Rolling Offset Conduit Calculator
Enter the vertical rise and sideways roll to turn them into one true offset. The result gives the bender clocking angle, mark spacing, shrink, and adjusted subtract-shrink reference for two equal bends.
True rolling offset (in)10
- True offset on a tape
- 10 in
- Bender clocking angle (degrees)
- 53.13
- Offset multiplier
- 2
- Distance between bend marks (in)
- 20
- Exact geometric shrink (in)
- 2.68
- Field-table shrink (in)
- 2.5
- Adjusted layout reference (in)
- 45.5
- Field layout summary
- Clock the bender 53.1° from the vertical-rise plane toward the horizontal roll. Make two 30° bends with marks 20 in apart; allow 2 1/2 in of field-table shrink and use 45 1/2 in as the adjusted subtract-shrink reference.
Exact trigonometric values and customary rounded field-table constants intentionally produce slightly different layouts.
How to use this calculator
- Measure the vertical rise and horizontal roll between the starting and ending conduit planes.
- Choose the angle for each of the two equal bends.
- Enter the distance from the conduit end to the target offset point.
- Choose field-table constants or exact trigonometry, then transfer the clocking, spacing, and adjusted reference to the conduit.
How a rolling conduit offset is calculated
A rolling offset changes elevation and moves sideways at the same time. Those two measured components form the legs of a right triangle. Let V be vertical rise, H be horizontal roll, and O be the true offset that the pair of bends must produce:
O = sqrt(V^2 + H^2)
The bender is rotated from the vertical-rise plane toward the horizontal-roll direction. Its clocking angle is:
clocking angle = atan2(H, V)
Using atan2 makes the zero-component cases clear. No roll gives 0 degrees and an ordinary vertical offset. No rise gives 90 degrees. The calculator uses magnitudes, so the installer must carry the actual left-or-right roll direction from the measured layout.
Bend spacing and shrink
For two equal bends at angle theta, the exact offset multiplier is 1 / sin(theta). The centerline distance between bend marks is:
spacing = O x multiplier
The exact geometric shrink along the original run is:
exact shrink = O x tan(theta / 2)
Field tables round these relationships to constants that are quick to use. The included multipliers for 10, 15, 22.5, 30, 45, and 60 degrees are 6.0, 3.9, 2.6, 2.0, 1.4, and 1.2. Their shrink allowances per inch of true offset are 1/16, 1/8, 3/16, 1/4, 3/8, and 1/2 inch. Field mode uses both customary constants; exact mode uses trigonometry. Both shrink results remain visible so the difference is deliberate rather than hidden.
With target distance D and the shrink selected by the layout method, the subtract-shrink reference is A = D - shrink. A negative answer means the allowance reaches behind the conduit end. The tape values in the instruction are rounded to the nearest 1/16 inch only after all geometry has been calculated at full precision.
What changes the layout most
Rise and roll combine into the true offset, so increasing either one increases both mark spacing and shrink. A shallow bend angle spreads the marks farther apart but uses less shrink. A steeper angle shortens the required spacing and increases shrink. Clocking depends on the ratio between roll and rise, not their overall size.
What the calculation leaves out
This is ideal centerline geometry. It does not compensate for the bender's centerline radius, shoe reference marks, conduit diameter, gain conventions, springback, material stiffness, or the operator's bend sequence. It also cannot determine whether the clocking direction should be left or right from unsigned measurements. Use the bender manufacturer's marks and instructions, keep both bends in one plane, and check a test piece before repeating the layout.
Worked example
For a 6 inch rise and 8 inch roll, the true offset is sqrt(6^2 + 8^2) = 10 in. The clocking angle is atan2(8, 6) = 53.13 degrees from the vertical-rise plane toward the roll.
With two 30 degree bends and the standard field table, the 2.0 multiplier puts the marks 20 inches apart. Field shrink is 10 x 1/4 = 2.5 in, so a 48 inch target becomes a 45.5 inch subtract-shrink reference. Exact 30 degree spacing is also 20 inches, while exact shrink is about 2.679 inches; exact mode would therefore use a 45.321 inch reference.
Common questions
How do you calculate the true offset for a rolling conduit bend?
Treat the vertical rise and horizontal roll as the legs of a right triangle. Square each measurement, add the results, and take the square root. A 6 inch rise and 8 inch roll therefore make a 10 inch true offset.
What angle should the bender be clocked for a rolling offset?
Measure from the vertical-rise plane toward the horizontal-roll direction. The clocking angle is the inverse tangent of roll divided by rise, evaluated with both components so a zero rise correctly gives 90 degrees. The calculator gives the magnitude; the physical layout determines whether to rotate left or right.
What multiplier is used for a 22.5-degree or 30-degree offset?
Common field tables use 2.6 for 22.5 degrees and 2.0 for 30 degrees. Exact trigonometric multipliers are about 2.613 for 22.5 degrees and exactly 2.0 for 30 degrees. Multiply the true rolling offset by the selected multiplier to get the distance between bend marks.
How is conduit shrink calculated on a rolling offset?
Apply shrink to the true offset, not separately to rise and roll. Exact geometric shrink is the true offset times the tangent of half the bend angle. Field tables instead use a customary per-inch constant, such as 1/4 inch per inch of offset for a 30-degree bend.
Should I use exact trigonometry or standard electrician multipliers?
Use the method that matches the bender marks and the layout practice for the job. Field constants are convenient rounded values, while exact trigonometry describes ideal centerline geometry. The small difference can matter on a large offset, so do not mix spacing from one method with shrink from the other.
Why can the finished bend differ from the calculated layout?
Real bends depend on conduit material and size, the bender radius and reference marks, springback, shoe wear, and operator technique. The formula describes ideal centerline geometry and cannot remove those variables. Verify the marking convention and make a test bend when fit or material cost matters.