ey guys i'm new to this, when a player gets natural 20 on an attack roll, do you add a damage dice or do you double them

for exemple if you normally have 2d8, do you roll 3d8 or 4d8

thanks for your time

You double the number of dice.

Normally you double the dice, not the result. Meaning that an attack with Greatsword which deals 2d6+3 damge deals 4d6+3 damage.

If I recall correctly, there is still some debate if other variables such as Divine Smite or Sneak Attack damage is also multiplied. Given that the chance is rather low, in my group I rule that all dice thrown as the result of an attack is doubled.

Also be aware that some features simply add dice, instead of doubling it, like the Savage Attacks trait of the Half-Orc or the Brutal Critical feature from the Barbarian. An added note; these add "one additional weapon damage die". This means that if you're using abovementioned Greatsword with 2d6+3 a crit deals 5d6+3, not 6d6+3. In previous editions simply doubled the roll, this is no longer the case.

And funny to note; you absolutely can crit spells that require an attack roll as well!

Quote from Stormknight >>

There's no real debate on whether sneak attack etc is also multiplied - the rules are quite specific on this. :)

Here's specifically what the rules have to say:

https://www.dndbeyond.com/compendium/rules/basic-rules/combat#Rolling1or20

https://www.dndbeyond.com/compendium/rules/basic-rules/combat#CriticalHits

When you score a critical hit, you get to roll extra dice for the attack's damage against the target. Roll all of the attack's damage dice twice and add them together. Then add any relevant modifiers as normal. To speed up play, you can roll all the damage dice at once.

For example, if you score a critical hit with a dagger, roll 2d4 for the damage, rather than 1d4, and then add your relevant ability modifier. If the attack involves other damage dice, such as from the rogue's Sneak Attack feature, you roll those dice twice as well.

 

 Well paint me blue and call me a berry... I completely missed that... I stand corrected sir Knight :P

Quote from Stormknight >>

No worries, it's something I have seen debated around the table as well, including a friend (who is a reasonably experienced DM) telling me that he knew for certain that sneak attacks and smites were NOT doubled.

I challenged him to go check the rulebook and then he was like, "Ohhh... damn, I should read this thing more often...."

 Yeah I can see why. I mean, the other day our lvl 5 paladin instakilled a Nimblewright with a crit (using a nonmagical battleaxe one handed I might add). I always ruled it that way too since it's more fun for the players to roll around loads of dice, but I always thought it wasn't RAW.

This is why you get the fancy orange letters :)