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The Dungeons & Dragons probability generator will determine exactly what the probability of getting a success is for D&D or any other d20 game, including Pathfinder. Unlike most RPGs, D&D does not actually have a probability curve; instead it has a probability line. This means the odds of getting a perfectly average success are exactly the same as the odds of rolling a crit or the odds of rolling a 1. In D&D terms, this means that your chances of rolling a 1, or a 20, or a 10, are excatly the same.
Thus the only factor in calculating the probability is what the target number you need to reach is, and what your skill modifier is.
Dungeons & Dragons Probability Calculator
To use this calculator simply enter the DC to succeed with an attack or a skill roll. Then enter your total skill bonus, taking both skill points, attribute modifier, and any other modifiers into account.
So if you wanted to calculate the probability of hitting an orc you'd enter the orc's AC (let's say it's 16) and if your fighter has a +3 base attack bonus, and an additional +2 from his strength, his total skill bonus would be 5. So you enter 16 as the DC, and 5 as the skill bonus, and discover that you have a 50% chance of success on any given roll.
D&D Failure and Auto Success Chances
It's worth noting that in D&D you always have a 5% chance of failure, since a roll of a 1 always fails. Furthermore you always have a 5% chance of success, since a roll of 20 is always a success.