For "at least one" probability problems, it's usually easier to start by calculating the chance that none of the dice crit, as that saves you the hassle of combining the probabilities of getting 1/2/3 crits.
Best of three rolls with 18-20 crit range: ~39% chance to crit
Chance that a single die will not crit: 17/20 = 0.85
Chance that all three dice will ...
The other answers do a good job of answering the question, but I'll point out how you can answer questions like this in the future:
https://anydice.com/ is a very powerful (if slightly complicated) calculator for these sorts of questions. In your case, you'd enter the query:
output [highest 1 of 3d20]
And then select "At Least" from options below to get ...
It has the property:
While you're wearing it, any critical hit against you becomes a normal
hit. (DMG 150)
It is only classified as uncommon, but it is still a magic item. Depending on how the DM handles that, the difficulty of acquiring one may change. You also need to be proficient in at least medium armor to properly use one.
Play as a Grave Cleric
The Grave Cleric domain from XGtE (p. 20) has the following ability at level 6:
As a reaction when you or a creature you can see within 30 feet of you suffers a critical hit, you can turn that hit into a normal hit.
This ability has a limited number of uses between long rests but unless you are getting critical hit too often, it ...
Player's Handbook, page 194:
Rolling 1 or 20
[...] If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. In addition, the attack is a critical hit, as explained later in this chapter.
If the d20 roll for an attack is a 1, the attack misses regardless of any modifiers or the target's AC.
Paralyzed and Unconscious both produce automatic crits
With the caveat that you have to be within short range, however.
From the Players Handbook -
A paralyzed creature is incapacitated (see the condition) and can’t move or speak.
• The creature automatically fails Strength and Dexterity saving throws.
• Attack rolls ...
It's probably fair, but also spectacularly unlikely to ever matter.
I've played dozens of sessions of 5e in the past year, and this has literally come up never.
Plus, if it ever does come up that someone rolls a double 20 or double 1, I'll probably just narrate awesomeness anyway, house-rule or no. You're probably just wasting your time preemptively coming ...
Probability of at least one crit
The Great Weapon Master feat allows making an extra attack when scoring a critical or reducing a creature to zero hit points. I'll not cover the latter case since its probability is very situational, and the former has no relevance here since the extra attack only happens if one already scored a critical hit. The easy way to ...
Both Sentinel at Death's Door and adamantine armor turn crits into regular hits
The Grave Domain cleric's Sentinel at Death's Door feature (XGtE, p. 20) says:
At 6th level, you gain the ability to impede death’s progress. As a reaction when you or a creature you can see within 30 feet of you suffers a critical hit, you can turn that hit into a normal hit....
This is a houserule
The section on Critical Hits states:
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 [...]
Note that you only ...
Temple of the Gods modifies the d20 roll, and modifiers don't cancel critical hits
The steps for making an attack are described as follows (emphasis added):
Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.
Choose a target. Pick a target within ...
The Dungeon Master's Guide has rules for handling this case:
On the Monsters and Critical Hits section (DMG page 248), it states what to do in the case of a monster critical hit when using average damage:
When the monster scores a critical hit, roll all the damage dice associated with the hit and add them to the average damage. For example, if a goblin ...
This is far, far too strong for a first-level spell, and arguably too strong for the entire game.
You've already done the math on how much it increases the probability of a crit, and that math demonstrates that this first-level spell would make crits more likely than any other mechanic in the game (with one exception, discussed below). In general, a ...
They are subject to critical hits!
The Dungeon Master's Guide (pg. 246) section on Objects never specifies that objects are immune to critical hits. It gives you, as the DM, suggestions of AC and hit points for the object based on its material and size. The "Statistics for Objects" section says:
When time is a factor, you can assign an Armor Class and ...
The optimal choice varies depending on how many more levels of barbarian you throw in and how hard it is to hit whatever you're currently fighting
I've used this anydice program to crunch some numbers on your problem and determine the expected damage on any given attack. I'd suggest looking at the "summary" view, in particular, to make it easy to compare ...
