Volume to Mass conversions are required when creating a new Waste Fraction, as this allows the system to normalise waste data by either a mass unit or a volume unit altogether.
We would recommend that you use the WRAP volume-to-mass conversion tool here when reviewing what your volume to mass conversion is: https://www.researchgate.net/profile/Manoj_Tiwari5/post/Is-anyone-familiar-with-WRAP-conversion-factor-for-construction-waste/attachment/59d64362c49f478072eabf64/AS%3A273809603530752%401442292704302/download/WRAP+waste+volume+to+mass+conversion+factors+-+July+2014.xlsx
Within the 'Conversion factors' tab you will notice an array of waste streams. Search for an appropriate description for your new Waste Fraction, and you will see the conversion factor stated in line with this waste stream.
If using this WRAP tool, then when inputting your volume-to-mass conversion, we would therefore suggest that, as per the below screenshot, A be stated as 'cubic metre', that B be stated as 'tonne' and C be stated as the conversion factor identified within the WRAP tool: