Why Can Rebar Lengths Differ in Revit? A Practical Solution

[Rediet366]

Why Can Rebar Lengths Differ in Revit? A Practical Solution

Have you ever noticed that the calculated length of a hooked rebar in Revit can differ from the length you get when you add the individual bar segments and hook lengths?

I came across this while working with reinforcement in a BIM workflow where rebar information needs to move from the model into schedules, quantities, fabrication documentation, and eventually to site.

The difference may look small on a single bar, but when the same difference occurs across hundreds or thousands of reinforcement elements, it can become significant.

### The difference

For example:

Revit calculated length:
3,190 mm

Segment-based calculation:
500 + 2,280 + 500 = 3,280 mm

Other examples:

3,190 mm → 3,280 mm
4,110 mm → 4,200 mm
1,700 mm → 1,760 mm

The important point is that these values represent different ways of reporting the bar length.

Revit provides the standard calculated bar length, while the rebar shape also contains individual segment and hook parameters that can be used for project-specific calculations. Autodesk's documentation confirms that rebar shape parameters include A, B, C... segment dimensions as well as start/end hook lengths.

### My approach

Instead of manually correcting the value in a schedule, I created an additional length parameter inside the Rebar Shape Family.

I called it:

T-Length

The parameter is formula-driven and sums the required segment and hook parameters.

For the example shown in the video, the basic formula is:

T-Length = A + B + C + D + E + F + G + H

Where required, the hook lengths can be assigned to the appropriate shape parameters, for example:

D = Start Hook Length
E = End Hook Length

This allows the segment-based value to be calculated automatically as part of the rebar shape definition.

### Workflow

1. Open the required Rebar Shape Family.
2. Open the Rebar Shape Parameters.
3. Assign the required hook lengths to the appropriate shape parameters.
4. Create a new Length parameter, such as T-Length.
5. Add the formula using the required segment parameters.
6. Load the modified shape back into the project.
7. Use the parameter in Rebar Tags and Schedules.

The result is an additional segment-based length that can be reported alongside Revit's standard bar-length information.

### Why I find this useful

When adopting BIM from traditional 2D reinforcement detailing, the challenge is not only creating the 3D geometry.

The information contained in the model also needs to be useful downstream:

2D Detailing
↓
BIM Model
↓
Rebar Schedules
↓
Quantity / BOQ
↓
Fabrication
↓
Site

A small difference in one bar can therefore become much more important when it is repeated throughout a project.

This approach is not intended to replace Revit's standard Total Bar Length in every workflow. It provides an additional project-specific value when a segment-based length is required for scheduling, quantity, fabrication, or site workflows.

### Video

I have attached the same short video showing the complete setup of the custom parameter inside the Rebar Shape Family.

The attached graphic also shows the comparison between the standard calculated length and the segment-based result.

I'd be interested to hear from other Revit Structure users:

Have you encountered a similar difference between calculated rebar length and your project's required segment-based length?

How do you handle it in your schedules or fabrication workflow?

[jay_colc]

If you use a regional rebar family you have a Total Bar Length Parameters built-in which correlate from a Shape Code based family against a Straight Bar with Hooks

You also have to be aware of rounding for different shapes and those with/without hooks