RAW: Roll the damage dice twice
The standard rule for critical hits from the basic rules states:
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 ...
Paralyzed and Unconscious Conditions do this
Any creature that suffers the effects of the Paralyzed or Unconscious condition converts any attack successfully delivered within 5' to a critical hit:
Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.
There are many ways in which you can get that condition, ...
I did some additional digging. Magic Items and Critical Hits (Dungeon Masters Guide Pg. 222)
Magic Weapons and Critical Hits: Some weapon qualities and some specific weapons have an extra effect on a critical hit. A flaming burst weapon, for example, does extra fire damage on a critical hit. This special effect functions against creatures not subject to ...
Yes, according to the PHB (p. 196) and basic rules, critical hits double the damage of all dice tied to an attack's damage (emphasis mine):
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 ...
You can use the feature until the DM declares the result
The intent of this features is to be used between the rolling of the dice and the DM declaring the results. If there was meant to be special behaviour on a 20 or a 1 the features would say so.
Natural 20's can still miss
It is possible for there to be situations where a natural 20 is not a ...
You don't reroll anything when you have Disadvantage. You roll two dice and use the lower one, so if one is a 20 and the other is a lower number (which it'll probably be) then yes, you use the lower one and won't score the automatic critical hit.
From the SRD
Sometimes a special ability or spell tells you that you have advantage or disadvantage on an ...
You got most of them, but you failed to consider magic items, you missed a couple of spells, and you were only thinking of creatures
There are two classes of targets that we want to consider here:
For objects, we obviously can't impose a condition, so we need to look at magic to help us here.
Fortunately there is a Magic Item ...
According to the lead game designer, subtracting from a roll is a modifier and thus you do not double the roll
The section on Damage Rolls states:
Each weapon, spell, and harmful monster ability specifies the damage it deals. You roll the damage die or dice, add any modifiers, and apply the damage to your target...
This tells us that first you roll the ...
Take the Lucky Feat
You can also spend one luck point when an attack roll is made against you. Roll a d20, and then choose whether the attack uses the attacker's roll or yours.
The Lucky feat allows, 3 times per long rest, a player to roll a d20 in addition to any d20s rolled as part of the attack. Afterwards, you get to choose whether your d20 or the ...
The rules use a certain pattern when discussing modifiers and Cutting Words adheres to that pattern. It is therefore a modifier to a roll result and does not replace the result.
Almost without fail:
If a modifier is a fixed value, the rules use the words "modifier" and "penalty."
See Raise Dead, Resurrection, Slow, Alter Self, Haste, Magic Weapon, ...
Are there official rules on this?
Yes, per Sdjz's answer..
But it requires a die roll.
You can do it without any dice rolling if you like.
What have I seen done that is fast and simple?
A critical hit when using average damage is "Max roll +1"
If I am doing 1d4, it becomes 5 + modifier.
If I am doing 1d6, it becomes 7 + modifier.
Easy to keep ...
This anydice program will tell you how many critical hits you can expect to score in various situations. Since in this particular case we're only trying to work out the odds of scoring critical hits at all (not how much damage we're dealing or anything much more variable like that) the program is quite simple.
The expression 1d20>=18 gives us the ...
As you quoted:
If the attack involves other damage dice, such as from the rogue’s Sneak Attack feature, you roll those dice twice as well.
Here is the definition I found for the term “damage dice” (under “damage rolls” in the PHB).
Each weapon, spell, and harmful monster ability
specifies the damage it deals. You roll the damage die
It is not doubled
On page 450 of the Core Rulebook we find the four steps of determining damage:
Roll dice + account for modifiers
Apply resistance or weakness
Doubling the damage (for any reason) is listed on the next page as the last type of modifier in step 1. Thus it happens before any weaknesses are applied in step 3.
Cutting Words wouldn't be able to affect the attack, only the resulting damage.
Cutting Words' timing is specified with regard to the three things you can use it on: attack rolls, ability checks,
You can choose to use this feature after the creature makes its roll,
but before the DM determines whether the attack roll or ability check
succeeds or ...