Methodology & data
Every number on OSRSCalc is either exact math or a cited fact. Here is which is which, where the data comes from, and what happens when Jagex changes something.
Probability math (exact)
Drop and pet calculators model each kill as an independent 1-in-N roll. The formulas are the standard binomial identities, computed with log-space arithmetic so very rare drops and very long streaks stay accurate to the last digit:
- Chance of at least one drop in K kills:
1 − (1 − 1/N)^K - Chance of still being dry after K kills:
(1 − 1/N)^K - Kills needed for probability p:
⌈ln(1 − p) ÷ ln(1 − 1/N)⌉ - Average (mean) kills:
N· Median kills:~0.693 × N
These are exact for any content that rolls a flat rate per kill. They are wrong for content that doesn't; see limitations.
XP curve (exact)
The experience table is generated from the documented RuneScape curve,
points(n) = ⌊n + 300 × 2^(n/7)⌋ accumulated and divided by 4, and
unit-tested against known anchors: level 2 = 83 XP, level 50 = 101,333 XP, level 92 =
6,517,253 XP, level 99 = 13,034,431 XP.
Combat formula (exact)
Combat level uses the formula documented on the OSRS Wiki:
0.25 × (Def + HP + ⌊Prayer/2⌋) + max(melee, ranged, magic), where each style
contributes 0.325 × (Attack + Strength),
0.325 × (⌊Ranged/2⌋ + Ranged) or 0.325 × (⌊Magic/2⌋ + Magic).
Tested anchors: all-99 stats = 126, all-1 stats = 3.
Where drop and pet rates come from
Preset rates are copied by hand from the official OSRS Wiki (oldschool.runescape.wiki) individual monster drop tables for uniques, and the wiki's pet table for pet rates. The full preset list was last verified on 2026-09-30, and each page shows that date.
Jagex does rebalance rates: the dragon warhammer moved from 1/5,000 to 1/3,000 in May 2024, for example. When a rate changes, we update the preset, note it on the tool page, and log it in the changelog. If we've missed one, tell us and it gets fixed fast; a wrong rate is the worst bug a calculator can have.
Limitations: what we deliberately don't model
- Non-flat mechanics. Barrows chest loot scales with reward potential; Chambers of Xeric and Tombs of Amascut roll loot from raid points; the Olmlet and Tumeken's guardian pets work differently from flat rates. None of these are preset, and entering a single "rate" for them would be misleading.
- Skilling pets. Their chance depends on your level and the action, not a flat per-kill rate, so they're excluded.
- GP/hour inputs are yours. We compute the arithmetic exactly, but loot values, kill rates and supply costs are estimates you supply. Grand Exchange prices and the 2% sales tax shift results over time.
- Independent rolls assumption. If specific content gains bad-luck mitigation mechanics, flat-rate math overstates dryness there. We follow the wiki's documentation of each item's mechanics.
How the site is tested
- Every formula is unit-tested against hand-checked and community-known values.
- Preset rates are spot-checked against their wiki pages before each release.
- AI assistance: calculators and copy were drafted with AI help, and every formula, rate and claim on this site was reviewed and tested by the site's author, who owns the correctness of the result. Errors are fixed publicly via the changelog.
Update schedule
Preset data is re-verified against the wiki at least quarterly, and immediately after any major game update that touches drop tables. The verification date shown on each tool page is the source of truth.