Using Percentile Dice in the HERO System
(Intended for Hero System 4th Edition, aka the “Big Blue Book”)
0. Introduction
The following rules allow Skill rolls, Attack rolls, and Characteristic and Perception rolls to be made on percentile dice (d100) rather than on 3d6. This has two advantages: First, it eliminates the ‘breakpoint’ values of Characteristics for these rolls. A Characteristic of 13 will no longer be a ‘magic number’ - Characteristics of 12 or 14 will be equally cost effective. Second, most people find it easier to understand probabilities expressed as percentages than as chances of rolling under a target number on 3d6; people have a better intuition for “74%” than for “12-” even though the two are almost identical.1. Skill Rolls
Each skill that had a 3d6 roll under standard HERO rules has a percentage score in this new system: E.g. Demolition 68% rather than Demolition 11-. The percentage scores are calculated as follows:- Familiarities have a flat 30% score.
- General skills have a 60% base score for 3 points, plus an extra 10% for every extra 2 points spent on improving them.
- Characteristic-based skills have a base score of [(2 × Characteristic) + 40%] for 3 points, plus an extra 10% for every 2 additional points spent.
- Background skills - Knowledge, Professional, and Science skills - have a base score of either 60% for 2 points or [(2 × Intelligence) + 40%] for 3 points, plus an extra 10% for every additional point spent on the skill.
The GM may have some background skills use a Characteristic other than Intelligence, or may have some skills use two Characteristics: For example, he may have PS: Surgery be based on (INT + DEX + 40%) for 3 points.
Using Skills
When a character uses a skill, the player rolls d100 against the skill percentage. A successful roll indicates successful use of the skill. Skill Levels add +10% to effective skill for each level used. In general, each +1 or -1 modifier under the 3d6 system gives +10% or -10% under the percentile system.Characters with a modified skill of 100% or more should be granted automatic success unless using an optional rule for “automatic failure” rolls. Characters with a base skill of 60% or better should be considered “competent” or “expert.” With the +40% bonus for routine tasks, they have a modified skill of 100% - don’t bother rolling the dice.
In addition, a failed roll should not usually indicate total, catastrophic failure. If the character has time to keep working on the situation, the GM should allow repeated rolls with a cumulative -10% modifier on each roll. However, if the character fetches tools, takes extra time, or otherwise acts to give himself a bonus, then the GM should allow the roll and not apply the -10% modifier.
Skill vs Skill
Skill versus skill situations can be handled by either of two different methods. The method the GM chooses will depend on what the GM prefers or what he feels is more appropriate for the given situation:- In the first method, the first character rolls against skill. On a successful roll, read the 10’s digit. (i.e. round the number rolled down to the nearest 10% - a 38 would round down to 30%). This becomes the penalty to the second character’s roll. If the first character’s roll is between 01 and 09, then the second character rolls with a 0% penalty. If the first character’s roll fails, then the second character doesn’t need to roll at all.
- In the second method, the first character rolls against skill. On a successful roll, the number rolled becomes the number that the second character has to beat: The second character must roll above the first character’s roll, but less than or equal to his own skill. For example, if the first character rolled a 38, and the second character had a 96% skill, then the second character would have to roll between 39 and 96 to succeed. Again, if the first character’s roll fails, then the second character doesn’t need to roll at all.
In both methods, skills above 100% “wrap around”: Rolls that succeed by 100 or more are read as the number rolled plus 100. For example, if the character has a skill of 124%, then a roll of 08 is read as “108,” a roll of 22 is read as “122” but a roll of 26 is still read as “26.”
This means that characters with very high skills will be hard to beat if they roll well. If the character has a 124% skill and rolls an 06, it is read as “106.” If using the first method, this rounds down to a -100% penalty on the second roll. If using the second method, the second roll has to beat 106, but still come in under the second character’s skill. In either case, the second character will have to have a skill over 100% himself.
Example (using method one): Andarra has Computer Programming at 60%. She tries to set up a security block on her computer system, taking extra time so as to get a +10% bonus. She thus needs to roll a 70 or less to successfully set up the block. She rolls a 36, setting up the block and giving any intruders a -30% penalty on their Computer Programming roll to hack into her system. If she had rolled 52, the penalty would have been -50% to any hackers, and if she had rolled 74, she would have failed to set up any block.
Later, Andarra has her friend Pfred set up a better block. With his levels and special equipment, Pfred’s modified Computer Programming skill for this task is 114%. Pfred rolls a 11. Normally this would give a block with a mere 10% penalty to hack through, but Pfred’s skill is high enough that wrap-around comes into play and the 11 is read as “111.” As a result, hackers attempting to attack this block have a -110% penalty to their Computer Programming skill.
