Late Extract Addition, IBU question.

I've got my first pale ale attempt (fourth batch ever!) in the primary at the moment, and I'm trying to figure out the IBU for future reference. I know it's in the 30-40range, but the beer recipator at hbd.org does not have an option for late extract addition.

The Recipe:
5gal batch, 3gal partial boil.
OG: 1.050

6lb Light Muntons DME
-8oz added at start of boil, after steep, the other 5.5 added at 15min from end of boil.
8oz Crystal 20L (steep 0.5hr)
8oz Biscuit (steep 0.5hr)
1oz Fuggles 4.5% at 60min
0.5oz Fuggles 4.5% at 30min
0.5oz Fuggles 4.5% at 15min
0.5oz Cascades 6.9% at 15min
0.5oz Cascades 6.9% at 5min
Maybe a dry-hop of 0.5-1oz of Cascades....still not sure...

According to the recipator, with only the steep and the 8oz light DME, I should have gotten 33.2IBU out of the first 45 minutes of the boil.

Now, tell me if I'm doing this right. What I did next was remove all of the hop information, up the extract to the full 6lbs, and only plug in the 15min and aroma hop additions, and came up with 7.2IBU. I then added this number to the 33.2IBU I got from the previous calculation to get a total of 40.4IBU. Am I doing this right?

By the way, it's 5 days in the fermentor, at a SG 1.015, and it tastes like a half-decent pale ale!

Thanks guys!
-Dom

Here is one of the more current formulas being used today for calculating IBU's.

You will do this calculation for each hop addition and then add it all up for the final IBU value for the whole brew.

First calculate the AAU's (Alpha Acid Units). Get the AA% off of the bag that your particular hop came in and plug it into this equation. AAU = Weight (oz) x % Alpha Acids

Now for the fun part - Calculate Utilization which takes into account time of boiling and gravity. You will need to plug in the time the hop was boiled(T) and the Original Gravity of the wort(Gb).

Utilization = f(G) x f(T) where:
f(G) = 1.65 x 0.000125^(Gb - 1)
f(T) = [1 - e^(-0.04 x T)] / 4.15

Notes: e = 2.7182818 rounded, and the carrot sign pointing up means "to the power of". If you're handy with EXCEL you can plug this formula in there and also record other brewing data for this particular batch.

Now that you have Utilization, the IBU equation for this particular hop addition is as follows:

IBU = AAU x U x 75 / Vrecipe

Vrecipe = volume of the batch in gallons.

75 is a constant for the conversion of English units to Metric. The proper units for IBUs are milligrams per liter, so to convert from ounces per gallon a conversion factor of 75 (74.89) is needed. For the metric world, using grams and liters, the factor is 10. (For those of you paying attention to the units, the missing factor of 100 was taken up by the % in the AAU calculation.)

Repeat this for each hop addition and add up all the individual IBS's for the total IBU's for the beer.

For something easier, GOOGLE "IBU calculators" and use one of the interactive web sites that do this for you.

Hope this helps.

Most excellent, thanks man! Glad to see it is something that is just added, not something that requires adjusting depending upon the gravity of the solution. IE: 30IBU's in a beer is still 30IBU's even if you were to add a sugar to it after boiling.
Cheers,
Dom

Your calculation is probably slightly on the low side, as the bittering hops will continue to contribute during the last 15 minutes at a reduced rate. For a 40 IBU beer, most, if not all, people will not be able to tell the difference. Otherwise, your method works fine.

[Should be noted that any calculation over 90 IBU is suspect.]