The VOB stipulates a specified opening measurement system. Depending on which trade the quantities are calculated for, there are different calculation rules especially in regard to the subtraction of openings (sub areas).
| Trades masonry, concrete and reinforced concrete works | Openings minor/equal 2,5 m² are ignored |
| Trades carpenter, wood construction, upholsterer, curtain wall, sealing, plumber, roof cladding and sealing | Openings minor/equal 2,5 m² are ignored |
| Trades tiles and plates, screed, flooring, wooden planks, asphalt, parquet works | Openings minor/equal 0,1 m² are ignored |
| Trades plaster and stucco, paint and varnish works | Openings, recesses, niches minor/equal 2,5 m² are ignored Openings, recesses etc. in floors minor/equal 0,5 m² are ignored |
| Trade natural stone works | Recesses, openings minor/equal 0.1 m² are ignored |
Initially, those rules have been defined to simplify the process of calculating quantities by hand, for example, to ignore smaller openings or to apply basic mathematical methods for calculating complex building parts.
As a CAD system allows a 100% precise quantity calculation, those VOB rules rather complicate the process, as they represent arbitrary rules which are only applied in specific, not always clearly defined cases. Therefore, a CAD-based quantity calculation cannot be performed automatically by the system in all cases, but requires also the interaction of the user.