The Tombs of Amascut (TOA) raid in Old School RuneScape offers some of the best in-slot gear and valuable rewards. A key factor in acquiring these rewards is understanding and maximizing your drop rate. This calculator helps you estimate your chances of obtaining a rare item based on various in-game mechanics.
Base Drop Rate
Every item in the Tombs of Amascut has a base chance of being dropped. This is the fundamental probability of receiving a specific item before any modifiers are applied. For instance, if an item has a 1/20 drop rate, its base drop rate is 0.05 (or 5%).
Number of Rolls/Attempts
The more times you successfully complete a raid encounter or a specific challenge within the raid, the more opportunities you have for a drop. This input represents the total number of times a drop could potentially occur during your raiding sessions. A higher number of rolls significantly increases your overall chance of seeing a rare drop.
Success Bonus
Certain actions or achievements within the Tombs of Amascut can grant a "Success Bonus" to your drop rate. This might come from completing waves without taking damage, using specific mechanics correctly, or achieving certain raid percentages. This bonus typically increases your effective drop rate for that particular roll. The value entered here is a percentage that will be added to the base drop rate.
Failure Penalty
Conversely, mistakes or failures during the raid can sometimes incur a "Failure Penalty," which reduces your effective drop rate for a given roll. This could be due to taking too much damage, failing a puzzle, or other negative outcomes. The value entered here is a percentage that will be subtracted from the base drop rate.
How the Calculator Works
The calculator first determines the "effective drop rate" for a single roll. It starts with the Base Drop Rate, then applies the Success Bonus (as a percentage of the base rate) if you've succeeded, or the Failure Penalty (as a percentage of the base rate) if you've failed.
The formula for the effective drop rate per roll is:
Effective Rate = Base Rate * (1 + Success Bonus / 100) (for a successful roll)
Effective Rate = Base Rate * (1 - Failure Penalty / 100) (for a failed roll)
Since we don't know the exact ratio of successes to failures in a given set of rolls, the calculator assumes a simplified approach for demonstration: it calculates the probability of NOT getting a drop for a single roll and then extrapolates that over the total number of rolls.
Probability of NOT getting a drop on one roll = 1 - Effective Rate.
Probability of NOT getting a drop over Number of Rolls = (1 - Effective Rate) ^ Number of Rolls.
Therefore, the estimated probability of getting at least one drop is:
1 - ( (1 - Effective Rate) ^ Number of Rolls )
The calculator displays the estimated percentage chance of obtaining at least one of the specified rare items after the given number of attempts, considering the modifiers.
Example Usage
Let's say you are targeting the 'Twisted Bow' from the Tombs of Amascut. The base drop rate for a rare item (like the Twisted Bow, assuming it's a unique drop) is approximately 1/300, so your Base Drop Rate is 0.00333. You plan to do 500 raids, and you're confident you can maintain a high success rate, earning a Success Bonus of 10%.
In this scenario, the calculator would show your estimated chance of getting at least one Twisted Bow after 500 raids.
function calculateDropRate() {
var baseDropRateInput = document.getElementById("baseDropRate").value;
var numberOfRollsInput = document.getElementById("numberOfRolls").value;
var successBonusInput = document.getElementById("successBonus").value;
var failurePenaltyInput = document.getElementById("failurePenalty").value;
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
// Validate inputs
if (isNaN(baseDropRateInput) || baseDropRateInput 1) {
resultDiv.innerHTML = "Please enter a valid Base Drop Rate between 0 and 1.";
return;
}
if (isNaN(numberOfRollsInput) || numberOfRollsInput <= 0) {
resultDiv.innerHTML = "Please enter a valid Number of Rolls greater than 0.";
return;
}
if (isNaN(successBonusInput) || successBonusInput < 0) {
resultDiv.innerHTML = "Please enter a valid Success Bonus (non-negative).";
return;
}
if (isNaN(failurePenaltyInput) || failurePenaltyInput 1) effectiveRateSuccess = 1; // Cap at 100%
var probNoDropSuccess = Math.pow(1 – effectiveRateSuccess, numRolls);
var probAtLeastOneDropSuccess = 1 – probNoDropSuccess;
// Scenario 2: Using the failure penalty (assuming a high failure rate)
var effectiveRateFailure = baseRate * (1 – failurePenalty);
if (effectiveRateFailure < 0) effectiveRateFailure = 0; // Cap at 0%
var probNoDropFailure = Math.pow(1 – effectiveRateFailure, numRolls);
var probAtLeastOneDropFailure = 1 – probNoDropFailure;
// Scenario 3: Just using the base rate for comparison
var probNoDropBase = Math.pow(1 – baseRate, numRolls);
var probAtLeastOneDropBase = 1 – probNoDropBase;
resultDiv.innerHTML += "