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:
What is the difference between gross and net floor space?
How Specter calculates both variants or imports them from the model.
What you should pay attention to when different floor space specifications are available.
1. What do net and gross mean in relation to area and volume?
Gross: Area or volume including all openings, protrusions, cavities, etc.
Net: Only the actual usable area or effective volume; openings are deducted (if modeled separately).
2. How does specter distinguish between these details?
All values calculated by specter are net values.
Openings (e.g., doors or windows) are only taken into account if they exist as separate elements in the model.
Unmodeled openings are included in the area/volume.
specter does not make any assumptions about “invisible” geometries—only what is present in the model counts.
3. When will gross values be available in Specter?
Gross dimensions are only available if the CAD software exports them as IFC Base Quantities or Properties.
Example: An IfcWall element can contain both a NetVolume and a GrossVolume.
4. How are the dimensions determined and what do they represent - dimensions at a glance
Height: Vertical extension of an object.
Length: Horizontal extension in the longer direction.
Width: Horizontal extension in the shorter direction. Note: These specifications can easily be misinterpreted – see examples with pillar and parapet.
Parapet:
Pillar:
Floor space
Gross: The total floor area of a property, including all openings. This is the maximum usable area, regardless of obstacles or cavities in the building component.
Net: The actual usable floor area, excluding openings such as recesses or unoccupied areas.
→ Image on the left (total floor area colored green).
→ Image on the right (only green areas, recesses are omitted).
Volume
Gross: total volume of an object, openings are ignored.
→ Image on the left
Net: volume minus modeled openings.
→ Image on the right
Side surface
Gross: all vertical surfaces, including openings.→ Image on the leftNet: vertical surfaces minus window and door openings.
→ Image on the right
(red = surfaces not taken into account)
(green = relevant surface(s)
Shell surface
Gross: Includes all external surface areas of an object, including all areas with openings.
→ Image on the left
Net: Only the usable external surfaces, i.e., excluding surfaces with openings such as windows or gaps.
→ Image on the right
Total area
Gross: Total area, including openings.
Net: Actual usable area, excluding openings.
5. Common misunderstandings
Gross values are often missing if the CAD does not export them – this is not an error on the part of specter.
Some CAD programs export gross and net values with incorrect labels.
Neither specter nor most CAD programs provide VOB-compliant billing values (exception: RIB iTwo).
6. Recommendations for everyday use
After splitting components, check the information in the info box—dimensions may differ there.
If in doubt, check the geometry in CAD to see whether openings are actually modeled.
If you need gross values, you must ensure that these are correctly exported as QTO in CAD.
7. Summary
specter always calculates net dimensions.
Gross values are only available if the CAD exports them.
Openings are only taken into account if they are actually included in the model.
For exact quantities, a clear distinction must be made between net and gross.
Practical example:
For a wall with a door opening, the following applies:
Gross side area = total formwork area including door opening.
Net volume = concrete volume minus the recess.
→ Relevant for formwork planning: gross value.
→ Relevant for concrete ordering: net value.