IBU' not what expected

[robange]

Below is a recipe for a Sierra Nevada I'm attempting from a BYO recipe (slightly altered the grain) Below is what I punched into BeerSmith and it only shows IBU's at 18.6? It should be around 37. Does anyone see what I missing from this or something I have inputted incorrectly. This is for a 5 gallon batch. Thanks



Ingredients:
------------
Amt Name Type # %/IBU
2 lbs 12.8 oz Light Dry Extract (8.0 SRM) Dry Extract 3
4 lbs Pale Liquid Extract (8.0 SRM) Extract 4
1 lbs 6.1 oz Pale Malt (2 Row) US (2.0 SRM) Grain 1
10.0 oz Caramel/Crystal Malt - 60L (60.0 SRM) Grain 2
0.18 oz Magnum [14.00 %] - Boil 60.0 min Hop 6
0.70 oz Perle [8.00 %] - Boil 60.0 min Hop 5
1.00 oz Cascade [5.50 %] - Boil 15.0 min Hop 7
0.75 oz Cascade [5.50 %] - Boil 0.0 min Hop 8
0.75 oz Cascade [5.50 %] - Dry Hop 7.0 Days Hop 10
1.0 pkg American Ale II (Wyeast Labs #1272) [124 Yeast 9 -

 

[JollyIsTheRoger]

. 18 oz of Magnum seems really low, that would be like 3 pellets, may want to check the numbers again

 

[S0meB0dy]

Is your boil volume 5 gallons or only the batch size? I put your recipe in beersmith and with a 5 gallon boil i was showing 36IBU but only 18.5IBU with a 2.5 gallon boil.

Also what IBU scale are you using? Rager, Garetz or Tinseth?

 

[robange]

it appears the issue was with the batch size. I need to tweek my equipment profile, which is proving to be a little more difficult than I thought. I found a couple good resources from old posts Im going to read thru a couple more times. Thanks