Page 1: Casting Spells in 8th Edition|Page 2: Casting Spells in 7th Edition
Casting
What's the optimum number of dice to use when casting a spell in Warhammer 7th edition?
This table shows the chance that you will successfully cast a spell with a certain casting value depending on how many dice you roll and what the optimum number of dice is for each casting value.
| Casting Value | Number of Dice | Optimum Number of Dice |
||||
| 1 | 2 | 3 | 4 | 5 | ||
| 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% | 86.8% | 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% | 52.5% | 75.2% | 5 |
| 15+ | 0 | 2.8% | 12.0% | 44.4% | 71.0% | 5 |
| Average Value | 3.5 | 7.0 | 10.5 | 14.0 | 17.5 | |
| Miscast | 0 | 2.78% | 7.41% | 13.19% | 19.62% | |
| Irresistable Force | 0 | 2.78% | 7.41% | 12.73% | 17.70% | |
These values include the following two conditions:
that an irresistable force is a successful cast even if the casting value is not reached; and
that a miscast takes precedence over an irresistable force.
(The possibility of casting the spell with Irresistable Force as a result of a Miscast has not been included.)
The table shows that when trying to cast a spell with a low casting value, rolling lots of dice is not a good idea, because there is a greater chance for a miscast.
If your opponent uses the same number of dice to dispel as you used to cast, he will dispel roughly 60% of your successful casts—for example, if you rolled 2 dice to cast, the average value you get is 7, and if he rolls 2 dice to dispel, he has a 58.3% chance to get 7+ and beat you. So, as a quick rule, multiply the value by 2.5 to see how many times you can expect to get a spell off in a game. 
For example, let's say you have a level 1 wizard with Fireball (casting value 5) and your opponent attempts to dispel with the same number of dice as you cast.
If you use 1 die to cast, you can expect to get the spell off 0.3 x 2.5 = 0.8 times during the game—so don't count on it.
If you use 2 dice to cast, you can expect to get it off 0.8 x 2.5 = 2.1 times.
And if you use the optimal 3 dice, you can expect to get if off 0.9 x 2.5 = 2.3 times.
This means you need to allow an appropriate number of dice for each cast in order to have a good chance of getting the spell off. This is no problem for low level wizards. However, if a level 4 wizard wants to cast all 4 of his spells, he only has an average of 1.5 dice per spell.
This approximation does not take into account the effects of irresistable force. The calculator below does take irresistable force into account.
Magic Items: extra dice vs. +1 to cast
Extending the previous example, we now consider the difference between a magic item giving an extra power die and a magic item giving +1 to cast.
We make the assumption that your opponent has as many dispel dice as you have power dice. Then, with an extra power die, you can effectively cast one spell where you use one more dice to cast than your opponent can use to dispel.
If you use 2 dice to cast and he uses 1 to dispel, you can expect to get the spell off 0.8 x 6 = 5.0 times.
If you use 3 and he uses 2, you can expect to get the spell off 0.9 x 5.3 = 4.9 times.
And if you use 4 and he uses 3, you can expect to get the spell off 0.9 x 4.9 = 4.3 times.
With +1 to cast, the multiplier would be 3.5 rather than 2.5.
If you use 1 die to cast, you can expect to get it off 0.5 x 3.5 = 1.8 times.
If you use 2 dice to cast, you can expect to get it off 0.9 x 3.5 = 3.2 times.
And if you use 3 dice to cast, you can expect to get it off 0.9 x 3.5 = 3.2 times.
The values for the extra die are more impressive at a glance, however the chance of a miscast increases with extra dice and it only affects one spell each turn. The +1 to cast affects all spells and so the more spells a wizard has, the more desirable it becomes.
Calculator
Here's a little calculator to play with the values:
| Casting value of spell: | |
| Casting bonus: | |
| Number of power dice used: | |
| Opponent's dispelling bonus: | |
| Opponent's dispel dice used: | |
| Number of turns in game: | |
| Expected number of casts: | |
Dispelling
The casting table can also be used for dispelling—for example, if your opponent tries to cast a spell and rolls a total of 6, you should optimally use 3 dice to dispel it.
This page was inspired by Avian's page.
Page 1: Casting Spells in 8th Edition|Page 2: Casting Spells in 7th Edition
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