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The D100 is Broken

thexar

Explorer
The d100 is forever off by one. This is a problem that programmers deal with constantly, but we seem to try to ignore it when we roll dice. Plus, the zero is usually a zero, but 1% of the time it’s a 10, sort of.

So let’s say we have a 60% chance to succeed. If I roll a 60, then I hit, but what about 59 or 61? Sadly, the 59 hits, and the 61 misses. If 60 is my target, then 01 to 60 succeeds, and 61 to 00 fails. Rolling a 20 on a d20 is good, but rolling a 00 (a “100”) on a d100 fails? Lame. Also, if the target is a multiple of 10 (as 60), then only the tens digit should matter, 1 to 6. But 6 and 0 depend on the one’s result. Zero fails if the one’s die is also a 0, and 6 only succeeds if the one’s die is a 0.

To hide the latter problem, we always roll two dice. Customarily the dice are of different color, and we declare the order before rolling. Or there’s a die marked with 10 to 00 to always represent the tens digit. I find them ugly and annoying to read. I tend to use dice that look the same, because it’s aesthetically pleasing. So I drop them one at a time, and read them as they drop (this is really what led me to the multiple of ten annoyance in the first place).

To fix the first problem we define the target number by failure rate. So if you have a 40% chance to fail, and you roll a 40, then you fail. Our target number to succeed is 41 or higher. It gets tedious when the failure rate isn’t a multiple of 10 or 5. When someone says “36”, does that mean the failure rate was 35%, or that the target number is really 37? This is why I consider d100 always off by one. It’s annoying to always specify “failure” or “target”, so we stop saying it. Then it’s doubly annoying to have to question, when the stated number was rolled, did I hit or miss?

With the shift to all things d20, we don’t see d100 very often, but I still want to fix it. It makes going retro (ADnD 2nd, Palladium, Advanced Marvel Super Heroes, Top Secret, etc.) more enticing.

Our goals are: the value of the die is consistent, the target number is never misunderstood, the math is easy for the roller, and it must be obvious whether you’re expecting to use this method or the traditional one. With these goals in mind I present d99.

D99 works as follows:
• There is a tens die and a ones die.
• Each die is read as 0 to 9, yielding a value 00 to 99.
• The base target number is 100.
• The target number can be modified by the Game Master if the situation is harder (raised) or easier (lowered) than normal. Usually these are external circumstances the player is unaware of.
• The chance to succeed is added to the roll in an attempt to meet the target number. Bonuses or penalties known to the roller are added or subtracted from this total.

Since the target is 100, without some chance of success, you can’t roll 100. With a success of 35%, a roll of 65 or more will score the 100 needed to hit. Most of the time only the tens digit matters, because a zero on either die is just a zero no matter what the other die is. If I had a 30% chance, then simply a 7, 8, or 9 will succeed.

The difference is subtle, but effective.

Thanks for playing.
 

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The d100 is forever off by one. This is a problem that programmers deal with constantly, but we seem to try to ignore it when we roll dice. Plus, the zero is usually a zero, but 1% of the time it’s a 10, sort of.

So let’s say we have a 60% chance to succeed. If I roll a 60, then I hit, but what about 59 or 61? Sadly, the 59 hits, and the 61 misses. If 60 is my target, then 01 to 60 succeeds, and 61 to 00 fails. Rolling a 20 on a d20 is good, but rolling a 00 (a “100”) on a d100 fails? Lame. Also, if the target is a multiple of 10 (as 60), then only the tens digit should matter, 1 to 6. But 6 and 0 depend on the one’s result. Zero fails if the one’s die is also a 0, and 6 only succeeds if the one’s die is a 0.

If the chance of success is exactly 60%, why not only roll that 'tens' die? Rolling 1-6 on d10 has exactly the same probability as rolling 01 - 60 on d%.

For that matter, why not simply invest in a real d100? Since the issue seems to bother you so much, isn't this a quicker and easier solution?

To hide the latter problem, we always roll two dice. Customarily the dice are of different color, and we declare the order before rolling. Or there’s a die marked with 10 to 00 to always represent the tens digit. I find them ugly and annoying to read. I tend to use dice that look the same, because it’s aesthetically pleasing.

Personally, I like the dedicated 'tens' die - at a stroke, it eliminated one of my power gamers' pet tricks. I'm also a great believer in colour-coded dice. I really don't like these matching dice sets, after too many instances of a player spending an age on every roll picking out the correct die to use. Though it was funny when he spent an entire combat rolling his attack rolls on a d12.

Our goals are: the value of the die is consistent, the target number is never misunderstood, the math is easy for the roller, and it must be obvious whether you’re expecting to use this method or the traditional one. With these goals in mind I present d99.

D99 works as follows:
• There is a tens die and a ones die.
• Each die is read as 0 to 9, yielding a value 00 to 99.
• The base target number is 100.
• The target number can be modified by the Game Master if the situation is harder (raised) or easier (lowered) than normal. Usually these are external circumstances the player is unaware of.
• The chance to succeed is added to the roll in an attempt to meet the target number. Bonuses or penalties known to the roller are added or subtracted from this total.

That works, but you'll find that for most people two-digit addition is surprisingly hard. Whereas a comparison will take but a moment, having to add the 36% chance of success to the 67% rolled will take considerably longer.
 


So let’s say we have a 60% chance to succeed. If I roll a 60, then I hit, but what about 59 or 61? Sadly, the 59 hits, and the 61 misses. If 60 is my target, then 01 to 60 succeeds, and 61 to 00 fails. Rolling a 20 on a d20 is good, but rolling a 00 (a “100”) on a d100 fails? Lame.

Because you chose an example where you were rolling under, not over. Rolling 1 on a d20 fails in many games, too.
 
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I don't think most people have this issue with percentile dice. It seems like it would be hard to convince an entire group to switch over to a method like this.
 

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