Every value has a 1 in 100 chance — 1% each. Average roll: 50.5.
The d100, or percentile die, is the one that speaks in plain percentages. A 40% chance is a roll of 40 or less. No conversion, no target number to work out — the number on the dice is the number in the rules.
That directness made it the backbone of an entire branch of roleplaying games, from Call of Cthulhu to Warhammer Fantasy Roleplay, and it makes it the default die for random tables long enough to hide something rare.
Most d20 games ask you to roll high against a difficulty. Percentile games usually invert it: your skill is a number out of 100, and you succeed by rolling that number or less. A Call of Cthulhu investigator with Library Use 65 finds the reference on a roll of 65 or under. The skill printed on the sheet is literally your chance of success, which makes those character sheets unusually easy to read.
Modern Call of Cthulhu layers degrees on top: rolling at or under half your skill is a hard success, at or under a fifth is an extreme success, and 96 to 100 fumbles. Warhammer does something similar, counting degrees of success from the tens digit. Either way, one roll tells you both whether you succeeded and how well.
At a physical table a d100 is almost always two d10s: one marked 00 to 90 for the tens, one marked 0 to 9 for the units. Decide which is which before you throw, read them together, and treat 00 with 0 as 100 rather than zero.
There is a real single-die version. The Zocchihedron, designed by Lou Zocchi, is a hundred-sided ball covered in small flat facets — the closest thing to a spherical die ever sold. It is also notorious: being nearly round, it rolls for an uncomfortably long time, and its internal weighting has been criticised for producing an uneven spread. This roller gives you an honest 1-to-100 result without either problem.
A hundred-entry table is the smallest one where a single line can be a genuine one-percent rarity. That is exactly what you want for the results players should see once a campaign — the wild magic surge that turns you blue, the treasure entry nobody expected, the encounter that rewrites the session.
DnD’s wild magic surge table, Rolemaster’s famously detailed critical hit charts, and countless encounter and loot tables all run on 1d100 for this reason. Put the same surprises on a d20 table and they come up too often to stay surprising.
Nominate one die as the tens and one as the units before you throw — percentile sets mark the tens die 00 to 90. Read them together: a 70 and a 3 is 73. Rolling 00 alongside 0 is read as 100.
On its own the tens die showing 00 contributes zero, so 00 with a 4 is simply 4. Only 00 together with 0 is special: that combination reads as 100, because percentile dice cover 1 to 100 rather than 0 to 99.
A genuine hundred-sided die designed by Lou Zocchi — a small ball covered in flat facets. It works, but being nearly spherical it rolls for a long time, and its balance has been questioned. Most players still use two d10s instead.
The maths behind why some rolls feel reliable and others feel like coin flips — with the numbers you can actually use at the table.
A fair question with a real answer. Here is how digital rolls are generated, where they can go wrong, and why physical dice are not the gold standard people assume.
Every rulebook writes rolls in the same compact shorthand. Here is how to read it, including the keep and drop notation that trips people up.
Last updated: August 20, 2026
