Spell ranks

Every spell has a rank from 1 to 20. A higher rank means a stronger spell and a higher mana cost, and it opens glyph slots (Glyphs).

Earning a rank. Raising a spell from one rank to the next needs three things:

Spell experience. You earn it by using the spell: casting damage spells that hit, healing real damage, using buffs in combat, and kills credited to the spell. Bigger kills give more. It is personal: no one else's hits count.
Gold. The cost grows with rank and with the spell's level.
A level gate. For ranks up to 10, your character level must be above the spell's level: one rank for every two levels. From rank 10 upward, only spell experience limits you.

Rank-ups are done in the spells window, anywhere, even in combat. Rank is permanent and never lost on death.

What a rank does. Each rank adds a multiplier to the spell's damage or healing. Mana cost also rises with rank, so a high-rank spell is strong but hungry. A spell trained to rank 20 gets a capstone bonus on top.

Bonus rank. Gear, the tree, the Spirit runeword and some uniques can add rank beyond what you trained. Rolled gear gives "+1 or +2 to the rank of [tag] spells" for one tag (fire, cold, lightning, earth, holy, death, physical, healing, projectile or summon) on caster wands, rods, spellbooks and amulets. Attack wands roll it only for projectile, physical, area, chain or wave spells, never melee. Bonus rank above 20 gives less damage per rank and keeps raising the cost, so there are diminishing returns. Bonus rank never unlocks glyph slots, which depend on trained rank only.

Pace. A main spell reaches rank 5 in minutes and rank 10 in an hour or so of use. Ranks 11 to 20 are a long road; a full kit matures over many hours.

Anti-AFK. Experience is earned only while you fight real monsters and are active. Casting at nothing earns nothing.