The estimated base rate of the item (e.g., Gold Bar ~0.2%).
How many times you plan to farm the quest.
2. Drop Rate Boosts
Total from Main/Support Summons (e.g., Kaguya, Rabbit).
Shop skill boost (e.g., Platinum Stage 20%).
Usually 20% for standard crew buffs.
Mainhand skills (e.g., Oliver, Septian Burner).
Treasure Hunt stacks (TH9 is approx 50%).
Passives like Tien/Esser (5% or 10%).
Probability of Getting At Least 1 Drop
0%
Total Drop Rate Boost:0%
Effective Drop Rate (per run):0%
Runs to reach 95% Confidence:0
Expected Drops (Average):0
Understanding GBF Drop Rates and Probabilities
In Granblue Fantasy (GBF), optimizing your farming efficiency ("grinding") requires a solid understanding of how drop rates are calculated and how probability works over large numbers of runs. Whether you are farming for Gold Bars in Ultimate Bahamut or collecting weapons for your grid, the math remains the same.
How Drop Rate Boosts Work
Most drop rate boosts in GBF are additive to a multiplier that applies to the base drop rate of a chest. The formula generally follows this structure:
Base Rate: The inherent probability that a specific chest will appear or an item will drop.
Boost Multiplier: Calculated as 1 + (Sum of all Boosts / 100).
Effective Rate:Base Rate × Boost Multiplier.
For example, if an item has a 1% base drop rate, and you have a 50% boost from Kaguya and a 20% boost from Journey Drops, your total boost is 70%. Your effective drop rate becomes 1% × 1.7 = 1.7%.
Types of Boosts
To maximize your drops, you should stack as many sources as possible:
Summons: Main and Support summons like Kaguya, Nobita, or White Rabbit.
Crew Skills: Standard crew buffs usually provide a 20% drop rate increase.
Journey Drops: Activated from the shop, providing significant boosts (up to 20% or more during campaigns).
Weapons: Mainhand weapons like Oliver or the Septian Burner.
Bounty (Treasure Hunt): Stacks applied during battle (TH1 to TH10). TH9 is generally estimated around 50%.
Characters: Passive skills from characters like Tien (Esser) or Richard.
The Binomial Distribution: "At Least One Drop"
Many Skyfarers fall into the "Gambler's Fallacy," believing that if a drop rate is 1% and they run the quest 100 times, they are guaranteed a drop. This is incorrect. Probability is calculated using the formula:
P(Success) = 1 - (1 - DropRate)^Runs
Even with a 1% drop rate, running a quest 100 times only gives you roughly a 63.4% chance of seeing the item. This calculator helps you manage expectations by showing the true probability of success over your planned farming session.
function calculateDropRate() {
// 1. Get Inputs
var baseRate = parseFloat(document.getElementById('baseRate').value);
var numRuns = parseInt(document.getElementById('numRuns').value);
// Boosts
var summonBoost = parseFloat(document.getElementById('summonBoost').value) || 0;
var jdBoost = parseFloat(document.getElementById('jdBoost').value) || 0;
var crewBoost = parseFloat(document.getElementById('crewBoost').value) || 0;
var weaponBoost = parseFloat(document.getElementById('weaponBoost').value) || 0;
var bountyBoost = parseFloat(document.getElementById('bountyBoost').value) || 0;
var charBoost = parseFloat(document.getElementById('charBoost').value) || 0;
// Validation
if (isNaN(baseRate) || baseRate <= 0) {
alert("Please enter a valid Base Drop Rate greater than 0.");
return;
}
if (isNaN(numRuns) || numRuns 100) {
effectiveRate = 100;
}
// 4. Calculate Probability of Success (Binomial: 1 – P(Fail)^n)
// P(Fail) per run
var probFailSingle = 1 – (effectiveRate / 100);
// P(Fail) over N runs
var probFailTotal = Math.pow(probFailSingle, numRuns);
// P(Success) over N runs (At least one drop)
var probSuccessTotal = 1 – probFailTotal;
// 5. Calculate Expected Value (Average drops)
var expectedDrops = (effectiveRate / 100) * numRuns;
// 6. Calculate Runs needed for 95% confidence
// 0.05 = (1 – p)^n => ln(0.05) = n * ln(1-p) => n = ln(0.05) / ln(1-p)
var runs95 = 0;
if (effectiveRate >= 100) {
runs95 = 1;
} else if (effectiveRate <= 0) {
runs95 = "Infinite";
} else {
runs95 = Math.ceil(Math.log(0.05) / Math.log(1 – (effectiveRate / 100)));
}
// 7. Update UI
document.getElementById('result').style.display = 'block';
document.getElementById('mainProbability').innerHTML = (probSuccessTotal * 100).toFixed(2) + "%";
document.getElementById('totalBoostDisplay').innerHTML = "+" + totalBoostPercent + "% (x" + boostMultiplier.toFixed(2) + ")";
document.getElementById('effectiveRateDisplay').innerHTML = effectiveRate.toFixed(4) + "%";
document.getElementById('expectedDrops').innerHTML = expectedDrops.toFixed(2);
document.getElementById('runsFor95').innerHTML = runs95;
}