Raid Lockout Calculator: Weeks to a Drop

Weekly lockouts change the math

A raid drop is not just about how rare the item is. It is also about how many times you are allowed to try per week. That is why a boss with a low drop chance can still be a reasonable farm if you get a fresh lockout every reset.

This guide shows the plain-English way to answer a simple question: if a boss has a certain drop chance per clear, and you can only clear it a set number of times each week, how many weeks do you need to reach a 50% chance of seeing at least one drop? How many for 90%?

The calculator does the arithmetic. The method is easy enough to understand on paper.

The core idea

The cleanest way to think about this is from the opposite direction.

Instead of asking, “What is the chance I get the drop?” ask, “What is the chance I do not get the drop?”

If the drop chance per clear is 10%, then the no-drop chance on one clear is 90%. If the drop chance per clear is 25%, then the no-drop chance on one clear is 75%.

When clears are independent, you multiply the no-drop chance across all your attempts.

So if:

• drop chance per clear = p
• no-drop chance per clear = 1 - p
• total clears = n

Then:

• chance of no drop after n clears = (1 - p)^n
• chance of at least one drop after n clears = 1 - (1 - p)^n

That is the whole trick.

How weekly lockouts fit in

Raid farming is usually limited by weekly clears, not by raw attempts.

If you can clear the boss 3 times per week, and you need 12 total clears to reach your target chance, then:

• 12 total clears ÷ 3 clears per week = 4 weeks

If the answer is not a whole number, you round up. You cannot do part of a lockout week in a practical farming plan.

So the process is:

1. Work out how many total clears you need for your target chance.
2. Divide that by clears allowed per week.
3. Round up to the next full week.

The paper method

Here is the simple version you could do by hand.

Step 1: Find the no-drop chance per clear

If the drop chance is 20%, the no-drop chance is 80%, or 0.8.

Step 2: Raise it to the number of clears

If you want to know the chance of no drop after 5 clears, calculate:

0.8 × 0.8 × 0.8 × 0.8 × 0.8 = 0.8^5

That gives the chance of still having nothing after 5 clears.

Step 3: Subtract from 1

If the no-drop chance after 5 clears is 0.32768, then the chance of at least one drop is:

1 - 0.32768 = 0.67232

So you have about a 67.2% chance of at least one drop after 5 clears.

Step 4: Repeat until you hit your target

For a 50% target, you keep increasing the number of clears until the chance of at least one drop is at least 50%. For a 90% target, you do the same until you reach at least 90%.

That is exactly what the calculator automates.

What the calculator gives you

Once you enter the drop chance per clear and the number of clears allowed per week, the tool returns:

• weeks to 50% chance of at least one drop
• weeks to 90% chance of at least one drop
• clears needed for 50%
• clears needed for 90%

That is useful because some players think in weeks, while others think in clears. Both answers matter.

A quick example

Say a boss has a 10% drop chance per clear, and you can clear it once per week.

The no-drop chance per clear is 90%, or 0.9.

After 1 clear:

• chance of at least one drop = 1 - 0.9^1 = 10%

After 5 clears:

• chance of at least one drop = 1 - 0.9^5 ≈ 41%

After 7 clears:

• chance of at least one drop = 1 - 0.9^7 ≈ 52%

So the 50% point is around 7 clears, which is about 7 weeks if you only get one clear per week.

If you had 2 clears per week instead, that same 7-clear target would take 4 weeks, because 7 ÷ 2 rounds up to 4.

What people get wrong

This kind of math is simple, but it is easy to misuse.

• A 50% chance does not mean you are “due” a drop on the next clear.
• A 90% chance does not mean the drop is guaranteed.
• Weekly lockouts matter because they cap your attempts, even if the drop rate itself never changes.
• More clears help, but they do not change the drop chance on any single clear.
• The calculator assumes each clear has the same drop chance.

What the numbers do not cover

These results are useful, but they are not the whole story.

They do not tell you:

• whether the drop is tradable
• whether your group can actually clear that often
• whether the boss has extra loot rules, such as personal loot, bonus rolls, or multiple items per kill
• whether the drop chance changes by difficulty, difficulty tier, or loot mode
• whether your run includes alts, split runs, or other special farming setups

They also do not tell you how long the farm will feel. A 90% target can still leave a real chance of bad luck. That is not a bug in the math; that is how probability works.

Why 50% and 90% are useful milestones

These two targets are popular because they answer different questions.

• 50% is the halfway point. It tells you when the drop is more likely than not.
• 90% is a high-confidence planning target. It tells you when the odds are strongly in your favor, though still not guaranteed.

If you are deciding whether to keep farming a weekly boss, those two numbers are often enough to make the decision clearer.

The short version

Use this rule:

• chance of at least one drop after n clears = 1 - (1 - drop chance)^n

Then:

• find the number of clears needed for 50% or 90%
• divide by clears allowed per week
• round up to get weeks

If you want the result without doing the exponent math yourself, the calculator above does it in seconds.