3 Calculators - 3 different results??

Ok, I'm trying to figure out what the gravity and IBU's are on this recipe and have used beercalculus, tastybrew, and brewpal (iphone) to try and estimate what my results should be. All three came up with wildly different results and I don't understand why.

Recipe - Rye Pale Partial Mash
-- 3 lbs Rahr 2-row
-- 1.5 lbs Weyermann Rye Malt
-- 0.5 lbs Carared
-- 6 lbs Amber malt syrup late addition (15 min)
-- 1 oz Summit 18AA (60 min) pellet
-- 1 oz Palisade 7.5AA (15 min) pellet
-- 0.5 oz Palisade 7.5AA (5 min) pellet
-- 0.5 oz Palisade 7.5AA (dry hop) pellet

Results:
Beercalculus.com:
OG: 1071
IBU: 46.8 ??
ABV: 6.9%

Tastybrew.com
OG: 1069
IBU: 97 ???
ABV: 6.7%

Brewpal
OG: 1067
IBU: 66 ??
ABV: 6.8%

Ok, I can live with the gravity being pretty close, but what is going on with the IBU's? I tripled checked to make sure I put in the numbers the same on each one but the results always came out this differently. What gives?

Cheers

I use brewpal and beertools and have also noticed in the past that they can sometimes come up with different measurements that I can't explain. That having been said the bitterness shouldn't be so wildly off. Here's what you want to look for:

There are different bitterness algorithms that can be used by these programs and you generally want to make sure that pick one and stick with it. I know brewpal let's you choose between the tinseth and rager formulas in the settings menu on the hops page.

You should also check to make sure that each program is making the same assumptions about factors like boiloff rate when it comes to gravity.

3 different methods to calculating IBU.

Rager
Tinseth
Gartz

I got 1.072 and 70.1 IBUs using ProMash @ 75%. Tinseth formula with a correction factor of 1.0

As some one told you in the other thread you started with the exact same topic. Different IBU rating scales. Tinseth, Rager, and Garetz (sp?)

IBU estimation is a black art. As noted, there are many different formulas for doing it that can vary wildly in their predictions--Tinseth, Rager, Mosher, Daniels, and Garetz are the most common, though there are others. Basic Brewing Radio did a show on this--they used several different formulas to guess, then actually had the real IBUs measured in a lab.

One recipe varied from 18 IBUs to 74 IBUs as estimates (actual: 34).
In some cases, the spread of the formulas didn't even contain the actual answer--e.g. a hefeweizen where estimates ranged from 12 IBUs to 17, but actually measured just 7 IBUs.

My personal takeaway was:
1. Pick one formula and use it the same way consistently; that way you'll have a consistent baseline that you'll learn how it comes out flavor-wise (I chose Tinseth, as it seemed most accurate to me); and
2. Only expect accuracy to about +/-30% in the estimates*. If the prediction is 50 IBUs, you can be reasonably confident that it's in the 35-65 IBU range, but don't expect it to be spot-on.

*Or even worse for unusual beers (e.g. anything with wheat/rye, partial boils, unusual hopping schedules, crazy yeast, etc)