Build the Dice Pool Before You Roll
Start by translating the activity into a precise pool: number of dice, sides on every die, one total modifier, roll mode, and optional success threshold. Matching dice are required in this calculator. A request for two six-sided dice is 2d6; a request for one twenty-sided die with a +5 bonus is 1d20+5.
Keep the game rule beside the setup. A modifier normally changes the final total once, while a success threshold examines each die separately. Confusing those roles can produce a plausible total that answers the wrong question.
- N is the number of dice.
- S is the number of faces on each die.
- The modifier is added once after the subtotal.
- The threshold counts retained faces at or above its value.
Read NdS Notation Without Guessing
Dice notation is compact because d separates count from sides. In 3d8, three independent integers are generated from 1 through 8 and added. In 3d8-2, the same three faces are added before 2 is subtracted. The possible total therefore shifts from 3-24 to 1-22 without changing any die's face range.
Free-form dice languages can also express rerolls, explosions, drops, arithmetic groups, and chained modifiers. This page does not parse those expressions. It displays simple NdS notation and gives two-pool keep behavior its own labeled control so the operation remains inspectable.
Understand the Three Roll Modes
Normal mode rolls one pool. Keep-higher and keep-lower each roll two complete pools with identical count, sides, and modifier. The calculator compares modified totals, retains the larger or smaller one, and displays both attempts. If totals tie, retaining the first attempt changes no numeric result.
For a one-die d20 check, this operation may match a game's advantage or disadvantage mechanic. It should not be described as universal advantage for every roll. Official systems can restrict eligible checks or add special rules for critical faces, multiple sources, cancellations, rerolls, and modifiers.
Follow the Random Face Generation
The browser fills integer arrays through crypto.getRandomValues(), which MDN and the Web Cryptography specification describe as a cryptographically strong random-value source. The engine then uses rejection sampling. It accepts only the largest source interval that divides evenly by the number of die sides and redraws values in the leftover interval.
This matters because applying a simple remainder operation to an uneven source range can favor some faces. Rejection removes that modulo bias. The implementation is appropriate for casual independent rolls, but it does not create a public seed, reproducible transcript, independent witness, or certified draw record.
Interpret Range, Mean, and Standard Deviation
N fair S-sided dice have a subtotal from N through N times S. One fair die has expected face (S + 1) / 2, and a normal pool has expected subtotal N times that mean. The total modifier is then added once. For 4d6+2, the range is 6-26 and the expected normal total is 16.
Standard deviation describes long-run spread around that expected total. It does not grade a roll as valid or invalid. A single result near either end of the possible range can occur naturally, and even several repeated values do not prove a generator is biased.
Count Successes in a Dice Pool
A threshold converts each retained die into success or failure. On a d10 with target 8, the successful faces are 8, 9, and 10, so the per-die chance is 3/10. Six such dice have 1.8 expected successes in one independent pool, although every observed roll must contain a whole-number count.
The chance of at least one success equals one minus the probability that every die fails. Keep modes choose the pool by modified total, not by success count. A retained higher total can occasionally have fewer successes than the discarded pool when face combinations differ.
Audit a Roll and Its History
Check the retained faces first, add them to reproduce the subtotal, and apply the modifier once. For a keep mode, repeat that arithmetic for both attempts and confirm that the labeled retained total follows the chosen comparison. The face-frequency table should reconstruct the retained subtotal when each face is multiplied by its count.
Recent history is a convenience for the open page session, not an audit log. It records a compact expression, mode, retained faces, and total, then disappears after a full reload. Copy or download the current result when the activity requires a durable note, while remembering that the export is not tamper-evident certification.
Choose the Right Randomness Standard
A browser roller is useful for tabletop play, board games, classroom demonstrations, casual prompts, and informal choices. It is not the right system when regulations, contest terms, security procedures, research protocols, or prize rules require a reproducible seed, witnessed draw, independent audit, secure ceremony, or approved random-number provider.
Match the standard to the consequence. For ordinary play, visible faces and unbiased sampling make the result easy to inspect. For money, legal rights, regulated gambling, valuable prizes, or security decisions, use the authorized process and retain the records that process requires.
- Use official game rules for special dice mechanics.
- Use repeated trials, not one roll, when studying frequency.
- Use a certified or documented system when the outcome has regulated consequences.
- Do not describe a casual page-session history as an audit trail.
Frequently asked questions
How do I roll dice online?
Choose the number of dice and sides, add an optional total modifier, select a mode, set the success threshold, and press Roll dice. The page shows every retained face, the subtotal, final total, and supporting probability context.
Can I roll a D20, D6, D100, or several dice at once?
Yes. Use the D4, D6, D8, D10, D12, D20, or D100 preset, or enter a custom die with 2 to 1,000 sides. You can roll 1 to 100 matching dice in one pool.
What does 2d6+3 mean?
It means roll two six-sided dice, add their faces, then add 3 once to the subtotal. Its possible total is 5 through 15 and its expected normal total is 10.
How do keep-higher and keep-lower modes work?
The calculator rolls two complete pools with identical settings. Keep-higher retains the larger modified total and keep-lower retains the smaller. Both attempts remain visible, and a tie retains the first attempt.
Is keep-higher the same as advantage in every game?
No. It resembles advantage for a one-die check when a game says to roll twice and keep the higher result, but game systems can define eligible rolls, modifiers, ties, critical results, rerolls, and special effects differently. Follow the official rules for the game.
Is this online dice roller fair and random?
Each face uses browser Web Crypto values and rejection sampling to avoid modulo bias. That is suitable for casual unbiased rolls, but a small sample cannot prove fairness and the page is not an independently certified draw service.
References
These sources support the method or guidance used for Dice Roller. Verify time-sensitive rules at the source.
Try the calculator
Open Dice Roller, enter your scenario, and compare its supporting rows with this guide's method and checks.