Percentile Dice and D100 Explained

Percentile dice produce a number from 1 to 100. They are commonly used for percentage-based skills: a character with a skill of 60 may need to roll 60 or less. The exact success rules belong to the game, but reading the dice is straightforward once you distinguish tens from units.

You can roll percentile dice physically or use a virtual d100. Math Rock Roller displays ordinary d100 results as paired tens and units dice, making the digits visible alongside the result.

How two d10s make a percentile result

Use one ten-sided die for the tens digit and another for units. A dedicated tens die is usually marked 00, 10, 20 … 90. The units die is marked 0–9. With two ordinary d10s, agree which is tens before rolling; do not choose afterwards.

Read them together rather than adding their face labels as two ordinary dice. Tens 40 and units 2 give 42. Tens 70 and units 0 give 70.

Under this common convention, the double zero is 100, not zero. That supplies exactly one hundred possible results. Do not treat every units zero as ten while also using the table above; mixing conventions produces incorrect readings.

Some tables or games use different conventions, including a range starting at 00. Follow their instructions. A percentile result is not automatically a percentage chance of something happening: it might instead select a numbered entry on a table.

What does 1d100 mean?

It means one random result from 1 to 100. It does not require a physical hundred-sided die. Paired tens and units are simply a practical way to produce that range.

2d10 ordinarily means add two ten-sided dice, giving a total from 2 to 20. It does not automatically mean percentile dice. In Math Rock Roller's Custom expression mode, use 1d100 for the usual percentile range. Its explicit pair(d10,d10) syntax also reads the two dice as digits, with double zero giving 100.

For other notation, see dice notation explained.

Making a roll-under check

Suppose a game asks for an ordinary roll at or below a skill of 60. A result of 42 succeeds, 60 succeeds and 73 fails. The target is included because the instruction says “at or below”.

If all hundred outcomes are equally likely and there are no exceptions, 60 of them meet that target. That gives a 60% chance. Criticals, automatic failures, opposed checks and difficulty adjustments can change how a particular game interprets the result.

A roll of 100 is therefore not universally a critical failure merely because it is the largest number. Read the game's rule. Likewise, rolling 01 does not automatically supply the same benefit in every percentile system.

Call of Cthulhu is more than a d100 roll

Chaosium's Call of Cthulhu Seventh Edition uses percentile checks with different difficulty levels. Regular checks compare with the full skill, Hard checks with half and Extreme checks with one fifth, rounding down where necessary.

For a skill of 63, those thresholds are 63, 31 and 12. A roll of 28 meets the Hard threshold; a roll of 15 does not meet Extreme. The game's critical, fumble and opposed-result rules add further detail.

Math Rock Roller's Seventh Edition profile calculates its supported difficulty thresholds and identifies criticals and fumbles. It also handles bonus and penalty tens dice. These are system-specific features, not properties of generic d100 notation.

Bonus and penalty dice use a shared units die

In that profile, extra tens dice create alternative percentile candidates while sharing one units result. A bonus favours the lower candidate; a penalty favours the higher.

For example, a shared units die of 7 and tens results of 20 and 60 make 27 and 67. A bonus selects 27; a penalty selects 67. This is not two wholly independent d100 rolls. Math Rock Roller labels the units and tens dice and distinguishes the selected tens result.

Check the display and the rule together

For an ordinary 1d100 roll, the app shows the result through separate tens and units faces. The display does not turn a plain roll into a complete skill check. Choose the Call of Cthulhu profile when you want its supported interpretation, and enter the relevant skill and difficulty.

For another game, consult its rules and use the appropriate profile or Custom expression. The supported systems page explains where automation stops.

Quick question: does 00 mean zero?

On the tens die alone, it contributes zero tens. Paired with a units 7, it means 7. Paired with units 0, the usual 1–100 convention reads it as 100.

Launch Math Rock Roller (opens in a new tab) to try a d100 roll and inspect the paired display.

Rules reference

Chaosium's game-system guide explains percentile dice, difficulty levels and bonus/penalty dice for Call of Cthulhu Seventh Edition.