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Dice probability calculator

Compute the exact probability distribution, odds and stats for any dice roll notation.

Input
Output
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Dice probability calculator
Type a dice roll in standard notation — 3d6+2, 2d20-1, d100 — and this tool computes the exact probability of every possible result, not a simulated approximation. It works out the full sum distribution by combinatorics, then reports the mean, median, standard deviation and the minimum/maximum, alongside a text histogram or a plain table of value-by-value odds. It suits tabletop players tuning a house rule, game designers balancing a roll table, and anyone who wants a fast alternative to a dice-probability spreadsheet.
The options cover mechanics real tables use. Keep dice computes a "highest 3 of 4d6" ability-score roll or a "lowest 2 of 5d10" damage roll, keeping only the top or bottom N dice. Advantage and disadvantage roll the whole expression twice and keep the higher or lower total, matching D&D 5e advantage. Reroll each 1 once applies a common house rule to every die before summing. A success threshold adds the probability of reaching or beating a target number, useful for checking the odds of hitting a DC or armor class. Cumulative probability adds an "at least this value" column, and the output format switches between a bar-chart histogram and a plain numeric table.
Notation parsing ignores extra spaces and line breaks, so "3d6 + 2" or a roll split across lines works the same as "3d6+2". Very large pools are capped rather than left to freeze the tab — the tool reports when a roll has too many combinations to compute exactly and asks for fewer dice or sides.
Everything runs locally in your browser; the notation you type is never uploaded or logged. Copy the result, download it as a .txt file, or send the output back into the input to compare a follow-up roll.
FAQ
What does a notation like 3d6+2 mean?
Roll 3 six-sided dice, add their results together, then add 2. The dice count before "d" is optional and defaults to 1, so "d20" means one twenty-sided die.
Is the distribution exact, or a simulation?
Exact. The tool computes every combination of dice outcomes combinatorially, so a probability like 21/1296 comes from counting, not from rolling millions of virtual dice.
How do advantage and disadvantage work here?
The whole roll (dice, keep rule and all) is evaluated twice independently, and advantage keeps the higher of the two totals while disadvantage keeps the lower — the modifier is applied once, after the higher or lower total is chosen.
What happens with a very large dice pool?
Exact computation for huge pools (hundreds of dice or dice with thousands of sides) can be too expensive to run instantly, so the tool shows a message asking you to lower the dice count or the number of sides instead of freezing the page.
Is my dice notation uploaded anywhere?
No. The entire calculation runs in your browser using local JavaScript — nothing you type is sent to a server.