Posted by: markfender | September 6, 2011

Why I Don’t Like Percentile Dice

So, I talked a bit last week about my dislike of percentile dice. I thought I’d expand on that some.

I get why they’re popular. It doesn’t take a math genius to understand the odds when I tell you you have a 68% chance of something. You have to have a grounding in dice and games to understand relatively easy odds with other types of dice. Knowing that 7 is the average result on 2d6 requires a deeper knowledge than simple percentiles (not much knowledge, granted, but more than your average non-gamer has). So I get the appeal. Everyone who has ever taken a math class understands percentiles.

My big complaints are three-fold. The first is that only 10% of the time does the ones place ever matter. If it’s a roll-under system, the fact that you have a 68% chance of succeeding only ever matters if the tens-place turns out to be a 6. Then, and only then, does the ones-place matter. It feels to me that the 1-100 range is more than the game needs. Reducing the die rolled to a single d10 and rolling under the 10s place would accomplish the same thing, except 10% of the time. Is that 10% worthy enough to include? The second part of that equation is how the game handles improvement. The FFG Warhammer games, for instance, give improvements in +10 or +20 to your chance. So, they’re not bothering with improving the ones-place either. Other games have steady and slow improvement in the ones-place, which changes your odds a whole 1%. Whoo.

The second reason I’m not a big fan of roll-under percentile is that it places a cap on how high you can improve a statistic. You can’t have a 110% chance of succeeding. A low-level human vs. a godlike entity can’t have a difference greater than 100%. Some systems work out alternate systems when you get above the 100% mark, but it seems to me that sort of thing could be avoided by adding a number to another number. That way, you never have a cap in your system.

My last complaint is in the math involved. Because almost every single roll-under system has some sort of degrees of success system in place. Not only do you need to roll below your percentile chance, but you also want to roll as well as possible (“well” varies between systems. Some look for the lowest possible result while others look for the closest to your target number without going over). And that gets into subtraction. If I roll a 33, how much better is that result if my chance was 68% versus my opponent who rolled a 26 and had a 55% chance? I’m not that math savvy, so I have to mentally stop all other thought processes and concentrate on that math to figure it out, and I’d rather not. I’d rather just see who rolled highest and call it a day. Granted, this isn’t hard math, but it always perplexes me when almost all roll-under systems then want me to figure out my degree of success. Your relatively easily graspable system just became “complicated” for no reason that couldn’t be solved easier and quicker with addition.

I realize these are pretty specific complaints that stem from my math-brain being undeveloped. Most other people would look at these complaints and dismiss my math skills (which wouldn’t be wrong). But my preference is for addition (or “count successes” sorta systems) and I’d rather not have to perform math operations that I don’t do well. And this being a hobby pastime, I’d rather just use math that’s comfortable for me.

So, I think what we learned is that math should be shunned.



  1. Luckily my Dark Heresy group has several math geeks in it who always figure out the degrees of success well before I do. Thus sparing me even having to try now. Which is good as my imagination works a lot better when it doesn’t have to stop & try to figure out some semi-complicated math all of the time.

  2. It became clear to me how weird stat escalation was in the percentile/roll-under system when I looked at the Ascension rules for Dark Heresy, which allowed you to effectively crank up your dodge to 90%. Some classes also had the ability to dodge several times, like 6-9 times, in a single round (Vindicare Assassin). It was at the point that I looked at the game and wondered what the point of rolling the dice was.

    I hear good things from a friend who has converted DH/RT/DW to Savage Worlds, but I also know that you’re not a fan of that system either.

    • That’s nothing – you should see the 140% chance a Space Marine sniper can get up to.

  3. I honestly haven’t looked at Deathwatch all that much. Role-playing as a space marine has never been particularly exciting to me…

    • You mean you don’t enjoy the role-play challenge of playing a brainwashed psychopath who’s only goal in life is to kill everything he is pointed at?

  4. 20% of the time the 1’s place matters. You need to roll under 68%? The 1’s matters if your first is a 6. It also matters if you rolled a 0.

    When rolling your d10s one at a time, getting that 0 is a petty moment of drama unavailable in other die mechanics, with a possible exception of “exploding” dice, but in practice very few tasks hinge on a die explosion in those games.

    • Wait, why does it matter if I rolled a 0? Any number I roll on the ones place at that point is a success (Unless it’s one of those games where 00 is 100, in which case it matters, but only 1% of the time). So if we’re using that math, shouldn’t it be – the ones place matters 11% of the time?

      • I don’t recall ever seeing a game where a ‘0’ by itself on a d10 roll didn’t count as a 10, or a d% game where ’00’ didn’t count as 100. That said, if you’re rolling the dice one at a time to get under 68, the second roll is necessary on a 0 or a 6, 20% of the time. If you’re rolling against 68 the ‘6’ only matters 2% of the time also, so I reject your reasoning there.

        If you’re rolling both dice at the same time like a reasonable person, there’s no drama or tension generated, of course.

  5. Burrowowl your math is wrong. Markfender math is much more correct. If you roll a 0, the second digit matters ONLY if it’s another 0 and NOT DEPENDING of your skill. UNLESS you are rolling a skill that’s under 10% of course.

    Why people use maths when they can’t understand the fundamentals of it? -__-

  6. I think most of these percentile systems you addressed are over-complicating things. The beauty of a percentile system isn’t in ease of the math, it’s in speed and adaptation during game events. Percentile as a means of determining things by comparison to 50/50 odds at level with no extra skills and then going from there based off a quick thinking GM to keep things moving.

    But the problem most percentile systems have going against them are they don’t mesh with overcomplicated systems meant to micromanage every aspect of play so everything can be agreed upon by everybody (which still doesn’t work with some groups, I might add. Our group saw a lot of arguing in the other D&D players where as we had no arguments. Especially not between players and GM). For example, in a system where you prepick all your skills and level them, how do you incorporate that without overcomplicated? But in a system run like Skyrim by the gm, where skills are earned as you do things in campaign, 1% to something can be a cool reward for nailing observation checks a lot.

    Have you ever played Man Myth & Magic? It’s an older system, but it’s the one my family started on when we started a group for me and my highschool friends. we played that system for 4 years. It was a blast. Part of that was because our GM was creative and adapted to anything we did as players. But It was a good percentile system to start from as a basis.

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