with the outer marks at 2.5 times the obstruction depth measured from the centre mark. Shrink is about 3/16 inch per inch of depth. Set the calculator above to three-point saddle and it produces those distances along with the mark positions from the end of the conduit. Worth knowing: a four-point saddle is easier to get right, uses more tube, and is the better choice whenever the obstruction is wide rather than narrow.
Why does my bend never come out at the marked depth?
Because the chart describes geometry and the bender introduces its own bias. Springback means the material relaxes a few degrees after the pressure comes off, and how much depends on the alloy and the wall thickness. The bender shoe grips over a length rather than at a point, so the effective bend location is not exactly at your mark. And the reference mark on the bender differs between bend types, which is the single most common source of an offset that comes out consistently wrong by the same amount. That consistency is the useful part: bend one in scrap, measure the error, and apply it as a correction every time. The adjustment field on this calculator exists for exactly that.