This tool calculates the probability of getting desired stats or traits when rerolling characters in video games, tabletop RPGs, or gacha titles. It helps gamers, game designers, and competitive players plan reroll sessions efficiently. Avoid wasting time on low-probability reroll attempts with accurate odds estimates.
Character Reroll Probability Calculator
Calculate odds for gacha, RPG, and tabletop rerolls
How to Use This Tool
Follow these steps to calculate your character reroll probabilities:
- Enter the success chance per roll: this is the percentage or decimal chance of getting your desired stat/trait in a single reroll. Use the dropdown to switch between percentage and decimal units.
- Input the total number of rerolls you plan to perform (e.g., 10 rerolls for a gacha session).
- Set your target number of successes: how many desired outcomes you want to achieve.
- Select your probability type: choose "Exactly k successes" for the chance of getting only that many, or "At least k successes" for the chance of getting that many or more.
- Click the Calculate Probability button to see your results. Use the Reset button to clear all inputs and start over.
- Use the Copy Results button to save your probability breakdown to your clipboard.
Formula and Logic
This tool uses the binomial probability formula, which calculates the odds of a fixed number of successes in a set of independent rerolls (each with the same success chance). The core formula for exactly k successes in N rerolls is:
P(X = k) = C(N, k) * p^k * (1-p)^(N-k)
Where:
- C(N, k) is the binomial coefficient (number of ways to get k successes in N rerolls)
- p is the success chance per roll (0-1 decimal)
- k is the target number of successes
- N is the total number of rerolls
For "at least k successes" calculations, the tool sums the probability of exactly k, k+1, ..., N successes. It also calculates expected successes (N*p) and standard deviation (√(N*p*(1-p))) to show result variability.
Practical Notes
Keep these gaming-specific factors in mind when using this tool:
- Gacha and RPG reroll chances are often patch-dependent: check the latest game updates, as developers may adjust drop rates without notice.
- Some games use weighted RNG (pity systems) that increase success chance after consecutive failures. This tool assumes independent, identical rerolls, so adjust your inputs if a pity system applies.
- Tabletop RPG rerolls (e.g., D&D stat rerolls) may have different rules for reroll sets: verify if your game allows rerolling individual stats or full character sheets before using results.
- Low-probability events (e.g., <1% chance) are rare but possible: don't assume a 0.5% chance means you'll never get a result in 100 rerolls.
Why This Tool Is Useful
This calculator helps gamers and game designers make informed decisions about reroll sessions:
- Gamers can avoid wasting hours on low-odds reroll attempts by checking if their target is realistic before starting.
- Competitive players can calculate the expected number of rerolls needed to hit meta-relevant stat thresholds for ranked play.
- Game designers can test balance for in-game reroll mechanics, ensuring drop rates are fair for casual and hardcore players.
- Streamers can use probability breakdowns to explain odds to viewers during gacha or RPG reroll streams.
Frequently Asked Questions
What if my game has a pity system for rerolls?
This tool assumes each reroll has identical, independent success odds. If your game increases success chance after a set number of failures (e.g., 5% chance becomes 10% after 50 rerolls), adjust your input p to the average pity-adjusted chance, or calculate odds for pre-pity and post-pity rerolls separately.
Can I use this for tabletop RPG character creation?
Yes, as long as your game uses independent rerolls for stats or traits. For example, if you reroll 6 D&D stats until you get 2 scores above 16, enter the chance of a single stat being above 16 as p, 6 as N, 2 as k, and select "at least 2 successes" to get your odds.
How accurate are the probability results?
Results are mathematically accurate for independent binomial events. However, they do not account for game-specific RNG manipulation, server-side roll modifications, or unannounced patch changes. Always verify in-game drop rates with official patch notes when possible.
Additional Guidance
Use these tips to get the most out of your reroll probability calculations:
- Test multiple scenarios: calculate odds for 10, 20, and 30 rerolls to see how much your success chance improves with more attempts.
- Compare "exactly" and "at least" results: you may find that aiming for 1 fewer success has a much higher probability, which is still viable for your gameplay needs.
- Save your results with the copy button to track odds across different game patches or balance changes.
- For game designers: use the expected successes and standard deviation to ensure reroll mechanics don't feel too grindy or too easy for players.