IBU calculation?

[bbrim]

Does anyone know the percent utilization for First Wort Hopping? I'm thinking about trying this on a Bitter I will make soon and I would like to know what to expect as the low gravity will allow the hop bitterness very quickly.
Thanks for any help.

 

[ChemE]

BeerSmith comes with a default value of 110%

 

[BeerSmith]

Hi,
Its a mixed bag. In terms of pure IBU measurement in a test-tube, in general First Wort hops (FWH) results in slightly higher IBU numbers than a comparable time boil.

However, the perception of the bitterness with FWH is generally that the hoppy flavor is smoother. In blind taste test, most people will say the FWH has a smoother, less sharp hops flavor. Therefore you may perceive the hop impact as lower than a normal boil hop, though again most people prefer the FWH smoothness.

For more here's an article I put together some time ago:
First Wort Hops

Cheers,
Brad