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Casting Spells in 7th Edition

by Avian

In 7th edition you get a dice pool to draw your dice from when you want to cast a spell. But how many dice should you take? As many as you can? The answer might very well be 'no'. I made a little algorithm that calculates the chance of rolling equal to or over over a certain sum when you roll a number of dice.

Now updated for 7th edition!


Chance of reaching the various casting values using a given number of dice

The chances were found by using a script who goes through all possible combinations and checks the results for each of them.


  Number of dice  
1 2 3 4 5 Suggested #
of dice
3+ 66.7% 97.2% 92.6% 86.8% 80.4% 2
4+ 50.0% 91.7% 92.6% 86.8% 80.4% 2
5+ 33.3% 83.3% 92.6% 86.8% 80.4% 3
6+ 16.7% 72.2% 91.2% 86.8% 80.4% 3
7+ 0 58.3% 88.0% 86.5% 80.4% 3
8+ 0 41.7% 82.4% 86.8% 80.4% 4
9+ 0 27.8% 74.1% 85.5% 80.4% 4
10+ 0 16.7% 62.5% 83.3% 80.0% 4
11+ 0 8.3% 50.0% 79.5% 80.0% 4
12+ 0 2.8% 37.5% 73.3% 79.3% 5
13+ 0 2.8% 25.9% 65.0% 77.9% 5
14+ 0 2.8% 17.6% 55.2% 75.2% 5
15+ 0 2.8% 12.0% 44.4% 71.0% 5
Average 3.5 7.0 10.5 14.0 17.5  
I. Force 0 2.8% 7.4% 12.7% 17.7%  
Miscast 0 2.8% 7.4% 13.2% 19.6%  


What should be most interesting is of course the best number of dice for each level. 5 dice doesn't give you a better chance of casting a 4+ than two dice, it's worse. The chance of a miscast is even as high as 20%. For high level spells this is pretty much like the old Waaagh! magic - roll a 1 and something nasty happens. This means that unless your wizard has something that protects him from miscasts he really shouldn't try to cast high level spells.

If you look up on the table you will se that roughly 60% of all spells cast will be dispelled if your opponent uses as many dispel dice as you used to cast it. As a rule of thumb you can look up a spell's casting value and cross refernce it with the number of dice you might usually spend each round to try to get it off, then multiply this by 2.5 to find a rough estimate of how many times you will get the spell off per battle. So if your wizard will on average expend 1 power dice each turn (also counting the turns he doesn't try to cast it) trying to cast the 'Flaming Sword of Rhuin' (CV 5+) he might reasonably expect to never get it off (about 40% chance for the entire battle). This means that you need to commit more dice to each spell. For low level wizards this is less of a problem as they will usually have 3 dice for their one spell per turn. A level 2 will have 2 dice per spell and a fourth level will have (on average) 1.5. The lesson should be that if you take a high level mage you should also take something that gives you more dice.


That Dreaded Thirteenth Spell

Because people asked for it. The table shows the chance of getting this 25+ spell cast using various numbers of dice. The first colum is the number of dice used, the second is the chance of getting it cast in total. The third column is the chance of getting it cast with Irresistible Force and the fourth is the chance of getting it cast without Irresistible Force (thus the second column is the sum of the third and fourth). The fifth column is the chance of miscasting.

The chance of casting the spell with 2, 3 or 4 dice can be found in the main table above (the chance being equal to the chance of an Irresistible Force).

Cast with Total chance IF Non-IF Miscast
5 dice 18.2% 17.8% 0.4% 19.6%
6 dice 28.6% 22.1% 6.5% 26.3%
7 dice 46.1% 25.3% 10.8% 33.0%
8 dice 55.4% 27.3% 28.1% 39.5%

I won't swear on the last figures being absolutely accurate as doing this kind of math in PHP isn't optimal, but I think they are all right.


Lore difficulty

You should also consider the average casting value of your chosen lore when deciding which to choose.

Lore Average CV Variance
Rulebook lores
Fire 8.17 8.6
Metal 7.50 5.9
Shadow 8.50 7.5
Beasts 6.67 3.5
Heavens 7.33 7.5
Light 6.50 4.3
Life 6.67 2.3
Death 8.17 3.0
Armybook lores
Little Waaagh! 8.17 3.0
Big Waaagh! 7.83 7.4
Nurgle 8.50 3.5
Tzeentch 8.29 7.2
Slaanesh 8.50 3.5
Athel Loren 7.50 5.9
Skaven 7.86 8.8
Necromancy 7.75 5.9
Dark Magic 8.33 6.7
High Magic 7.67 7.0

For those of you who haven't had much statistics the variance is an indication of how much the numbers are spread out. A high number indicates a large spread.

For spells with different casting levels (such as the Invocation of Nehek, which can be cast on a 3+, 7+ or 11+), each casting level is treated as a separate spell.

As you see the different lores are, well, different. Fire has pretty high casting values and a large spread so if you've only got a low level wizard you might consider something else as he'll have a hard time getting anything off (too little fizz in his staff, heh heh :] ). Beasts on the other hand has low casting values and low spread so it's less of a gamble.

To continue the example from above you will see that a level 1 mage using the lore of Light may reasonably expect to get his one spell off at least two times per battle (if his opponent uses an average of three dice to dispel his spell each time it succeeeds. The same wizard using the lore of fire, on the other hand will probably get his spell off less than two times per battle. One trick you might try with a level two wizard is to try to get one low level (5+ or less) spell and one high level spell (8+ or higher). If your first spell comes up as a 7+ or higher, take the lowest one instead (unless it's something you really want). This way you can spend one die per turn on the low level and three on the high level, giving you an about equal chance of getting each one off.

When you have decided how many dice you want to use on each spell you should try to cast the easiest one first (i.e. the one you have most chance of getting off with the dice you are willing to use) and leave the biggest gamble for last. This means that your opponent will either have to gamble as well, which is a good thing, I think...


Average casting values in 6th edition

For those who want to compare these things...

Lore Average CV Variance
Fire 8.5 7.5
Metal 6.2 9.3
Shadow 7.7 6.7
Beasts 6.8 2.2
Heavens 7.7 3.9
Light 6.7 2.7
Life 7.3 1.1
Death 8.0 7.6


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