Probability Calculator
This tool has been used … times
Instantly work out your odds: the chance of winning at least once over N draws, the chance of winning several times in a row, or the number of draws needed to hit a target. For example, pulling a 1% gacha 100 times gives about a 63% chance of at least one win. Enter odds as a fraction or a percentage. Free.
Win probability
—%
Miss probability
—%
How to Use
With or Without Replacement
This tool assumes every draw has the same probability — drawing with replacement, like gacha. If you draw without replacement from a fixed pool of tickets (so the odds change after each draw), the result will differ.
Input Mode: Fraction
Enter a numerator and denominator to specify the probability. For example, “1 / 100” equals a 1% chance per draw.
Input Mode: Percent
Enter the probability directly as a percentage. For example, entering “1” means a 1% chance per draw.
Calculation Mode: At Least Once
Calculates the probability of winning at least once across N draws, and the probability of getting no wins at all. Great for calculating gacha or lottery odds.
Calculation Mode: Consecutive
Calculates the probability of winning on every single one of N consecutive draws. Useful when all attempts must succeed.
Calculation Mode: Min. Draws
Calculates the minimum number of draws needed to reach a target win probability (e.g., 80%). Enter a price per draw to also see the estimated total budget.
Save Conditions
Press “Save” to keep the current settings as a favorite. Add a name to compare multiple gacha conditions, and press “Load” in the list to restore them anytime. Saved data stays only in this browser.
Frequently Asked Questions
If I pull a 1% gacha 100 times, am I guaranteed a win?
No. The chance of winning at least once is 1 − (1 − 0.01)^100 ≈ 63.4%, not 100%. More draws raise the odds but never make a win certain. Use the “At Least Once” mode to calculate this figure automatically.
How do I convert a fraction to a percentage?
Divide the numerator by the denominator and multiply by 100. For example, 1/100 = 1% and 3/200 = 1.5%. This tool lets you switch between “Fraction” and “Percent” input modes, so you can enter odds either way.
What is the formula for winning at least once in N draws?
1 − (1 − p)^N, where p is the per-draw probability. It equals 1 minus the chance of missing every time, (1 − p)^N. Winning on every one of N consecutive draws is instead p^N.
How many draws do I need for a target probability?
Use “Min. Draws” mode and enter your target (e.g. 80%). It back-calculates the minimum number of draws, using N ≥ log(1 − target) ÷ log(1 − p). Enter a price per draw to also see the estimated budget.