OSRS Barrows Guide: Sourced Unique Odds & Route Logs

Three evidence layers

A defensible Barrows analysis has three separate layers. Game mechanics determine equipment probability. Observations determine your route time, costs, Reward Potential, and chest mix. Valuation determines what those items are worth at a named time or under a named personal objective. Blending the layers makes the result impossible to update.

The Barrows Calculator encodes only the stable equipment formula and the rune-only diary multiplier. It asks the user to supply the other layers.

Derive any-equipment probability

The OSRS Wiki chest mechanics give a per-roll equipment chance of P(N) = 1 ÷ (450 − 58N). The chest has N + 1 reward rolls. Under six brothers, P(6) = 1/102 and there are seven rolls.

The chance of at least one equipment success is the complement of all seven rolls missing: 1 − (101/102)^7 ≈ 6.67%. Do not multiply 24 pieces by 1/102; 1/102 is the equipment check on a roll, after which one available piece is selected.

Derive one named-piece probability

Each killed brother adds four pieces to the selection pool. With N brothers, 4N pieces are available. A specific available piece has per-chest probability (N + 1) × P(N) ÷ (4N). For six brothers, the result is 7/2448 ≈ 0.286%. This agrees with the Wiki's item rows.

If the target's brother was not killed, the target is not in the pool. Record that as zero rather than averaging it with available pieces.

Reward Potential is a different mechanism

When an equipment check misses, Reward Potential helps determine other rewards and quantities. It is based on slain monsters and has its own thresholds. That makes “average common chest value” setup-dependent. Two players can have the same equipment odds after killing six brothers but different rune, bolt-rack, key, and coin outcomes because they opened the chest at different potential.

Choose the route question first. If comparing an 880-point route with a maximum-potential route, log both separately. Do not change the equipment probability between them unless brother count also changes.

Design a route log

Start with an empty CSV header:

timestamp_utc,version,brothers_killed,reward_potential,hard_diary,unique_item,rune_quantities,other_rewards,cycle_seconds,supply_cost_gp,price_snapshot,notes

Record consecutive chests rather than only memorable drops. Define the start and end of a cycle before timing. Include failures, deaths, banking, and slow tunnels if they are part of the route you want to evaluate. Preserve raw quantities so a later price snapshot can revalue the same observations.

Calculate current expected value

1. List the four pieces for every killed brother and collect their current values from one timestamp.
2. Average those available values. Multiply by expected equipment count, (N + 1) × P(N).
3. Estimate the rune and other non-unique components from a source matching your Reward Potential.
4. If the Hard Diary is complete, multiply only the pre-diary rune component by 1.5.
5. Add components, subtract measured per-cycle cost, and multiply by 3600 ÷ measured cycle seconds.

Report the timestamp and sample window beside the answer. A GP/hour result without those two fields is not reproducible.

Handle price changes

Market prices can move faster than a guide. Keep item quantities or component averages separate from prices. When revaluing, copy the calculation, change the price snapshot, and preserve the original. Do not claim a background API refresh unless the page actually performs one; this tool does not.

Apply the Hard Diary correctly

The Morytania Hard Diary grants 50% more runes from Barrows. It is a Hard-tier reward and affects runes, not every chest item. Keep a pre-diary rune component so the adjustment is visible and auditable.

Dry streaks and cumulative scenarios

For a constant per-chest chance p, the probability of at least one result in n chests is 1 − (1 − p)^n. This does not create a guarantee. To combine two routes with different probabilities, multiply their miss probabilities: 1 − (1 − p₁)^n₁ × (1 − p₂)^n₂.

Audit checklist

• Brother count matches the equipment formula.
• The named target belongs to a killed brother.
• Reward Potential is recorded with non-unique observations.
• Diary applies only to rune value.
• Prices share a timestamp and valuation method.
• Time covers a full repeatable cycle.
• Costs come from the same route window.
• Sample rows are real observations, not fabricated examples.