Here are the details out of the Revit help.
Revit Architecture measures the perimeter of a room at a defined distance above the base level of the room. This distance is the computation height. It is used to compute the room perimeter, area, and volume. By default, the computation height is 4’ or 1200 mm above the base level of the room (the height of the default cut plane).
For buildings with vertical walls, the default computation height usually gives accurate results. However, if a building includes sloped walls or other atypical features, you may need to adjust the computation height to achieve more accurate room areas and volumes.
For example, the following drawing shows a section of a room with a sloped wall. The dotted line across the lower part of the room indicates the current computation height. (This line displays when you select the room.) Revit Architecture uses the perimeter of the room at the computation height when computing the room area and volume.
Changing the computation height affects the room perimeter, and thus the room area and volume. For example, the following drawing shows the same room, but the computation height has been moved lower (indicated by the dashed line). The room tag shows the changed room area and volume.
The computation height is defined as a parameter of a level family. If needed, you can change this parameter. You can also create multiple level families that use different computation heights. For example, you may want to define one level family for stories and another level family for plenums.
Changing the Computation Height
- Open a section view (or any view in which you can see defined levels).
(Optional) Make rooms visible in the section view.
- Right-click a level datum in the view, and click Element Properties.
- In the Element Properties dialog, click Edit/New.
- In the Type Properties dialog, under Dimensions, do one of the following:
- To use a default computation height, select Automatic Room Computation Height.
- To specify a computation height, clear Automatic Room Computation Height. For Computation Height, enter the distance above the base level to use when computing the room area and perimeter.
If the room includes a sloped wall, consider using a computation height of 0 (zero).
- Click OK.
The change in computation height affects all rooms that use this level type family for its base level.
Automatic Computation Height
In most cases, the automatic computation height is 4’ (1200 mm) above the base level of the room. In some situations, however, the automatic computation height is different.
- Suppose the top of a room-bounding wall is lower than 4’ (1200 mm above the base level of the room), and it is joined to a room separation line. In this case, the computation height defaults to 0 (zero) to bound rooms by joined lower walls for that level.
- Suppose the top of a room-bounding wall is higher than 4’ (1200 mm), and its bottom is above the base level of the room. In this case, the automatic computation height is 4’ (1200 mm).