Advantage & Disadvantage, Explained
The simplest good rule in modern d20 games — and the math behind it.
Advantage and disadvantage are the d20 game's elegant way of saying "this is easier" or "this is harder" without fiddly bonuses. The rule is tiny: roll two d20s instead of one. With advantage, keep the higher die. With disadvantage, keep the lower. That's the whole thing — and you can do it in one tap on the dice roller.
When do you get it?
The GM grants advantage when circumstances clearly favor you: attacking a surprised enemy, helping an ally, the right tool for the job. Disadvantage comes from anything that hinders you: attacking in the dark, firing at long range, acting while poisoned or frightened. You don't add a number — you just change how the die is rolled.
The math: advantage is not a flat +5
A fair d20 averages 10.5. Keeping the higher of two independent d20s averages 13.825; keeping the lower averages 7.175. That is a change of 3.325 in the average die result. The change in your chance of success depends on the number you need, so it cannot be replaced by a fixed modifier.
Let p be the chance that one die meets your target after the modifier. With advantage, both dice must fail for the check to fail, so success is 1 − (1 − p)². With disadvantage, both must succeed, so success is p². This counts all 400 equally likely pairs; it is not a simulation.
| Die needed | One d20 | Advantage | Disadvantage |
|---|---|---|---|
| 6+ | 75% | 93.75% | 56.25% |
| 11+ | 50% | 75% | 25% |
| 16+ | 25% | 43.75% | 6.25% |
| 20 | 5% | 9.75% | 0.25% |
At 11+, advantage adds 25 percentage points: the same success-rate increase as +5 on one d20 for this particular target. At 20, it adds only 4.75 percentage points. Advantage also changes critical chances: the kept die is a natural 20 in 39 of 400 pairs, and a natural 1 in just one pair.
Check your own odds
Enter the target number (DC) and the bonus added to the die. This is a mathematical threshold check: a natural 1 or 20 has no automatic effect. If your game uses automatic failure or success, apply those rules separately.
DC 15 with +4 needs 11+: one d20 50%; advantage 75%; disadvantage 25%.
Two common conventions
The following are common d20 conventions, not universal rules for every tabletop system. Your game's rulebook and table agreement take precedence.
- It doesn't stack. Three sources of advantage still means roll two dice, keep the higher. You never roll three and keep the best.
- They cancel out. If you have advantage and disadvantage at the same time, they cancel completely — you roll one normal d20, no matter how many of each you have.
Rolling it without the headache
Tracking two physical dice and remembering which to keep is exactly the kind of thing that slows combat down. Set the dice roller to Advantage or Disadvantage and tap d20 — it rolls both, keeps the right one, and crosses out the die you dropped so the table can see it was fair.
- Does advantage apply to damage?
- No. Advantage and disadvantage only affect a single d20 roll — an attack, a check or a save. Damage pools like 2d6 are rolled normally.
- What if I have advantage from two things?
- Still just two dice, keep the higher. Multiple sources don't add up.
- What happens with both advantage and disadvantage?
- They cancel out entirely and you roll a single normal d20.