Drop Rate

A Drop Rate is a value that indicates how often an item has a chance to drop from a game event such as defeating an enemy.

Simple Drop Rate

The simplest implementation of a drop rate is to store the rate as a percentage or ratio, and then generate a random number and check if it is lower than the drop rate. If it is, the item drops. This is similar to rolling a dice and dropping the item if certain numbers come up.

// 1 in 20 drop rate (could also set to 0.05f directly)
float DropRate = 1f / 20f;

void OnEnemyKill()
{
    var n = random.NextDouble();
    if (n < DropRate)
    {
        DropItem();
    }
}

• Pros
• Cons
  • For small drop rates and rare items, there is a large variation from player to player in how often the item drops.
  • If the item is something that the player is intended to grind for, the grind may take multiple times longer than average.

Sampling without Replacement

For rare items that the player cannot trade or must obtain for progression, sampling without replacement provides a more consistent experience for obtaining an item. Each time the item fails to drop, the chance of failure decreases. This is similar to picking a card from a deck and removing the card from the deck once picked.

// 1 in 20 drop rate (numerator)
float DropRateSuccess = 1f;
// 1 in 20 drop rate (denominator)
int DropRateTotal = 20;

// number of times the item has failed to drop since last success
int DropFailures = 0;

void OnEnemyKill()
{
    var n = random.NextDouble();
    if (n < DropRateSuccess / (DropRateTotal - DropFailures))
    {
        DropItem();
        DropFailures = 0;
    }
    else
    {
        DropFailures++;
    }
}

Example: Dropping a key for a door

Suppose a game has a room with a locked door that spawns endless waves of monsters. Killing a monster has a 10% chance to drop a key that opens the door. How many monsters need to be killed to open the door?

Total Kills	Simple Drop Rate	Sampling without Replacement
	Players w/ Key	Players w/ Key
1	10%	10%
2	19%	20%
3	27%	30%
4	34%	40%
5	41%	50%
6	47%	60%
7	52%	70%
8	57%	80%
9	61%	90%
10	65%	100%
20	88%
30	96%
40	99%