Specter provides you with various geometric measurements such as surface areas and volumes. These can either be calculated automatically or transferred from the IFC model. In this series of articles, we explain how these values are generated, what you should pay attention to, and why the same “surface area” is not always to be understood in the same way.

In this article, we will show you:

Which dimensions specter calculates automatically and which are taken from the IFC model.
How to recognize the differences between calculated and transferred values.
What to look out for when interpreting the same unit of measurement.

1. Overview: What are IFC Base Quantities?

IFC Base Quantities (also known as Qto) are standardized quantity specifications that can be provided by the CAD modeler when exporting a model. They provide information on the length, area, or volume of an object, for example. These dimensions are stored in a structured manner in the IFC data model and depend on the respective IfcType class (e.g., IfcWall, IfcBeam, etc.).

2. What sizes are available?

The following fields are typical IFC BaseQuantities that Specter can import, provided they are contained in the model or in the IFC file:

(The “Field” column describes the partially manual designation of the dimension description from the IFC: IFCQUANTITYVOLUME('NetVolume',$,$,2.09249999999999))

Field - Description	Type	Description
Length	IfcQuantityLength	Length of the object (e.g., wall length)
Height	IfcQuantityLength	Height of the object (e.g., floor height, component height)
Width	IfcQuantityLength	Width of the object (e.g., wall thickness, component width)
Perimeter	IfcQuantityLength	Perimeter / outer length (e.g., polygon outline of an area)
FinishFloorHeight	IfcQuantityLength	Finished floor height (FFH)
FinishCeilingHeight	IfcQuantityLength	Finished ceiling height (room height to bottom edge of ceiling)
Area	IfcQuantityArea	General area without differentiation
NetArea	IfcQuantityArea	Net area taking into account openings
GrossArea	IfcQuantityArea	Gross area: total area without deductions
NetFootprintArea	IfcQuantityArea	Net floor space (footprint) including deductions
GrossFootprintArea	IfcQuantityArea	Gross floor area (footprint) without deductions
NetSideArea	IfcQuantityArea	Net side area (e.g., walls) including openings
GrossSideArea	IfcQuantityArea	Gross side area without openings
NetFloorArea	IfcQuantityArea	Net floor area (taking openings into account)
GrossFloorArea	IfcQuantityArea	Gross floor area without deductions
NetWallArea	IfcQuantityArea	Net wall area (with openings)
GrossWallArea	IfcQuantityArea	Gross wall area (without deductions)
NetVolume	IfcQuantityVolume	Net volume (minus openings and cavities)
GrossVolume	IfcQuantityVolume	Gross volume of the property (total volume without deductions)

specter can support additional sizes such as CrossSectionArea, OuterSurfaceArea, GrossSurfaceArea, or NetSurfaceArea - all you need to do is define which dimension the respective size should be assigned to.

The following dimensions apply as standard for the four classic components: wall, column, slab, and beam.

Maß	Beam	Column	Slab	Wall
Width			X	X
Height				X
Length	X	X
Net area			X
Outer surface area	X	X
Gross footprint area				X
Net volume	X	X	X	X
Net weight	X	X	X	X

For any further documentation on other components, see also:

6.1.5.15 Qto_WallBaseQuantities - IFC 4.3.2 Documentation

3. Important information regarding use

Whether this data is included in the IFC model depends on how the exporting CAD software has been configured. Not all software automatically exports base quantities, and sometimes they are missing entirely or are incomplete.

In addition, the definitions differ between tools: for example, “Length” may have a different meaning in Revit than in Archicad. The presence of openings (doors, windows) also influences the net/gross data—provided they have been modeled correctly.

In addition, an IfcQuantity can still contain “free text.” Example: IFCQUANTITYVOLUME(‘NetVolume’,$,$,2.09249999999999);

This “NetVolume” or, for example, “GrossVolume” is fairly standardized. This tells us whether it is net or gross.

4. How does specter handle this data?

If base quantities are included in the model, specter displays them in the info box as additional dimensions. They are then shown next to the automatically calculated values. By default, however, specter always uses the dimensions it calculates itself if no external data is available or reliable.

If desired, the source of the dimensions can be activated by a specter admin. The source is then displayed behind the dimension.

The two additional dimensions “Height at top” and “Area” are from so-called IFC properties.