How Many IBU's?

When I run the recipe below in ProMash and in the beer recipator I get pretty different IBU numbers. Was wondering which one is more accurate. ProMash says 37 and Beer Recipator says 23.

Recipe for Partial Mash Porter:

2lbs 2 Row
1lbs Munich
8oz Crystal 60
6oz Chocolate
2oz Black Patent

3.5lbs Dark DME

1oz Kent Goldings (5%) 60min
1oz Kent Goldings (5%) 30min

Wyeast 1028 London Ale

My boil size will be 3 gallons. Batch size is 5 gallon.

Differing software might be assuming a different point in the boil where you add the extract? (i.e. maybe Recipator thinks you're adding it at 60, and ProMash thinks you're adding it at end?) Just an idea. *shrug*

Or, different boil volumes....

There are a couple of different formulas, but I wouldn't imagine they'd make that much of a difference.

Check the AA levels in the hops menu. The levels set may be different. You can always enter a new hop with the measured AA level if you know what it is. There are also corrections for whole versus pellet. The boil time should be an entry you specify.

Check and see how they are doing the calculations. There are three popular formulas for calculating IBU bitterness. These are Rager, Tinseth and Garetz. They all give different results. Tinseth is probably the most accurate as it uses non-linear utilization factors.