Thanks for the great question! Usually our users would seek the advice of their structural engineers to give them an estimate on quantities of concrete/steel used for a similar project.
If the project is only at design review stage and if for some reason an estimate is unobtainable at the time of assessment, you may consider taking on the assumption that the CLT is replaced by the same volume of reinforced concrete. Typically you’ll need less concrete (in volume) than CLT for the equivalent structural integrity. Therefore please bear in mind the GWP savings may be overstated with this assumption if you’re using it for any point/credit calculation predictions.