Box Offset Calculator | Conduit Bend Marks, Travel & Shrink

Result

Formulas

\(D_{\text{offset}}=\text{box offset depth},\quad \theta=\text{bend angle},\quad M_{\text{first}}=\text{first bend mark}\)
\(M_{\text{offset}} = \frac{1}{\sin(\theta)}\)
\(T = D_{\text{offset}} \times M_{\text{offset}}\)
\(M_{\text{second}} = M_{\text{first}} + T\)
\(S_{\text{estimated}} = D_{\text{offset}} \times \tan\left(\frac{\theta}{2}\right)\)

A box offset is a two-bend conduit offset used when a raceway must clear the face, depth, or obstruction of an electrical box, coupling, fitting, or similar transition point. The layout value needed before bending is the travel between bends—the developed conduit distance from one bend mark to the next.

The worksheet uses Box offset depth, Bend angle, and First mark to establish the second bend location. Distance to box remains visible as a reference measurement for the overall conduit run, but it does not change the offset travel calculation.

For the entered values, the calculator produces:

Second mark: 18 in
Travel between bends: 8 in
Offset multiplier: 2 x
Shrink estimate: 1.0718 in

The second mark is placed along the conduit from the same reference used for the first mark. In the example, the first bend mark is 10 in from the reference point and the second bend mark is 18 in from that same point.

Box Offset Geometry

A conduit box offset creates a parallel displacement using two equal bends in opposite directions. The required displacement is entered as Box offset depth. The required distance between the two bend marks depends on the angle selected for each bend.

The calculator uses:

\(\displaystyle \text{Travel between bends} = \frac{\text{Box offset depth}}{\sin(\text{Bend angle})}\)

The offset multiplier is the reciprocal of the sine of the bend angle:

\(\displaystyle \text{Offset multiplier} = \frac{1}{\sin(\text{Bend angle})}\)

Therefore:

\(\displaystyle \text{Travel between bends} = \text{Box offset depth} \times \text{Offset multiplier}\)

For a 30-degree bend angle:

\(\displaystyle \frac{1}{\sin(30^\circ)} = 2\)

A 30-degree box offset therefore uses a 2 x multiplier. Every 1 inch of required offset depth requires 2 inches of travel between the bend marks.

Calculator Inputs

Field	Electrical layout use
Box offset depth (in)	The perpendicular conduit displacement needed to clear the box, fitting, or obstruction
Distance to box (in)	The distance from the selected reference point to the box; retained for layout context
Bend angle (deg)	The angle used for each of the two bends
First mark (in)	The reference distance to the first bend mark

The Box offset depth is not the overall length of conduit to the box. It is the depth or side-step the conduit must make. Measuring the actual clearance requirement is necessary because a nominal box depth may not reflect cover thickness, connector position, knockout orientation, mounting hardware, or an obstruction adjacent to the enclosure.

The First mark is the established location of the first bend on the conduit. Once the calculator determines the required travel, that travel is added to the first mark to locate the second bend mark.

\(\displaystyle \text{Second mark} = \text{First mark} + \text{Travel between bends}\)

Calculation Example

Use the following layout values:

Input	Value
Box offset depth	4 in
Distance to box	30 in
Bend angle	30°
First mark	10 in

Travel Between Bends

At a 30-degree bend angle, the multiplier is 2.

\(\displaystyle \text{Travel} = 4\text{ in} \times 2 = 8\text{ in}\)

The conduit requires 8 in of travel between bends.

Second Bend Mark

\(\displaystyle \text{Second mark} = 10\text{ in} + 8\text{ in} = 18\text{ in}\)

Mark the first bend at 10 in and the second bend at 18 in, measured from the same reference end or layout point.

Shrink Estimate

The worksheet reports Shrink estimate: 1.0718 in for this geometry. Shrink is the reduction in the conduit’s effective run length caused by forming the offset. It affects final box alignment when the conduit is being cut or fitted to a fixed endpoint.

The stated Distance to box used: 30 in provides run context, but the calculator does not use that measurement to determine the 8-inch travel between bends. The physical fit-up still depends on how the first mark was established, where the bender reference is placed, and how the box connection is measured.

Bending Layout and Verification

Make both bends at the entered Bend angle and maintain the same conduit orientation between bends. A box offset requires the bends to oppose each other; reversing the second bend incorrectly, rotating the conduit, or placing the bender on the wrong side of the mark can create a dogleg rather than a parallel offset.

Verify the completed offset against the actual installation before cutting a fixed-end conduit run or making a final connection. Confirm:

The box or fitting depth and actual clearance requirement
The reference point used for First mark
The direction of both bends
The bender’s arrow, star, rim notch, or other manufacturer-specific reference
Conduit springback and the bender’s actual bend behavior
Available straight length on both sides of the offset
Box connector, coupling, locknut, and enclosure geometry

The calculator provides a geometric layout estimate. It does not determine bender deductions, take-up, conduit cut length, raceway fill, conductor fill, conductor pulling clearance, or whether the final raceway arrangement complies with project requirements or AHJ inspection expectations.

Does distance to box change the offset travel?

The entered distance is shown for layout context. In this simple model, offset travel is determined by box offset depth and bend angle.

Does this include the box fitting take-up?

No. Confirm fitting dimensions, bender take-up, and the field reference convention separately.