Does polymorph use a PC’s CR or its level?

Question

I was discussing the spell with my DM relating to this question Can a Wizard polymorph low-level PCs into over-powered beasts? about the polymorph spell.

According to Polymorph's description,

The new form can be any beast whose challenge rating is equal to or less than the target’s (or the target’s level, if it doesn't have a challenge rating).

I was excited about the idea to turn one of my fellow group members into a giant ape or T-rex, but my DM told me that anyway with our current level (lvl 9) I would only be able to turn them into a CR 4 monster.
He bases his judgment on this part:

The new form can be any beast whose challenge rating is equal to or less than the target’s

From my understanding, PC's don't have a CR - CR seems to have been invented to adapt monsters' levels to players' levels (originally in D&D 3e). In other case, we could have just put experience levels to monsters instead of creating an alternate ranking system. And why does the spell specify:

(or the target’s level, if it doesn't have a challenge rating).

I'm still relatively new in D&D 5e world so I'm maybe wrong. I understand that at the end of the day the DM's word is law and I got no problem with that. But I was wondering if there was any precision points in the rules that I could show him if it's just a problem of interpretation?

I think that my DM isn’t making a house rule on purpose and is just misunderstanding the rules. I think he used a "conversion array" that he found on google: something like a level X creature is a normal threat for 4 level X PCs. Right now he tells me that he is right and the spell is just badly written.

So the question is does a PC fall under the

The new form can be any beast whose challenge rating is equal to or less than the target’s

part of the spell's rule or under the

(or the target’s level, if it doesn't have a challenge rating).

part of the spell's rule? Please justify.

Answer 1

Your DM has chosen to vary from the RAW

You correctly point out that by the letter of the rules, his ruling does not match the rules text for the polymorph spell.

The spell as written uses the character's level as the basis for the CR of the beast that the PC is polymorphed into.

The new form can be any beast whose challenge rating is equal to or less than the target’s (or the target’s level, if it doesn't have a challenge rating). (spell description, SRD p. 169)

The bolded part is the key.

You are also correct in understanding that the DM's ruling is how the game is played at his table. Now that you know what the ruling is, you know how powerful polymorph is in your DM's world.

What do I do?

I'd suggest that you discuss with your DM why he feels that the bolded part (above) of the spell text should not apply to a PC being polymorphed. Find out if a "balance¹" motive is the case, or if it's a simple misunderstanding.

I'd further recommend you do this away from the table (not during a game session) and work with your DM to discover the process he goes through to determine the CR level of the PC under a polymorph spell. There is a section in the DMG (pages 273-285) that addresses how to build a monster or NPC and assign a CR to it. The DM may feel that the spell should work a different way, or, he may be using an unofficial "conversion array" as you mentioned in a comment, or this may be a simple misunderstanding. The DMG and the game's developers point out that the rules serve the game at the table, and the that DM is Master of Rules(DMG p. 5) so for the moment play within his guidance until that is revised (if it is revised).

Beyond that discussion, and finding out if the DM agrees on the rules text and makes a change, what you can do is:

1. Check with the DM ahead of time on what the estimated CR is of a PC so that you know what you can try and polmorph them into.
2. Play and have fun.

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¹ As an aside:
There is a school of thought that I have seen among various players and DMs in on-line discussions; they have fallen into the "polymorph is overpowered" school of thought. Those folks see a CR adjustment as a remedy to a perceived imbalance or loophole in the game. DM's do things like that for a wide variety of reasons with the intent of making the game play more smoothely/better at their table.

Answer 2

What the Spell Does, Rules as Written

The new form can be any beast whose challenge rating is equal to or less than the target’s (or the target’s level, if it doesn't have a challenge rating).

This means exactly what it says. When the target is a player, who doesn't have a CR, you let the player ploymorph into something that has a CR equal to or less than the player's level.

How to have the Conversation

You may try, away from the table, to convince the DM they are incorrect. The best way to start that conversation is to talk about how you feel about the ruling, rather than coming at them with statements that could be seen as an attack. Starting with "The internet says your wrong..." or even "Jeremy Crawford says in sage advice compendium..." is a good way to make them angry and shutdown the lines of communication. Instead, try opening with something short sweet and to the point like, "In the last game you made a ruling about the polymorph spell, and it feels (he can't argue you don't feel that way) a little unfair and complicated. and reading up on it I'm not sure we're (note "we" not "you") doing this right."

Acknowledge his perspective, to get a "that's right" reaction to work forward from. "Man, you work hard away from the table preparing adventures for us, dealing with personal interaction issues, always looking up rules and monsters and settings that would be cool. Even when the time at the table turns out epic, that part we don't see has sometimes gotta be a thankless job."

Pause, let them talk. Listen to then. Empathize with them.

Then preface your next statements by acknowledging what you think will by their largest objection will be, without denying it, because denial will plant the objection if it isn't already there. "I know it seem like I'm nitpicking or attacking your decision." Don't say "but" following this, or it turns into a denial. Pause. Uncomfortable silence for a heartbeat or two. You may get them to give you, "No, I get it" or deep inhale waiting for the shoe fall after this. Live in those two heartbeats of silence before landing on open ended "How" questions leading them to where you want to go. Maybe questions like:

• "How am I supposed to know ahead of time if the polymorph I'm attempting is valid if there is table you have behind the screen?"
• "How am are we supposed to beat a [CR X] creature if waste our [only/few] level 4 spell turning into a creature that has a CR of [X/4]?"

Then, flat out, ask for his help in the matter, appealing to his authority. "I know I'm being pain about this. Can you help me out?"

Maintain a trust communication. Make sure your questions aren't being lobbed like accusations but sound like you're asking for help in this situation -- which you are. The more you let the DM feel in control, the more they will feel they are "winning" and the more likely they are to hedge their position and help you.

What Your DM does

If he's unconvinced, and your talk didn't work: let it go, don't bother him about it again. If you have a good game otherwise, this isn't the hill you want to die on, is it? Moreover, what the DM says at the table is the rules for the table. You've made your case; now, don't flog the deceased equine.