Battle calculator

Damage is the stack's roll multiplied by the game's own damage modifier, (20 + attacker's Attack) ÷ (20 + target's Defense). That formula and its constant come from the game's own detailed stat tooltip, not from Heroes 3 — Olden Era has no ±5%-per-point rule and no damage clamp, so a Heroes 3 calculator gives the wrong answer here for every matchup.

A hero adds the whole of their Attack and Defense to every creature they command. Specialisations, artifacts, spells and laws are applied on top: flat bonuses first, then proportional ones, which is the order the game's own artifact text states (“+4 Attack, then +15% total Attack”).

Modifiers of the same kind add together, and the attacker's outgoing bonus multiplies the defender's incoming one. The game files store those as separate values and never say how they combine, so this is the one assumption here that is reasoned rather than read — the two factors are listed separately above so you can see both.

Not included: Morale (an extra turn is not a damage number), creature abilities with their own damage, attacks that splash onto neighbouring hexes, and anything whose value only exists mid-battle. Only spells, artifacts and laws that change a damage number are offered — the rest of the compendium is a click away on each entry.