NPC drops

Most enemies drop coins when they are killed, and many also have a chance of dropping various items.

Chance of Obtaining a Drop
To determine the chance for at least one specific item to drop for a given number of enemies with the same chance of dropping said item, use the equation:

$$1-(1-\text{drop chance})^\text{number of enemies}$$
 * Example: What is the chance of obtaining a Rod of Discord after defeating 300 Chaos Elementals? Its drop chance is 0.002 (0.2% or 1/500).
 * , which means the chance is about 45.2%.

To find how many enemies are needed to obtain a certain "goal" chance it has dropped at least once, use:

$$\frac{\ln{(1-\text{goal chance})}}{\ln{(1-\text{drop chance})}}$$
 * Example: How many Chaos Elementals have to be defeated to have a chance of 90% (0.9) of obtaining a Rod of Discord? Its drop chance is 0.002 (0.2% or 1/500).
 * $$\frac{\ln{(1-0.9)}}{\ln{(1-0.002)}}$$, which means 1151 Chaos Elementals have to be defeated.

Note that all chances are in decimal, i.e. 70% = 0.7 etc.

Item Drops
Some items are dropped by all enemies (except Meteor Heads) under certain conditions. Those can be found below.

Coin Drops
Most enemies will typically drop a few stacks of coins after dying. Enemies that drop a large number of coins have a higher chance of dropping multiple stacks.

The total amount of money dropped is calculated using a "base value", which depends on the enemy, and in expert mode, the "base value" is multiplied by 2.5 and weak enemies receive an additional multiplier to it in Hardmode and after Plantera has been defeated depending on their normal mode stats. A Skeleton for example, has a "base value" on normal mode of, and on expert mode, in Pre-Hardmode,  in Hardmode, and  after Plantera has been defeated. The exact amount of money dropped is this number multiplied by 80-120%.

After the amount of money is determined, there are then a few chances for the number to be increased.


 * 20% chance of increasing by 5-10%
 * 10% chance of increasing by 10-20%
 * 6.67% chance of increasing by 15-30%
 * 5% chance of increasing by 20-40%

These increases are not mutually exclusive, are multiplicative, and an enemy may have its money increased several times. It is possible to get all increases at once, as each one is rolled individually. Additionally, the Midas debuff increases the money dropped by 10-49%. The average amount dropped, therefore, is 106.14% of the "base" value, or 137.45% if the enemy is afflicted with Midas. It is possible to get it increased even more by the Lucky Coin.

Double Loot
Old-gen console versions of the game give players a chance to have double the loot drop from any enemy. The exact chance of this happening is 1/1000. Players can easily see the effect when farming for more common loot such as souls or gel. It can be extremely useful if farming for rare items, like the Rod of Discord or Sniper Rifle, as two can be obtained from one kill. However, this obviously requires a lot of luck, as the chance of this occurring is at about 0.5%.

History
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