Calculate your probability of getting a specific item drop based on attempts and percentages.
Enter the percentage chance (e.g., 5 for 5%)
How many times will you kill the boss or open the chest?
Calculation Results
Probability of At Least One Drop:0%
Probability of Zero Drops (Bad Luck):0%
Approx. Runs for 99% Confidence:0
function calculateDropOdds() {
var dropRateInput = document.getElementById("dropRate").value;
var attemptsInput = document.getElementById("attempts").value;
// Validation
if (dropRateInput === "" || attemptsInput === "") {
alert("Please enter both the drop rate and the number of attempts.");
return;
}
var dropRate = parseFloat(dropRateInput);
var attempts = parseInt(attemptsInput);
if (isNaN(dropRate) || isNaN(attempts) || dropRate < 0 || attempts 0 && dropRate = 100) {
runsFor99 = 1;
} else {
runsFor99 = "Infinite";
}
// Convert to Percentages for display
var displaySuccess = (totalSuccessProb * 100).toFixed(2);
var displayFail = (totalFailProb * 100).toFixed(2);
// Update DOM
document.getElementById("probSuccess").innerHTML = displaySuccess + "%";
document.getElementById("probFail").innerHTML = displayFail + "%";
document.getElementById("runs99").innerHTML = runsFor99;
document.getElementById("results").style.display = "block";
// Dynamic Interpretation
var interpretText = "";
if (totalSuccessProb < 0.5) {
interpretText = "It is more likely that you will NOT get the drop in " + attempts + " runs.";
} else if (totalSuccessProb >= 0.5 && totalSuccessProb < 0.9) {
interpretText = "You have a favorable chance, but failure is still a realistic possibility.";
} else {
interpretText = "You are statistically very likely to see the item dropped.";
}
document.getElementById("interpretation").innerHTML = interpretText;
}
How to Calculate Drop Rate Odds
Whether you are farming for a rare mount in an MMORPG, hunting for shiny Pokémon, or trying to pull a specific character in a Gacha game, understanding the mathematics behind RNG (Random Number Generation) is crucial. Many players fall into the trap of thinking that a 1% drop rate means you are guaranteed a drop in 100 runs. This calculator shows why that isn't true.
The Mathematics of RNG
Drop rates generally follow a binomial distribution. Instead of calculating the chance that you will get the item, it is mathematically simpler to calculate the chance that you won't get the item, and then subtract that from 100%.
The Formula: P = 1 – (1 – DropChance)Attempts
Here is how the logic breaks down:
Single Event Failure: If an item has a 1% drop rate, there is a 99% (0.99) chance you will fail to get it on a single run.
Compound Failure: To fail twice in a row, you multiply the probabilities: 0.99 * 0.99 = 0.9801.
Success Probability: The chance of getting at least one drop is 1 minus the chance of failing every single time.
Why 100 Runs Doesn't Guarantee a 1% Drop
Using the logic above, if you run a dungeon 100 times for an item with a 1% drop rate, the math looks like this:
Chance to fail once: 99%
Chance to fail 100 times: 0.99100 ≈ 36.6%
Chance to succeed: 100% – 36.6% = 63.4%
This means that even after 100 attempts at a "1 in 100" drop, roughly 1 out of every 3 players will still not have the item. This statistical reality is often referred to as being "dry" on a drop.
Common Drop Rate Tiers
Understanding the number of attempts required to reach "statistical confidence" (usually 90% or 99%) helps in planning your farming sessions.
Common (10%): You need roughly 22 runs to have a 90% probability of success.
Rare (1%): You need roughly 230 runs to have a 90% probability of success.
Ultra Rare (0.1%): You need roughly 2,300 runs to have a 90% probability of success.
Tips for Efficient Farming
While you cannot control the RNG, you can control your efficiency. Focus on reducing the time per run (Kill Time) rather than obsessing over the drop rate. If you can double the speed of your runs, you effectively double your chances of seeing the drop within the same timeframe.
Use the calculator above to set realistic expectations before you start grinding. Knowing that a 1% drop rate usually requires significantly more than 100 runs can save you from frustration.