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Inventory
Items you own but haven't equipped. Tick Try on for one or more to see what equipping them would do.
Character stats
Totals from the in-game Character Details screen. Leave blank anything you haven't checked.
Skills and paragon
From an imported build, drawn from your own game data file. Point at any node for its description; scroll to zoom and drag to move around.
Damage calculator
Estimates one hit of your main skill from everything entered above. Point at a bucket to see every bonus that feeds it.
Conditions for this hit
Turn on what's true when the hit lands. Bonuses tied to a condition only count while it's on.
How this build gets these conditions
What's worth more right now
Damage gained from adding each of these to your current setup and conditions.
Other bonuses
Paragon, skill passives, glyphs and anything else not on your gear.
Where every number comes from
Formula and assumptions
Hit = weapon damage × skill damage × main stat × (1 + additive bonuses) × each damage multiplier group × each separate [x] bonus × Vulnerable × Overpower. Critical Strikes add ×(1 + critical multiplier) and their own bonuses; the average hit weighs crits by your crit chance. Values from the Character stats section are used for main stat, crit chance and attack speed when filled in, since they already include everything. Gear multipliers of the same name add together before multiplying, as Blizzard describes for the 3.0 gear rework.
Main stat gives 1% damage per 8 points for every class except Barbarian, which gets 1% per 9, following Avarilyn's damage sheet for Lord of Hatred. Maxroll's beginner guide says 1% per 10 for Barbarian, so that one is disputed.
Base weapon damage for imported builds comes from the same sheet (via jlian's d4-damage-calc, MIT License): the average damage per weapon type at item power 900, fully masterworked. The game computes it in code, so it isn't in the game data.