From weights to chances
Give every pet in an egg a weight. Its chance is its weight divided by the sum of all weights. With Common 60, Uncommon 25, Rare 10, Epic 4, Legendary 0.9 and Mythic 0.1, the weights add up to 100, so a Mythic is 0.1%: 1 in 1,000.
“1 in 1,000” does not mean a Mythic every 1,000 hatches. The chance of at least one in n hatches is 1 − (1 − p)n: about 63% after 1,000 hatches. Half of players have one by hatch 693, nine in ten by 2,302, and one in a hundred is still waiting at 4,603.
What a luck boost changes
The usual luck boost multiplies the weights of the rarer pets and leaves the rest. With 2× luck on Rare and above, the Mythic weight becomes 0.2 while Common stays 60, so the total grows and every chance is recalculated. The Mythic goes from 0.10% to about 0.17%, not to 0.20%: a “2×” boost is less than double. Say so in the item’s description, or players will feel cheated.
Mutations as a second roll
Golden, Rainbow and similar variants are usually rolled after the pet: first the egg picks a Mythic (1 in 1,000), then a Rainbow roll succeeds 1 in 3,000. A Rainbow Mythic is 1 in 3,000,000. Roll the rarest mutation first, so a lucky player gets the best one.
Roblox’s paid random items policy
If the egg is bought with Robux, or with in-game currency that can be bought with Robux, Roblox asks you to:
- show every possible outcome and its odds before the purchase, as percentages;
- make the percentages add up to exactly 100%; long decimals may be rounded to four or more places;
- put a long list behind a clearly labelled “Info” or “Details” button; an (i) icon alone is not enough;
- disclose the odds before the random part happens even when the purchase came earlier, such as a bought seed that is planted later;
- show the changed odds as numbers when you sell something that boosts them, like a luck potion.
The odds calculator rounds to four decimals so the list adds up to exactly 100%, prints the boosted odds too, and writes a Luau table with the weights and the numbers to show.
Icons for every rarity and mutation
A pet with 8 mutations needs 9 icons, and a game with 60 pets needs 540. Pet Factory renders all of them with the same pose, angle, frame and outline, and writes a Pets.lua table with the file names. In Roblox Studio, the Asset Manager’s Bulk Import takes up to 50 images at a time; then swap each file name in the table for its asset id.
Robux and your share
From game passes and developer products, Roblox keeps 30% of the Robux and the creator gets 70%. If an egg costs 100 Robux and the Mythic is 1 in 1,000, an average Mythic costs a player 100,000 Robux, of which you receive about 70,000. Let numbers like that guide where you put the rarest pets, and how generous the free path is.