When you have Flacs simulation results including 3D output for the velocity vector (VVEC)
Use the r3file utility like this (example on Linux, similar on Windows):
run r3file r3010101.dat3 region=0,10,-10,6,5,30 name=AVERAGES force
In the output file a3010101.AVERAGES you can find information about the open and total volume of your specified region. The blocked volume will then be the total volume minus the open volume. For more details, check the guidance in the section "Air change rate" in the FLACS User's Manual.
When you have run a Flacs simulation with a volume monitor region
For a standard gas monitor region, which obviously has to be the same region as the one you want to know the open volume of, the output file rt010101.FUEL (example when the job number is 010101) contains the information:
# NET VOLUME = 5195.4 [m3]
# TOTAL VOLUME = 5474.0 [m3]
The blocked volume within the region will then be TOTAL VOLUME minus NET VOLUME (i.e. the NET VOLUME is the same as open volume).
When you have a custom volume monitor region, the output file rt010101.MON.custom01 (example the custom monitor region with the name "custom01") will contain similar information for NET VOLUME and TOTAL VOLUME. For more details check the guidance in the section "Custom gas monitor regions" in the FLACS User's Manual.
The reported open volume of your region may depend to a certain degree on the chosen grid resolution and the porcalc version employed. It is recommended to use the default Porcalc (porcalc_64) version for such estimates.