Calculate the odds of getting your desired character stats, gear, or traits when rerolling in video games, tabletop RPGs, or gacha titles. This tool helps gamers, game designers, and competitive players plan reroll sessions more efficiently. It accounts for common RNG mechanics used in modern gaming systems.
How to Use This Tool
Follow these steps to calculate your character reroll probabilities accurately:
- Select your input method: choose between entering favorable/total outcomes per roll or a direct success probability.
- Fill in the required input fields based on your selected method. For favorable/total, enter how many outcomes count as a success and the total possible outcomes per roll.
- Enter the total number of roll attempts (including your initial roll and any rerolls).
- Select the calculation type: probability of at least 1 success, 0 successes, or exactly K successes.
- If calculating exactly K successes, enter the value of K (number of successful rolls you want).
- Click the Calculate button to view your detailed results. Use the Reset button to clear all inputs.
Formula and Logic
This tool uses standard binomial probability formulas to calculate reroll odds, as most gaming RNG systems use independent, identical trials for rerolls:
- Probability of success per roll (p) = Favorable Outcomes / Total Outcomes (or direct input value, converted to decimal).
- Probability of failure per roll (q) = 1 - p.
- Probability of 0 successes in n rolls: qⁿ
- Probability of at least 1 success in n rolls: 1 - qⁿ
- Probability of exactly K successes in n rolls: C(n,K) * pᴷ * qⁿ⁻ᴷ, where C(n,K) is the number of combinations of n items taken K at a time.
- Expected number of successes: n * p
All results are rounded to two decimal places for readability.
Practical Notes
Keep these gaming-specific factors in mind when using this tool:
- Many gacha and RPG games use pseudo-RNG rather than true random, but these calculations still provide a close approximation for planning purposes.
- Some games have "pity" systems that increase success probability after a set number of failed rolls — this tool assumes fixed probability per roll, so adjust your inputs if a pity system applies.
- For tabletop games using dice (e.g., D&D, Pathfinder), total outcomes are the number of sides on the die, and favorable outcomes are the number of sides that count as a success.
- Game patches may adjust drop rates or RNG mechanics — always use the most recent in-game values for accurate calculations.
- Competitive players often use these calculations to determine if rerolling for a specific stat or gear piece is worth the time investment.
Why This Tool Is Useful
This calculator saves time and reduces guesswork for multiple gaming use cases:
- Gamers can plan reroll sessions for gacha games, avoiding wasted time on impossible rolls.
- Game designers can balance RNG mechanics by testing how probability changes with different roll counts.
- Streamers can use real-time probability data to explain odds to their audience during live gameplay.
- Competitive players can optimize their reroll strategy for maximum efficiency in ranked or tournament settings.
Frequently Asked Questions
Does this tool account for gacha game pity systems?
No, this tool assumes a fixed probability per roll. If your game has a pity system that increases drop rates after failed attempts, you will need to adjust your input probability manually for later rolls, or calculate each roll segment separately.
What is the difference between "total roll attempts" and "number of rerolls"?
Total roll attempts include your initial roll plus any additional rerolls. For example, if you have 1 initial roll and 3 rerolls, your total roll attempts are 4.
Can I use this for tabletop RPG dice rolls?
Yes, this tool works for any independent RNG roll. For a D20 roll where you need a 18 or higher to hit, favorable outcomes are 3 (18,19,20) and total outcomes are 20, so p = 0.15 per roll.
Additional Guidance
Follow these tips to get the most accurate results:
- Always verify in-game drop rates or RNG values from official patch notes, as community-reported values may be outdated.
- For multi-step reroll processes (e.g., rerolling both character and weapon), calculate each step separately then multiply the probabilities if the rolls are independent.
- If you are calculating odds for a tournament or competitive event, double-check that the game's current patch has not changed the RNG mechanics you are using.
- Save your input values if you plan to recalculate with adjusted roll counts or probability values.