How it works
True offset O = √(rise² + roll²). Roll angle = atan(roll ÷ rise) from vertical. Travel = O ÷ sin θ. Run = O ÷ tan θ. Shrink = travel − run.
Start with why the rise alone is the wrong number. A conduit that has to climb 3 in and step 4 in sideways is not making a 3 in jog or a 4 in jog: it travels along the hypotenuse of the two, √(3² + 4²) = 5 in. At 30° that puts the second mark 10.00 in from the first. Treat the 4 in roll as a plain offset and you would mark 8.00 in; work from the 3 in rise and you would mark 6.00 in — either way the second bend comes up short and the far end of the stick misses its target. Once the rise and roll are combined, the bend itself is nothing exotic: it is an ordinary two-bend offset, made in a plane tilted atan(roll ÷ rise) away from vertical — 53.1° for the 3-and-4 case. That is why the roll angle is a result here rather than a footnote. It is a setup step: rotate the stick that far toward the roll side before the first bend, and hold the same rotation for the second. Shrink comes out of the same triangle as a subtraction. Inside the tilted plane the pipe spends the travel, O ÷ sin θ, to move forward only the run, O ÷ tan θ, so the free end pulls back by exactly the difference — 10.00 − 8.66 = 1.34 in for the example. The readout prints 1.25 in, and that is deliberate: at 30° a bender chart says a quarter inch per inch of offset, where the geometry says 0.268. We print the chart number because that is what your bender chart says; at 30° the two stay within a sixteenth until the true offset passes about 3½ in, and at 10° the chart rounds much harder — 0.0625 against 0.0875 — so on very shallow bends measure after the first one. Pipefitters read the same triangle with different words: travel is the centre-to-centre length an electrician calls the distance between bends, run is the advance, roll is the set or horizontal offset, and the true offset is the resultant. A fitter joining two 45° elbows gets the same travel and run from this readout and subtracts fitting make-up instead of shrink. Two checks come from the code tables. Each bend arc needs R × tan(θ ÷ 2) of straight pipe on either side of its mark, where R is the NEC Chapter 9, Table 2 radius for your conduit and trade size, so the travel must be at least twice that or the two bends overlap on the shoe; and the pair adds 2θ toward the 360° allowed between pull points. Once the bends are in, confirm the wires still fit with the conduit fill calculator.
Code references
- Right-triangle trigonometry (sine, tangent) OpenStax, Algebra and Trigonometry 2e (Abramson, 2021), §7.2 Right Triangle Trigonometry — sin t = opposite / hypotenuse, tan t = opposite / adjacent
- Pythagorean theorem (combining rise and roll) OpenStax, Prealgebra 2e (Marecek et al., 2020), §9.3 — in any right triangle a² + b² = c², where c is the hypotenuse
- Radius of conduit and tubing bends NEC 2023, Chapter 9, Table 2 — centerline bend radii by trade size
- EMT / IMC / RMC minimum bending radius NEC 2023, 358.24 / 342.24 / 344.24
- Maximum 360° of bends between pull points NEC 2023, 300.24 (general) — EMT 358.26, IMC 342.26(B), RMC 344.26(B)
FAQ
What is a rolling offset, and how is it different from a regular offset?
A regular offset jogs the conduit in one flat plane — straight up over a pipe, or straight sideways around a column — so one measurement describes it. A rolling offset is what you need when the obstacle is both above the run and beside it, such as a beam flange that sits higher and further out than the conduit's current line. The pipe has to move in two directions at once, so you measure the rise and the roll separately, combine them into the true offset, and make both bends in a plane tilted between vertical and horizontal. Marks, shrink and the bender setup all follow from that true offset, not from either measurement alone.
How far do I roll the conduit in the bender?
Rotate it atan(roll ÷ rise) from vertical, toward the side the conduit has to move. For 3 in of rise and 4 in of roll that is 53.1°, a little past halfway between straight up and lying flat. Set the rotation before the first bend and keep exactly the same rotation for the second; a straight line drawn along the top of the stick before you start makes the rotation easy to see and repeat. The classic mistake is rolling only the second bend: the two bends then sit in different planes, the far end comes out twisted, and the run will not line up with the box or strut it was meant to reach.
Why use the true offset instead of the rise for the multiplier and the shrink?
Because the multiplier and the shrink apply to the distance the bend actually covers, and in a rolling offset that is the hypotenuse, not either leg. With 3 in of rise and 4 in of roll at 30°, the true offset is 5 in, so the marks go 10.00 in apart and the shrink is 1.25 in. Working from the 4 in roll alone gives 8.00 in and 1.00 in; working from the 3 in rise alone gives 6.00 in and 0.75 in. Both put the second bend short of where it belongs, and the error grows with the size of the leg you left out.
What do travel, run and advance mean, and do plumbers and electricians mean the same thing?
Mostly, yes. Travel is the length of pipe between the two bends or fittings, measured centre to centre — the number an electrician marks as the distance between bends. Run, also called advance, is how far that travel carries you forward along the original line. Roll is the sideways leg, also called the set or horizontal offset, and the true offset is the resultant of rise and roll. The numbers agree across trades; the last step differs. A pipefitter subtracts the make-up of two elbows from the travel to get a cut length, while an electrician bending one stick subtracts shrink from the measured run.
Can I use two different bend angles for a rolling offset?
Not if the conduit has to leave parallel to the line it started on. An offset works because the second bend undoes the first: equal angles, opposite directions, the same roll. Make one bend 30° and the other 22.5° and the pipe leaves at 7.5° to the original run, which is a compound turn rather than an offset, and it drifts further from the wall or strut the longer it goes. Pick one angle for both bends. If 30° makes the travel too long for the space, step up to 45° for both rather than mixing the two.
My rise is zero and only the roll moved — is that still a rolling offset?
Strictly, no. With no rise the conduit only moves sideways, which is a flat kick, and the roll angle comes out at 90°: turn the stick a quarter turn so both bends go in the horizontal plane. The math is identical and the calculator handles it as is — the true offset is simply the roll. The mirror case is zero roll, a plain vertical offset with a 0° roll angle; there the marks and shrink match the conduit bend calculator to the hundredth of an inch, and that page also covers saddles, 90° stubs and back-to-back bends.
This calculator is provided for estimation purposes. Rolling-offset marks depend on bender shoe, spring-back and how accurately the roll is held between the two bends; always verify with a test bend, and confirm radius and 360° limits against the current NEC edition and local amendments with a licensed electrician before bending.