Every task-resolution system has to compare a random roll against a character’s ability. There are two natural ways to do it. In a roll-over system you add your skill to the die and try to beat a target number — higher is better. In a roll-under system your skill is the target, and you try to roll at or below it — lower is better. D&D is the archetypal roll-over game; Call of Cthulhu and Warhammer Fantasy Roleplay are the best-known roll-under ones.
The two archetypes
| Roll-over (d20) | Roll-under (d100) | |
|---|---|---|
| Example games | Dungeons & Dragons, Pathfinder | Call of Cthulhu, Warhammer Fantasy Roleplay |
| You want to roll | High — meet or beat a target | Low — at or under your skill |
| Where the number lives | The difficulty is the target; skill is a bonus you add | The skill is the target; difficulty shifts it |
| Typical die | A single d20 | Percentile d100 (two d10 read as 1–100) |
In a roll-over game a Perception check might be “d20 + 5, beat 15”. In a roll-under game the same check is “roll d100 at or under your Perception of 55”. Both ask the same thing; they just put the character’s number on opposite sides of the comparison.
How modifiers flip meaning
This is the point that trips up players moving between systems: a bonus in one is a penalty-shaped number in the other. In roll-over, making a task easier means lowering the target or adding to the roll — anything that pushes your total higher helps. In roll-under, making a task easier means raising the number you must stay under, so a “+20” bonus increases your target percentage, while a difficult task subtracts from it. A helpful modifier therefore moves the number up in both systems, but what that number represents — your roll versus your target — is reversed. Keeping straight whether a plus makes the die more generous or the target more generous is the main mental adjustment.
Margins and degrees of success
Both approaches can measure how well you did, not just pass or fail, and this is where roll-under has a quiet elegance. Because you and your target are on the same 1–100 scale, the margin falls out for free: roll a 12 against a skill of 55 and you succeeded by a wide margin; roll a 54 and you scraped it. Many roll-under games formalise this into degrees of success. Call of Cthulhu, for instance, rewards rolling at or under half your skill with a “hard” success and at or under a fifth with an “extreme” one, so a single d100 tells you both whether you succeeded and how impressively.
Roll-over systems get degrees too, usually by comparing how far your total beat the target — Pathfinder’s beating a DC by 10 or more is a classic example — but it takes a subtraction, whereas a roll-under margin is simply the gap between two numbers on one track.
The intuitiveness trade-off
Neither approach is objectively simpler; they are intuitive in different places.
- Roll-over feels natural at the table because “roll high, add your bonus, beat the number” matches how most people expect dice and rewards to work. Bigger is better, full stop.
- Roll-under feels natural on the character sheet because a skill of 65 is your 65% chance — the percentage is right there, no arithmetic required. You always know your odds before you roll.
The cost of roll-under is the mental flip on modifiers and the momentary oddness of cheering for a low number. The cost of roll-over is that your actual chance of success is hidden until you subtract, and that opposed contests can need extra comparison steps. Designers pick based on what they want to be effortless — the moment of rolling, or the reading of the sheet.
Which should you prefer?
If you like transparent odds and detailed degrees of success from one die, roll-under (and percentile dice generally) will feel tidy. If you like a punchy “beat the number” moment and don’t mind that your odds are implicit, roll-over will feel more dramatic. Plenty of gamers happily switch between the two, and once the modifier direction clicks, both become second nature.
Whichever camp your game sits in, it helps to see the numbers land in real time — you can roll a d20 or a percentile d100 and read the result against any target you set.
Try it yourself. Dice Drop rolls real physics dice in your browser — pools, exploding dice and modifiers built in.
🎲 Roll dice freeKeep reading: how dice pools resolve the same question by counting, or the maths of crits and fumbles.