The “meta-rule” here is that the higher the roll the better one has performed - as long as the roll doesn’t go over one’s skill percent.
Complementary Skills
When using a Complementary skill, roll against skill as normal. If the roll is successful, read the 10’s digit on the dice, divide by two, and round up to the nearest 10%. This becomes the bonus given to the main skill. For example, if a “54” is rolled for the complementary skill (and this is a success) then the main skill gets a 30% bonus: Half of 5, rounded up, is 3 - which translates to a 30% bonus.Remember to apply “wrap-around” for skills above 100%: A roll of 23 against a skill of 130% is really “123” and gives a bonus of 60% to the main skill: Half of 12 is 6 - 60%
Note that the minimum bonus given by a successful complementary skill roll is 10%: A roll of 01-09 (without wrap-around) will give a bonus of 10% rather than 0%.
Skill Modifiers
As noted above, each +1 or -1 worth of “standard” modifier gives a +10% or -10% modifier in the percentile system. For example, the Skill Modifier table on page 18 (4th edition) would translate as follows:| Modifier | Circumstance |
|---|---|
| +30% to +50% | Routine |
| +10% to +30% | Easy |
| -10% to -30% | Difficult |
| -30% to -50% | Extremely Difficult |
| -50% or more | Sheer Folly |
| +10% per level | Extra Time (one or more levels down on the time chart) |
| +10% to +30% | Character has extensive knowledge of the object of his Skill Roll |
| +10% to +30% | Character roleplays the Skill use well |
| +10% to +30% | Using good equipment in connection with the Skill Roll |
| +10% to +30% | Excellent conditions for performing the Skill |
| -10% to -50% | Poor conditions for performing the Skill |
| -10% to -50% | Extremely strange or weird (to the character) object on which to perform the skill |
| -10% to -50% | Lack of proper equipment (if appropriate) |
| -10% to -30% | Combat conditions, for Skills not normally used in combat |
2. Characteristic and Perception Rolls
When using percentile dice, a Characteristic roll is [(2 × Characteristic) + 40%]. An Intelligence or Perception roll, for example, would be [(2 × INT) + 40%]. As is the case with skills, each +1 or -1 modifier in the standard HERO rules would translate into plus or minus 10% on the percentage Characteristic roll.3. Attack Rolls
Percentage Combat Values are calculated as follows:- Defensive Combat Value (DCV): (3 × DEX)%
- Offensive Combat Value (OCV): [(3 × DEX) + 60%]
For Ego Combat Values, substitute EGO for DEX in the above formulae.
When making an Attack Roll using percentile dice, a successful hit requires the character to roll over the target’s DCV but less than or equal to the character’s own OCV. Example: George (OCV 96%) attacks Agent Fred (DCV 30%). George’s d100 Attack Roll must be greater than 30 and less than or equal to 96 for the attack to hit.
Combat Levels and other modifiers mostly apply as usual: Each +1 or -1 modifier in the standard HERO rules gives a 10% modifier in the percentile system. However, note that some modifiers should be switched to apply to the attacker’s OCV rather than to the target’s DCV. In particular, the DCV penalties from a target’s Growth become bonuses to the attacker’s OCV under this system, and likewise the DCV bonuses from Shrinking become OCV penalties to the attacker.
If DCV penalties reduce a target’s DCV below zero, treat the target as having 0% DCV - there are limits to how easy one can make oneself to hit. Before doing this, though, be sure that none of the DCV penalties to the target are really more appropriately applied as OCV bonuses to the attacker (as in the case of Growth, above).
If a character has an OCV greater than 100%, it “wraps around” (and effectively eats into the target’s DCV from the bottom). Example: Speedy Sam (OCV 129%) attacks George (DCV 36%). For Sam, rolls of 01-29 count as “101-129.” Since rolls of 101-129 are above George’s DCV and less than or equal to Sam’s OCV, they hit.
If both OCV and DCV are above 100%, simply drop the 100’s digit from both. I.e. OCV 160% vs DCV 132% simplifies to OCV 60% vs DCV 32%
When making an autofire attack, each full 20 points by which the d100 roll exceeds the target’s DCV indicates an additional hit on the target. For OCV 87% vs DCV 30%, a roll of 31+ indicates one hit, 50+ indicates 2 hits, 70+ indicates 3 hits - and 90+ indicates a clear miss. If the attacker’s OCV “wraps around”, then the autofire count does too: For OCV 115% vs DCV 30%, a 31+ equals 1 hit, 50+ equals 2 hits, 70+ indicates 3 hits, 90+ or 01-09 (read as 101-109) indicates 4 hits, and a roll of 10-15 (read as 110-115) indicates 5 hits. A roll of 16 (too large to wrap-around) indicates a clear miss.
