This calculator helps you estimate the number of kills required to obtain a rare item in Old School RuneScape, based on its drop rate. Understanding drop rates can help manage expectations and plan your PVM or skilling activities.
function calculateDropRate() {
var dropRateNumerator = parseFloat(document.getElementById("dropRateNumerator").value);
var dropRateDenominator = parseFloat(document.getElementById("dropRateDenominator").value);
var desiredDrops = parseFloat(document.getElementById("desiredDrops").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
if (isNaN(dropRateNumerator) || dropRateNumerator <= 0 ||
isNaN(dropRateDenominator) || dropRateDenominator <= 0 ||
isNaN(desiredDrops) || desiredDrops <= 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields.";
return;
}
// The probability of getting the drop in a single kill
var dropProbability = dropRateNumerator / dropRateDenominator;
// The expected number of kills to get one drop is the inverse of the probability
var expectedKillsPerDrop = dropRateDenominator / dropRateNumerator;
// The expected number of kills to get the desired number of drops
var totalExpectedKills = expectedKillsPerDrop * desiredDrops;
// To estimate the number of kills to reach a 90% chance of getting at least one drop,
// we use the formula: K = -ln(1-P) / p, where P is the desired probability (0.90)
// and p is the probability of success (dropProbability).
var probabilityToReach = 0.90; // 90% confidence level
var killsFor90Percent = -Math.log(1 – probabilityToReach) / dropProbability;
resultDiv.innerHTML =
"Estimated Kills for " + desiredDrops + " Drop(s): " + Math.round(totalExpectedKills).toLocaleString() + "" +
"(This is the average number of kills expected. You may get your drop much sooner or later.)" +
"Estimated Kills to have a 90% chance of obtaining at least one drop: " + Math.round(killsFor90Percent).toLocaleString() + "";
}
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font-family: sans-serif;
border: 1px solid #ccc;
padding: 20px;
border-radius: 8px;
max-width: 600px;
margin: 20px auto;
background-color: #f9f9f9;
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.osrs-drop-calculator-container h2 {
text-align: center;
color: #333;
margin-bottom: 15px;
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.osrs-drop-calculator-container p {
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margin-bottom: 20px;
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display: block;
margin-bottom: 8px;
font-weight: bold;
color: #444;
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width: calc(100% – 22px);
padding: 10px;
margin-bottom: 15px;
border: 1px solid #ddd;
border-radius: 4px;
box-sizing: border-box;
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button {
display: block;
width: 100%;
padding: 12px 20px;
background-color: #4CAF50;
color: white;
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border-radius: 4px;
cursor: pointer;
font-size: 16px;
transition: background-color 0.3s ease;
}
button:hover {
background-color: #45a049;
}
.result-section {
margin-top: 20px;
padding: 15px;
background-color: #e7f3fe;
border: 1px solid #b3d1f2;
border-radius: 4px;
}
.result-section p {
margin-bottom: 10px;
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.result-section p:last-child {
margin-bottom: 0;
}
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color: #0056b3;
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color: red;
font-weight: bold;
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