Weights, not percentages
Many games store loot chances as weights rather than percentages. A weight is a relative number: it says how large an entry’s share is compared with the others. The chance of an entry is its weight divided by the total of all weights in the table.
The reason is practical. With weights, a designer can add a new item without rebalancing everything else to sum to exactly 100. The game simply adds the new weight to the total.
A worked example
Here is a table with illustrative weights that happen to add up to 100.
| Rarity | Weight | Chance |
|---|---|---|
| Common | 60 | 60% |
| Uncommon | 25 | 25% |
| Rare | 10 | 10% |
| Epic | 4 | 4% |
| Legendary | 1 | 1% |
To make a choice, the game can draw a whole number from 0 to 99 and see which band it falls into:
- 0 to 59 (60 values): Common
- 60 to 84 (25 values): Uncommon
- 85 to 94 (10 values): Rare
- 95 to 98 (4 values): Epic
- 99 (1 value): Legendary
Each entry owns a stretch of the number line as long as its weight, so a bigger weight simply means a longer stretch.
When the weights do not sum to 100, the method is the same. A table with weights of 12, 6 and 2 has a total of 20. The chances are 12 ÷ 20 = 60%, 6 ÷ 20 = 30% and 2 ÷ 20 = 10%.
Adding an item changes everything else
Because chances are shares of a total, changing one weight changes all the others. Suppose a new Common item with weight 5 is added to the table above. The total becomes 105.
- Common chance falls from 60/100 to 60/105, about 57.1%, and the new item takes 5/105, about 4.8%.
- Legendary falls from 1/100 = 1% to 1/105, about 0.95%.
Nothing was changed for the Legendary entry, yet its chance dropped slightly. This is why patch notes that “add items to the pool” can quietly make existing items a little harder to find. It also shows why a larger item pool with equal weights thins out the chance of any one item.
Bonuses do not scale chances neatly
Luck stats and event bonuses often work by multiplying weights. Imagine a luck effect that doubles the weight of Rare, Epic and Legendary entries but leaves Common and Uncommon alone.
- New weights: Common 60, Uncommon 25, Rare 20, Epic 8, Legendary 2
- New total: 60 + 25 + 20 + 8 + 2 = 115
- Legendary chance: 2 ÷ 115, about 1.74%
Doubling the weight raised the Legendary chance from 1% to about 1.74%, not to 2%, because the total grew too. Whether a game’s bonus works on weights, directly on percentages or by rolling twice and keeping the better result is a design choice, and the details are often not shown.
Nested tables: a tier, then an item
Many games roll in two stages. The first roll picks a tier, and a second roll picks an item inside that tier. The overall chance of a specific item is the product of the two stages, an application of the multiplication principle.
Example: the Rare tier is 10% of rolls, and it contains four equally weighted items. Each Rare item has a chance of 10% × 1/4 = 2.5% per roll.
This is also why a headline rarity can mislead. “Epic: 4%” says nothing about the item you actually want. If the Epic tier holds eight equally weighted items, each has a 0.5% chance. If one of them has a larger weight, its chance is higher and the others are lower.
Reading a table sensibly
- Find the total. Divide each weight by it. If a table is already in percentages, check that it adds to about 100%.
- Check the unit. A table might describe one roll per kill, one roll per chest or several rolls per boss. Several rolls change the chance of getting at least one.
- Look for stages. Is the tier chance followed by an item chance, or is the list already combined?
- Look for conditions. Level requirements, difficulty settings, luck bonuses and event effects can all change the weights that apply.
- Remember what a percentage means. A 4% entry is a per-roll chance. Over 25 independent rolls, the chance of seeing it at least once is 1 − 0.96²⁵, which is about 64%, not near-certain. The article on loot drop rates covers this kind of repeated-roll arithmetic in more detail.
Why designers like weights
Weights make tables easy to edit and combine. A designer can build one table for a region, another for an enemy type, then merge them or scale one up for a special event. They also allow “no drop” to be included as an entry, so the chance of getting nothing is set in the same way as the chance of getting anything.
For players, the main takeaway is that every chance in a table is relative. A rare item is rare because its weight is small compared with the total, so adding items, boosting other entries or merging tables can change its chance without anyone touching its own number. Whenever a game publishes full odds, as covered in the article on loot box odds, they are the result of this kind of calculation.