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 4 drop rate (could also set to 0.25f directly)
float DropRate = 1f / 4f;

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

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
int NumDropFailures = 0;

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

Example: Dropping a key for a door

Let's say a game has a room that spawns endless waves of monsters and a locked door. 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%