Pc --> cr

[willyryan]

how do I convert a playable character to a challenge rating I can't find it anywhere

[Beheld]

A PC's challenge rating is equal to their level.

You can derive this from a couple ways:

1) NPCs with PC classes of level X are CR X. The DMG specifically describes how NPCs have less wealth than PCs, but use more of it in consumables to provide the challenge necessary.
2) A party of 4 is party level equal to the PCs level. But a party of half size is -2 Party Level. So a party of 2 level 10 PCs is Party level 8, and a party of 1 level 10 PC is Party level 6. The DMG defines that a Party Level 6 is supposed to have a 50% chance of win and a 50% chance of loss against a CR 10. So a single level 10 PC is supposed to be exactly even against a CR 10 (and win more against CR 9, and lose more against CR 11). So since a level 10 PC is going to go perfectly even against itself, it would make sense that it is CR 10.

[KillianHawkeye]

Officially, a character of any of the standard races with PC classes has a CR equal to their character level (or one less if they have NPC classes), but this doesn't take into account any power differences between the classes (so a level 10 human Monk and a level 10 human Wizard are both CR 10 but vastly different challenges) or any of the character's build choices (so a level 6 Fighter who took spiked chain proficiency, Combat Reflexes, Improved Trip, and Weapon Focus/Weapon Specialization plus other useful feats has the exact same CR as a level 6 Fighter who wields a club, forgot to put on armor today, and blew all his feats on Toughness).

So it's really up to the DM to determine if any particular encounter is going to be more or less of a challenge than the CRs would indicate and adjust accordingly (and this goes for monsters as well as humanoids).