Drop rate calculator

Why the drop rate is not the whole story

A 1 in 512 drop does not mean you get one every 512 kills. It means each kill independently has a 1 in 512 chance, and the outcomes spread far wider than most people expect.

At exactly 512 kills on a 1/512 drop, your chance of having seen it is about 63%. That leaves more than a third of players still empty-handed at the point they feel they are “owed” one. Go to 1,024 kills — double the rate — and 13.5% of players still have nothing.

The maths

Each kill is an independent trial, so the probability of getting no drop at all across n kills is simply the miss chance multiplied by itself n times:

P(dry after n kills)   = (1 - 1/rate)^n
P(at least one drop)   = 1 - (1 - 1/rate)^n
Kills for X% chance    = ln(1 - X) / ln(1 - 1/rate)

There is no pity mechanic

Old School RuneScape does not track how long you have gone without a drop, and your odds on the next kill are identical to your odds on the first. Every kill is a fresh roll.

This cuts both ways. A run of bad luck does not make you “due” — but it also means a dry streak carries no information about the future. The only thing that raises your chance of a drop is more kills.

Using this for planning

The most useful output above is the kill count for a 90% or 99% chance. If you are deciding whether to chase a specific item, that number tells you the realistic worst case rather than the average. Combined with a GP/hr estimate for the boss, it turns “how long will this take” into something you can actually answer.