Understanding the Proficiency bump on Level Up for untrained skills

I'm experienced with DnD3.0, DnD3.5, and PF1e, but I am just starting to delve into Pathfinder 2e and I have a question on Proficiency for Skills.

It says when you Level Up your character, you "Increase all of your proficiency bonuses by 1 from your new level". Does this only apply to trained (or higher) skills? Or does this apply to all Skills regardless of training? If I am untrained, why would my proficiency in that skill go up if I haven't been training in it.

If you later train in a skill at a higher level, for example my character becomes "trained" in Medicine at Level 5, would the proficiency bonus be +3 (+2 for "Trained" and +1 for the level up to 5th) or would it be +7 (+2 for "Trained" and +5 for being Level 5)? Thematically I would go with the former. If you delay training in something, you don't instantly become really good at it when you finally do start training.

Is this the same then for other things with Proficiency modifiers, such as Perception, Saving Throws, Weapons, and Armor?

Some of that text in leveling up seems like a holdover from the playtest when your level was always added to proficiency. Rather than using that section for explaining your proficiency bonus, lets consult the section on proficiency.

Proficiency is a system that measures a character’s aptitude at a specific task or quality, and it has five ranks: untrained, trained, expert, master, and legendary. Proficiency gives you a bonus that’s added when determining the following modifiers and statistics: AC, attack rolls, Perception, saving throws, skills, and the effectiveness of spells. If you’re untrained, your proficiency bonus is +0. If you’re trained, expert, master, or legendary, your proficiency bonus equals your level plus 2, 4, 6, or 8, respectively.

To help understand this, we can rearrange it into a table, like so:

$$\begin{array}{|c|c|} \hline \textbf{Proficiency} & \textbf{Bonus}\\ \hline \text{Untrained} & \text{0}\\ \hline \text{Trained} & \text{Level + 2} \\ \hline \text{Expert} & \text{Level + 4} \\ \hline \text{Master} & \text{Level + 6} \\ \hline \text{Legendary} & \text{Level + 8} \\ \hline \end{array}$$

So to use your example, if you go from Untrained to Trained in medicine at level 5, you would go from have a +0 bonus to a +7 bonus. This would work similarly for other proficiency modifiers as well.

Proficiency bonus is calculated as character level (1-20) + training bonus (+2 for trained, +4 for expert, and so on). A 3rd level character that is trained in a skill is 3+2=5. A 3rd level character that has expert uses 3+4=7. If you are not trained, then the proficiency bonus is 0.

Adding a skill as trained later on does indeed cause a large spike in the bonus.

While it is often a lot of numbers to keep up with, it is intended to be simple it doesn't track what level you took trained in a skill.

• I guess the large spike in a skill bonus could be achieved in DnDv3 and PF1e by throwing all your skill ranks into a single skill when you level (up to your current level). Apr 15, 2020 at 14:33
• Yes, this would represent you "practicing" with that skill for two levels; you weren't proficient enough before to risk using it, but now your general world experience combined with on-topic knowledge allows you to learn in an accelerated way. It's not the most realistic thing, but Paizo's new release seems to care more about mechanic balance than "making sense" and is allowing groups to decide how to justify things themselves Apr 15, 2020 at 15:53

Is this the same then for other things with Proficiency modifiers, such as Perception, Saving Throws, Weapons, and Armor?

Yes - everything works pretty much the exact same way. The one exception is in agency - generally if you roll with a modifier, it indicates that you are the one acting. Saving Throws reverse this aspect.

From the perspective of a GM, the fact that these all work the same way, on the exact same scale, has the advantage that you can roll anything against any DC.

N.b. If you want to calculate a DC from a skill (which is occasionally called for, e.g. grapple and trip), simply add ten to the value. For example, if my character's Fortitude is +6, then his Fortitude DC is 16.

Because everything works the same way, you might have a situation in which you want to roll against a player's Arcana*, for example. In that case you would calculate the player's Arcana DC (by adding 10 to their Arcana skill), and have the NPC roll against that target.

The rules generally advise against using opposed checks if you can avoid it. For example if you had a wrestling match, you could use the GM Guide's victory point side system. Have each character (NPC or not) take turns rolling their athletics skill against their opponent's athletics DC, and have the winner be the first character to be 3 or higher, and also 2 ahead of their opponent at the end of a full round.

*Which might happen if say some NPC believed some incorrect Arcana knowledge and was telling a player about it - the NPC isn't intending to deceive them.