D&D advantage and disadvantage: which d20 counts?

Keep one die before adding the modifier

Under the D&D basic rules, advantage uses the higher of two d20 results; disadvantage uses the lower. Choose that die, then apply the modifier once. For a 4 and 17 with a +3 modifier, the respective totals would be 20 and 7.

This explains the basic operation, not every feature that can alter it. For rerolls, replacements or other exceptions, use the rules that apply at your table.

Source: D&D Beyond · Basic Rules · Playing the Game

Work from the die result you actually need

If the target total is 15 and your modifier is +3, a die result of 12 or above reaches it. A fair d20 has nine such faces, so the ordinary chance is 9/20, or 45%.

With advantage you fail only if both dice are below 12: 1 − (11/20)² = 69.75%. With disadvantage both must reach 12: (9/20)² = 20.25%. These calculations assume independent fair dice and a simple numerical threshold, without additional success, failure or reroll rules.

Why 'advantage equals +5' is not a general rule

For an ordinary threshold roll, adding five changes which faces pass. Advantage changes how two rolls are combined. At a threshold of 11, advantage raises 50% to 75%; at a threshold of 20, it raises 5% to 9.75%. Those changes are very different.

An arithmetic shortcut may be useful in a particular rule or approximation, but it cannot replace this calculation for every target. Also distinguish the raw die threshold from a target that already includes your modifiers.

Use the individual dice in Picknado

Set number of dice to 2 and sides per die to 20. The large headline is their sum; for this purpose, ignore it and read the two individual results. Select the higher or lower manually and add your modifier afterwards.

Announce what the roll represents before generating it. Two d20s are not an invitation to choose advantage after seeing which outcome would be convenient. Keep a copy of the individual results if the group needs to check the arithmetic later.