D100 roller: percentile dice online

How to use this d100 roller

At the top of the page you have two ten-sided dice already set up as percentile dice. One is numbered 00, 10, 20… 90 and gives the tens; the other runs from 0 to 9 and gives the units. When they land, the page adds the two together and shows the final result between 1 and 100, using the rule that 00 and 0 together make 100.

1. Press Roll to throw both dice at once.
2. If you would rather throw them by hand, drag them with your finger or mouse and let go over the tray: the speed and direction of your swipe change how they tumble.
3. Once they stop, read the tens die, the units die and the total.

Each roll is independent of the last. The dice move in a physics simulation, so the number is not decided before they land, and each of the 100 results has exactly the same chance: 1%.

How to read percentile dice

A d100 is hardly ever a single die. The usual approach is two d10s in different colours or, better still, an ordinary d10 (0 to 9) plus a second one with two-digit numbers (00 to 90), which is what this page uses. The two-digit die always gives the tens, so there is nothing to agree before you roll.

You read the result by putting the two digits together. If the tens die shows 40 and the units die shows 7, you have rolled 47. The only special case is the double zero: 00 on the tens die and 0 on the units die is not zero, it is 100. With that rule the possible results run from 1 to 100, exactly a hundred of them.

Using two ordinary d10s

With two dice numbered 0 to 9, say which one is the tens before you roll, for example ‘red is tens’. Then you read them the same way: red 4 and white 7 is 47; red 0 and white 3 is 3; both zeros is 100. Some groups read the double zero as 00 or 0 and play on a 0 to 99 scale. That works too, as long as everyone knows beforehand and the table you are rolling on is written for that range.

Bonus and penalty dice

In Call of Cthulhu (7th edition) you sometimes roll an extra tens die. With a bonus die you keep whichever tens die gives the lower, and therefore better, result; with a penalty die you keep the higher one. There is still only one units die, shared by both. One bonus die and one penalty die cancel each other out.

Is there a 100-sided die? The Zocchihedron

There is, although it is not what most tables use. It is called the Zocchihedron, it was designed by Lou Zocchi and went on sale in 1985. It looks like a ball covered in a hundred small flat spots, each with a number, which is why it is also known as ‘Zocchi’s golf ball’. Inside there is a filling of loose weights that brakes the ball, because a sphere takes a very long time to come to rest.

The trouble is that it does not spread its results evenly. You cannot place a hundred identical faces on a sphere with perfect symmetry, so some faces end up packed more tightly than others. White Dwarf, the British magazine, published a test of 5,164 rolls with the first version: numbers above 93 and below 8 came up noticeably less often than the middle numbers, which sat around the ball's ‘equator’. The numbering was then changed to mix high and low values near the poles. The faces at the poles still come up less often, but they no longer favour one end of the scale.

One enthusiast logged 5,150 rolls of a Zocchihedron bought in 2005, already with the new numbering. It showed no bias towards high or low numbers, but the distribution was still clearly uneven, with 1 and 100 among the faces that came up least. It is also awkward to read: on occasion it comes to rest on the curved strip between two faces.

That is why the practical way to roll a fair d100 is still a pair of d10s. A d10 has ten identical faces (it is a pentagonal trapezohedron), and every combination of the two dice comes up equally often.

Games that use the d100

The d100 is the core of any system where skills are written as percentages: if your Library Use is 60, you succeed by rolling 60 or under. It also turns up in plenty of random tables with a hundred entries.

Call of Cthulhu and the BRP system

Chaosium published RuneQuest in 1978, and its rules became the Basic Role-Playing (BRP) system that Call of Cthulhu has used since 1981. In the 7th edition of Call of Cthulhu you roll 1d100 against your skill:

• Regular success: roll your skill or under.
• Hard success: roll half your skill or under.
• Extreme success: roll a fifth of your skill or under.
• Critical: a roll of 01.
• Fumble: a roll of 100; if the skill is below 50, any roll from 96 to 100.

A player can also ‘push’ a failed roll by explaining how the investigator tries again, but if the pushed roll fails too, the Keeper inflicts a serious consequence. In opposed rolls the higher level of success wins; on a tie, the higher skill wins.

RuneQuest

RuneQuest is percentile too, with its own levels of success: a critical is a roll of a twentieth of your skill or less (01 is always a critical) and a special is a fifth or less. Fumble ranges vary with the edition and with the skill level, so check the rulebook for the one you play.

Warhammer Fantasy Roleplay

In the 4th edition you also roll under your characteristic or skill. 01 to 05 always succeeds and 96 to 100 always fails. In tests where the margin matters you work out Success Levels by subtracting the tens digit of the roll from the tens digit of your target. In combat, a success on a double (11, 22, 33…) is a critical and a failure on a double is a fumble. To find where a blow lands, you reverse the attack roll: a 37 becomes 73 on the hit location table.

Dungeons & Dragons and random tables

D&D runs mostly on the d20, but it uses the d100 for tables. In the 2014 fifth edition Player's Handbook, a Wild Magic sorcerer rolls a d20 after casting a sorcerer spell of 1st level or higher; on a 1, they roll on the Wild Magic Surge table, which has 50 effects, each covering two numbers on the d100. The Dungeon Master's Guide uses the d100 for treasure hoards and for the magic item tables.

Other uses

Many game masters use it for home-made tables of encounters, rumours, weather or loot, and for any question with a percentage attached: ‘there is a 30% chance of rain’ is settled by rolling and checking whether you get 30 or under.

d100 probabilities

With both d10s read correctly, every number from 1 to 100 has a 1% chance. That makes the d100 the easiest die to interpret: the chance of rolling X or under is exactly X %. In roll-under systems, your skill is your percentage chance of success.

Levels of success in Call of Cthulhu (7th edition)

The chance of each level for a given skill. Half and one fifth are rounded down, as on the character sheet.

A critical (01) is always 1%, whatever the skill.

How much a bonus die helps

The chance of rolling your skill or under with an extra tens die, worked out across all 1,000 combinations of three dice:

At low skill a bonus die nearly doubles your chances; at high skill it is the penalty die that hurts most.

How many rolls before you see a 100

Even though it comes up once in a hundred rolls on average, over a hundred rolls there is better than a one-in-three chance of not seeing it at all. The same goes for 01 or any other specific number. In Warhammer, the chance of rolling a double (11, 22… 99 and 100) is 10%.

Making a d100 from other dice, and how not to

Gamers were rolling percentages long before the modern d10. In the late sixties and through the seventies, twenty-sided dice numbered 0 to 9 twice were in circulation, and they were rolled twice (or two at once in different colours) to make a number from 1 to 100. TSR's early dice sets for Dungeons & Dragons included these percentile d20s, and the d10 only joined the company's sets in the 1980s.

With a d20 and a d10

If you are missing the tens die, an ordinary d20 will do: take only its last digit (13 counts as 3, 20 as 0) and use it as the tens, with the d10 as the units. Because every digit from 0 to 9 appears exactly twice on a d20, the result is just as even as with two d10s. We checked all 200 possible combinations: every number from 1 to 100 comes up exactly twice.

Methods that look right and are not

The flaw in adding dice is always the same: far more combinations produce middle results than extreme ones. With ten d10s there is just one way to roll 10 (all ones) and more than 432 million ways to roll 55. If a 30% skill were resolved like that, you would almost never succeed.

d100, d% and other names

All of these names refer to the same thing: an even result from 1 to 100.

When a rulebook asks for ‘1d100’ you do not need a special die: a pair of d10s or the roller on this page is all it takes.

Practical tips for playing with d100