Quick Answer: A natural 20 (critical hit) occurs 5% of the time — 1 in every 20 attacks. With Advantage, the probability rises to 9.75% (1 - (19/20)² = 9.75%). A Champion Fighter's Improved Critical hit on 19-20 gives 10% base, and with Advantage: approximately 19%.
Base Critical Hit Probability
In D&D 5e, a natural 20 on an attack roll is always a critical hit regardless of the target's Armor Class. The probability is always 1/20 = 5%. Additionally, attacks always miss on a natural 1 and always hit on a natural 20 — these are absolute rules that modifiers cannot override.
| Condition | Crit Range | Base Probability | With Advantage |
|---|---|---|---|
| Standard | 20 | 5% | 9.75% |
| Champion Fighter (3rd level) | 19-20 | 10% | 19% |
| Champion Fighter (15th level) | 18-20 | 15% | 27.75% |
| Hexblade Warlock (Hex Warrior) | 20 | 5% | 9.75% |
| Paladin Divine Smite on Crit | 20 | 5% | Bonus damage doubling |
Critical Hit Damage Calculation
On a critical hit, you roll all the weapon's damage dice twice. A longsword normally does 1d8 damage — on a crit, you roll 2d8 + modifier. A greatsword (2d6) crits for 4d6 + modifier. Spell attacks also crit and double their damage dice.
Lucky Feat vs Advantage for Crits
The Lucky feat lets you add a third d20 to any roll and choose which to use. This gives you three chances at a 20 rather than two. P(crit with Lucky) = 1 - (19/20)³ ≈ 14.3% — higher than Advantage's 9.75%. However Lucky is limited to 3 uses per long rest, while Advantage can apply every attack.