How is drop chance over many kills calculated?

Each kill is an independent Bernoulli trial with probability p = 1/rate. The chance of at least one drop in N kills is 1 - (1 - p)^N. The chance of going completely dry is (1 - p)^N.

If the drop is 1/128, am I guaranteed it in 128 kills?

No. With p = 1/128 over 128 kills the chance of at least one drop is 1 - (127/128)^128, which is about 63.5%, not 100%. Drops never become guaranteed; each kill resets the odds.

What does 'going dry' mean?

Going dry means doing many attempts past the drop rate without receiving the item. The 'chance of going dry' figure is (1 - p)^N — the probability of zero drops in N kills. It shrinks as N grows but never reaches zero.

How many kills do I need for a 90% chance?

Solve 1 - (1 - p)^N >= 0.90 for N, giving N = ln(0.10) / ln(1 - p), rounded up. For a 1/5000 drop that is about 11,512 kills for 90% confidence and roughly 23,025 for 99%.

Does this work for clue scrolls and pets?

Yes — any fixed per-kill rate works, including pets, unique boss drops, and clue rewards. For rates that change with thresholds or rolls-on-table mechanics, use the effective single-roll rate from the OSRS Wiki.

Old School RuneScape Drop Rate & Probability Calculator

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The Old School RuneScape Drop Rate & Probability Calculator answers the question every grinder asks: “I’ve done X kills at a 1/Y drop — what are my real odds?” It uses the exact independent-trial maths the game’s RNG follows, so the numbers match what you would compute from the OSRS Wiki drop tables.

How it works

Every kill is an independent roll with success probability p = 1 / rate. Over N kills the outcomes follow a binomial process, and the two figures that matter are:

P(at least one drop) = 1 - (1 - p)^N
P(zero drops / dry)  = (1 - p)^N

To find how many kills you need for a confidence level c (say 90%), solve for N:

N = ceil( ln(1 - c) / ln(1 - p) )

Worked example

For a 1/512 drop after 512 kills, p = 1/512 and the chance of at least one drop is 1 - (511/512)^512 ≈ 63.3%. That is why hitting the exact drop rate in kills only gives you roughly a 63% chance — the famous “drop rate ≠ guarantee” rule. To be 99% sure you would need about ceil(ln(0.01)/ln(511/512)) ≈ 2,357 kills.