Advantage vs flat bonuses: the real maths

In Dungeons & Dragons 5th edition, advantage means you roll two d20 and take the higher result; disadvantage means you take the lower. It’s the game’s cleanest mechanic — no arithmetic, just grab a second die. But players often overrate or underrate it because its value isn’t a fixed number.

The average swing

A single d20 averages 10.5. Take the higher of two d20 and the average jumps to about 13.8 — a raw increase of roughly +3.3 on the number you roll. That’s the figure people quote, and it’s a fair headline. But “average roll” isn’t what wins fights. What matters is your chance of beating a target, and there advantage behaves very differently depending on the target.

Why the middle is where advantage shines

Advantage helps most when your success is close to a coin-flip. If you need an 11 or higher — a 50/50 roll — a second die is most likely to rescue a near-miss, and advantage lifts your success chance by a full 25 percentage points. That’s the equivalent of about a +5 flat bonus.

Now go to the extremes. If you only need a 2 to succeed, you were going to hit anyway 95% of the time — the second die rarely changes anything, so advantage is worth barely +1. Likewise, if you need a natural 20, advantage roughly doubles a tiny chance but still leaves you almost certain to fail; again worth about +1. The maths is symmetric: advantage’s equivalent bonus rises from the edges to a peak in the middle.

Approximate value of advantage by target

These are rounded “equivalent flat bonus” figures — how big a static modifier would give the same jump in success chance. Read the table as a curve: near-worthless at the edges, worth about +5 in the middle.

When a flat +2 beats advantage

Because advantage is unreliable at the extremes, a small guaranteed bonus can outperform it there:

• When you barely need it. If you already succeed on a 3+, a flat +2 that widens the crit range or pushes past a damage threshold does more than a second die that mostly agrees with the first.
• When you’re a long shot. Chasing an 18+ target, +2 shifts every outcome up by two; advantage just gives a second slim chance.
• For consistency. A flat bonus applies to every roll, all day. Advantage often has to be earned each turn (flanking a target, help actions, specific spells) and can be cancelled.

Around the middle, advantage clearly wins — a one-time +5-equivalent is huge. It’s at the tails where a humble modifier quietly pulls ahead.

Stacking: advantage doesn’t pile up

A crucial rule: advantage never stacks. Three different sources of advantage on the same roll still means you roll exactly two d20 and take the higher — not three, not four. And advantage and disadvantage cancel: if you have at least one of each, you have neither, and roll a single normal d20 regardless of how many of each apply. Flat bonuses, by contrast, do add together, which is another reason a stack of small modifiers can quietly outpace a single grant of advantage.

The takeaway

Advantage is best thought of as “up to +5, when the roll is a coin flip.” Seek it out for the tense, even-odds moments — the clutch save, the tight attack. For the rolls you were always going to make or miss, a reliable flat bonus is often the smarter investment. If you want to see the spread for yourself, roll a few hundred with advantage on and off in Dice Drop and watch the totals shift.

Keep reading: probability, variance & luck in dice — the maths behind why some rolls feel streaky.