DBs will pretty much always be less than barbell. As for comparison, that can't be done really. If you are going from DB to barbell, there is a good amount of carryover and you will probably be able to put up around the same weight if not more. Going from barbell to DBs, you will be able to put up significantly less weight -- all depending on how strong your stabilizers are.
Stabilizer strength plays largest factor in putting up weight between different exercises like DB and barbell, but it can't really be quantified unless you actually do it. Let's remember though that the more you do an exercise, the more your CNS and muscle activation patterns adapt to it -- so the specific strength of certain exercises might get easier, while your absolute strength and stablizers may or may not be going up.