Item drops

Items that drop from monsters or item boxes all follow a common set of drop rate calculations that determine which item drops, if one drops at all. Items obtained via other methods, such as gambling Meseta with Coren or redeeming Lucky Coins in Gallon's Shop, use their own unique sets of calculations. If chance is involved in obtaining items, the mechanism by which the drop is determined is commonly referred to as a roll.

Item rolls

A roll can be envisioned as rolling a many-sided die, where rolling a 1 means the item will drop. For example, if a monster drops an item with a 1/200 drop rate, it is equivalent to rolling a 200-sided die.

Monsters

Each time a monster is defeated, the game rolls to determine whether an item will drop, and if so, what that item will be, in the following order:

1. The game rolls for event drops, such as Event Egg, Halloween Cookie, or Present (Christmas)
2. The game rolls for music disks (1/600 from any monster)
3. The game rolls based on the monster's drop anything rate (DAR) to determine if the monster has the possibility of dropping anything else
4. The game rolls to decide if the monster will drop its specified rare item (depending on area, section ID, and difficulty)
5. The game rolls to determine whether the drop is Meseta, a tool, or the monster's set drop. Each of these categories has an equivalent 1/3 chance of being rolled

The chance to obtain items from monsters can be increased with drop rate boosts. DAR boosts increase the chance to obtain any item at all from monsters, which can also help with its designated rare drop. Rare drop rate (RDR) boosts only increase the chance for the designated rare drop. However, a monster can't have more than 100% DAR. Certain monsters already have 100% (bosses, rare monsters) or otherwise high DAR, meaning strong DAR boosts like the 25% weekly boost won't have their full effect. A similar rule applies to RDR, in which the maximum RDR can only be boosted to 7/8 (87.50%). This, however, applies to very few monsters (e.g. rare Rappies from VR Temple).

A monster's set drop is the type of item that it will drop if it fails its rare item roll and doesn't drop either Meseta or a tool. A set drop is a category of item, not a specific one; for example, a Bartle's set drop is a weapon, so it can drop any weapon that would be able to be dropped in the area and section ID it appears in. A set drop can be a weapon, a frame, a barrier, a unit, or nothing.

Item box

Some item boxes may be set to drop specific items (e.g. some boxes in 1-1:Planet Ragol will only ever drop Monomates), or specific categories of items (e.g. some boxes in 1-3:Subterranean Den will only ever drop tools).

For non-drop-specific item boxes, each time an item box is opened, the game rolls to determine whether an item will drop, and if so, what that item will be in the following order:

1. The game rolls to decide if the box will drop its specified rare items (depending on area, section ID, and difficulty). In many areas, item boxes have multiple rare drops assigned to them. The roll order is from top to bottom in the drop charts
2. The game rolls to determine whether the drop is a weapon, frame, barrier, unit, tool, Meseta, or nothing (depending on area and difficulty; Yellowboze has a higher chance to obtain Meseta from item boxes).

Item box drops are not affected by any drop rate bonuses, including (but not limited to):

• drop anything rate (DAR);
• rare drop rate (RDR);
• Anguish common weapon rates;
• Photon Drop drop bonuses;
• event drop rate bonuses;
• altered weapon percentage patterns from Anguish

Calculation

The overall chance to obtain an item is best explained in an example. Assuming the Easter event is currently active, Event Eggs can be obtained by killing monsters. Claws have weapons as a set drop. The calculation of receiving a weapon box from killing a Claw on Ultimate is executed by following the roll order from above:

• Roll for Event Egg fails: 1 - 1/500 = 499/500
• Roll for music disk fails: 1 - 1/600 = 599/600
• DAR roll successful: 3/10 (30%)
• RDR roll fails: 1 - 1/64 = 63/64
• Roll for set drop successful: 1/3

The overall chance can now be calculated by multiplying the above rates:

499/500 * 599/600 * 3/10 * 59/64 * 1/3 = 1/10.89 (9.18%)

From this point, further calculations can be performed in a similar fashion to determine more specific drop rates, e.g. which weapon will drop and with what attributes (see weapon attribute drop tables for further information).

Multiple rolls

The overall chance of obtaining items by killing several monsters or destroying several boxes can also be calculated. For multiple rolls, the overall chance to receive a drop naturally increases. However, this increase is not linear. The increase is determined by following the binomial distribution formula:

chance of at least 1 drop = 1 - (1 - drop rate) ^ (number of events)

Using the above example, the overall chance of receiving at least 1 weapon box from a Claw by killing it 10 times is:

chance of at least 1 drop = 1 - (1 - 1/10.89) ^ (10) = 1/1.62 (61.83%)

Killing more enemies naturally increases the likelihood of receiving an item, but it will never be 100% guaranteed. This is contrary to the popular belief that killing a monster X times for its 1/X item drop will give you a guaranteed drop. In reality, the overall chance in this case always comes out to roughly 63% (approaching 1 - 1/e, where e = Euler's number). This is statistically defined as the average chance where at least 1 drop will occur, and is particularly useful when performing calculations such as "how many runs will it take to get this item?"

Combined drop rates

Some monsters drop the same item but with different drop rates. For a drop rate A and a drop rate B the combined drop rate C is:

C = A + B - A * B

For instance, if one enemy drops an item with a drop rate of 1/2 and another with a drop rate of 1/4, the overall chance of receiving at least 1 drop from killing both monsters is:

1/2 + 1/4 - 1/2 * 1/4 = 1/1.6 (62.5%)