Using measurements, many plastic plugs allow the use of different screw diameters, enabling a wide selection of screws to be applied.  (Example: fixing Ø 8 mm – possible screw Ø 4,5 mm, 5 mm and 6 mm). The highest working load of a fixing is achieved with the largest possible screw Ø. If a smaller screw Ø is used, the working loads are correspondingly lower.  This must be taken into account.

The type of screw thread frequently plays a role. Chipboard screws have a thinner screw core than, for example, wood screws and hangar bolts with the same diameter. This can affect the expanding force with which the fixing expands against the drill hole.  Metric threaded screws are almost exclusively designed for metal fixings. 

In order to function properly, many fixings require a screw with a minimum length  (e.g. for knotting in perforated building materials. The following tool is meant to provide assistance with the calculation.

Determining the minimum screw length*
Fixing length 50 mm
+ Thickness of the object to be mounted  (mounted object) 10 mm
+ Screw diameter 6 mm
+ Wall layers possibly non-supporting 10 mm
Screw length = 76 mm
Round up the calculated screw length to the next standard length! = 80 mm
(*) Example standard fixing