Exact model
Avoid a natural 1 on a d20 attack roll with advantage
- Probability:
- 99.75%
- Odds:
- 399 to 1
- dBan
- +26 dBan
Of the 400 equally likely pairs rolled with advantage, 399 avoid a natural 1 because the higher result is used. A natural 1 remains only when both dice show 1.
Calculation
p = 1 − (1/20)² = 399/400
Assumptions
The dice are fair and independent, and the higher roll is used for one attack. Avoiding a natural 1 does not guarantee a hit. Modifiers, rerolls, and other game systems are excluded.