How does IV inheritance work?

Without an item, three random IVs are inherited from the two parents and the rest are rolled randomly. With a Destiny-Knot-style item, five of the six IVs are inherited from the combined parent pool, dramatically raising the odds of passing target stats to the offspring.

How is the perfect-egg probability calculated?

For each target perfect stat the chance it is inherited is the number of inherited slots divided by six, and the chance an un-inherited stat still rolls perfect is one in 32. The calculator combines these across your target stats to find the probability a single egg is perfect in all of them.

Why does a Destiny-Knot item help so much?

It increases inherited IVs from three to five out of six, so each parent stat is far likelier to carry over. For a 5-IV target the per-egg odds improve by orders of magnitude, cutting expected eggs from thousands to dozens.

How many eggs will I need on average?

The expected number of eggs is one divided by the per-egg success probability. If each egg is perfect with probability one in 50, you should expect about 50 eggs, though variance means some runs take far more or fewer.

What is the one-in-32 random roll?

Any IV not inherited is rolled uniformly from 0 to 31, so the chance of a random perfect 31 is one in 32. Covering a stat through a parent and the inheritance item is far more reliable than hoping for that random roll.

RuneScape 3 Breeding & IV Calculator

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Breeding a perfect-IV specimen is a numbers game, and the difference between using an inheritance item or not is enormous. This calculator models the real inheritance mechanic to give your per-egg perfect odds and the expected number of eggs you will need to hatch.

How it works

A held inheritance item raises inherited IVs from three to five out of six. Each target stat is perfect if inherited from a parent (probability inherited / 6) or, failing that, randomly rolled perfect (probability 1/32):

pStat       = inherited/6 + (1 − inherited/6) × 1/32
pPerfectEgg = pStat ^ (number of target perfect IVs)
expectedEggs= 1 / pPerfectEgg

The exponent is the count of stats you need perfect, so each extra target stat multiplies the difficulty.

Example and tips

Targeting 5 perfect IVs with a Destiny-Knot-style item (5 inherited of 6), each stat carries over with about an 84 percent chance, and a single egg is fully perfect with probability around 0.84^5, roughly 41 percent — only a handful of eggs on average. Drop the item and inherited IVs fall to 3, sending expected eggs into the thousands. Always breed with the item and the best combined parent IVs you can assemble.