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Help needed to calculate d16 and d12 dice odds

Started by antema, March 15, 2017, 08:19:24 AM

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antema

So, on my homebrewed RPG system NPCs "level" is determined by the dice they use. Players always use d20, but weaker or stronger enemies can use any dice from d6 to d30. Critical hit is natural 20 for players, but for NPCs with different dice the crit chance must still be the same as for the players, 5%. Getting a 16 on d16 is 6,25% chance and a 12 on a d12 is a 8,333% chance. So the dice has to be rethrown, and what number must the player get to get a 5% crit chance? And how do you calculate this?

Thank You!



Xanther

Quote from: antema;951582So, on my homebrewed RPG system NPCs "level" is determined by the dice they use. Players always use d20, but weaker or stronger enemies can use any dice from d6 to d30. Critical hit is natural 20 for players, but for NPCs with different dice the crit chance must still be the same as for the players, 5%. Getting a 16 on d16 is 6,25% chance and a 12 on a d12 is a 8,333% chance. So the dice has to be rethrown, and what number must the player get to get a 5% crit chance? And how do you calculate this?

Thank You!

Think this is an easy one.  For example on a d16, a 16 is a 6,25% chance.   So you want a roll with odds m, where m x 6.25 = 5.   The answer in this case in m=0,8, or 80%.  Yu could have the second roll be on a d20, a 16 or higher means it is a crit.  for you 8,33% example, m = 0,6 or 60%, a 12 or higher on a d20.
 

spaceLem

Unfortunately, a d16 or a d12 cannot give exactly a 5% chance of success (because of different prime factors etc.). So there's no way to do it with just one die type.

However there is one very simple, consistent, no maths involved way: in addition to your regular roll, roll a d20. If that d20 comes up 20, then you crit. There is always a 1 in 20 chance of this happening, so it's always the same chance, regardless of power level.

The complication is if your d20 comes up a 20, but your other die misses, then what? If you rule that the other die also has to succeed, then it means that 5% of hits are crits (as opposed to 5% of attacks if you crit whether the other die hits or not). If you rule that a crit happens regardless, then some fails are being upgraded to crits (however since this is only an extra (chance of failure x 5%) success, it's probably something you can live with).
Currently playing: Shadowrun 3e, Star Wars: Edge of the Empire, Half-Life 2 post apocalypse homebrew
Currently running: nothing currently

Skarg

Quote from: spaceLem;951816...
The complication is if your d20 comes up a 20, but your other die misses, then what? If you rule that the other die also has to succeed, then it means that 5% of hits are crits (as opposed to 5% of attacks if you crit whether the other die hits or not). If you rule that a crit happens regardless, then some fails are being upgraded to crits (however since this is only an extra (chance of failure x 5%) success, it's probably something you can live with).

I think 5% of hits are crits is better anyway.

Shawn Driscoll

Simple. Player characters don't get critical hits if rolling weaker dice.

spaceLem

Quote from: Shawn Driscoll;952218Simple. Player characters don't get critical hits if rolling weaker dice.

Well that rules out David & Goliath.

The 5% of hits are crits method is fine. If you've got a weaker die, then you're less likely to hit, and so less likely to crit.
Currently playing: Shadowrun 3e, Star Wars: Edge of the Empire, Half-Life 2 post apocalypse homebrew
Currently running: nothing currently

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