Blood Bowl Basics: Fouling

Everybody loves a good foul. Having a half dozen goblins hold a guy down while another jumps up and down on his head is both hilarious and an objectively great idea.

Great question, Aaröngandr! It turns out that the answer to that question is the same as the answer to all questions: LET'S LOOK AT THE MATH!

How A Foul Actually Works
Once per turn, you can foul an opposing player, making an armor roll against him. Like all armor rolls, you roll 2d6 and if the result is higher than the player's armor then he has to make an injury roll. There's one way that the foul roll is different than other injury rolls, though: your teammates can provide assists! Foul assists work just like blocking assists, which is to say that each of your teammates projecting a tackle zone onto the fouled enemy and not in any other enemy's tackle zones will provide a +1 bonus to the foul roll.
Since the lowest you can roll on 2d6 is a 2, a guaranteed armor break on a foul requires (enemy AV - 1) assists.
Now that's neat, but it takes a lot of dudes to guarantee an armor break on even the relatively crummy AV of 7. You can't always spare that many players, since a player assisting a foul is a player who's not really contributing to the rest of the game. So how many assists does it take to make an armor break likely?

It turns out: not very many! (Some rounding ahead.)

Enemy AV - # of assists	Armor Break %
1	100
2	97
3	92
4	83
5	72
6	58
7	42
8	28
9	17
10	8

An armor break is more likely than not at 6, which is just one or two assists respectively on the common AVs 7 and 8. So you can probably get an armor break... but then what's the chance of killing the guy you're fouling?

How An Injury Roll Actually Works
If you break a player's armor, that player has to make an injury roll on 2d6. Here's the short version of the injury table:

2d6 Roll	% of Occurance	Result
2-7	58	Stunned
8-9	25	KO
10-12	17	Casualty

There are a few common skills which alter the injury roll: Thick Skull moves the 8 result into the Stunned category (making Stunned 72% likely and reducing KO to 11%), while Stunty adds 1 to the injury roll to produce this table:

2d6 Roll	% of Occurance	Result
2-6	42	Stunned
7-8	30	KO
9-12	28	Casualty

What About The Ref?
The ref will get up the courage to eject your player from the game if you roll doubles on the armor break roll and/or the injury roll. If you don't break armor, you have a 1/6 (~17%) chance to be ejected. If you do break armor and as such cause an injury roll, you have a 11/36 (~30%) chance to be ejected.

Tying It All Together
Rolling the (non-stunty) injury results and ejection chances into our armor break table from before gives us this (including some smallish propagating rounding errors):

Enemy AV - # of assists	KO %	Cas %	Ejection %
1	25	17	30
2	24	16	30
3	23	15	29
4	21	14	28
5	18	12	26
6	15	10	25
7	11	7	22
8	7	5	21
9	4	3	19
10	2	1	18

TL;DR
You're more likely to remove the opposing player from the field than to get ejected if (enemy AV - # of assists) < 6. That's not really the whole of the consideration, though. You also have to be concerned about the fact that getting ejected causes a turnover (so do important moves before fouling!), the value difference between the fouling player and the fouled player (because it's generally worthwhile to trade a halfling for a wardancer or a gutter runner, for example), and any bribes you may have (each of which has an ~83% to cancel an ejection). Smart fouling will increase your game win percentage as well as your opponent rage percentage, so figure out the math and stomp some faces!