Spell Damage Increase

Spell Damage Increase (SDI) is an item property that increases the damage of most magical attacks that are cast. This property is capped at 25% in PVP provided you dont have more than 30% of a skill in two different spell lines or Animal Taming or Parry. Then, you will become "focused" to that spell line and can display it from your titles menu, under overhead name, skills. So, if you have Imbuing, mysticism and focus you can be a focused mystic, but not if you have mysticism and 31 in evaluating intelligence. It's noteworthy that chivalry is amongst the magical lines even though it has no spell affected by SDI. So, if you have 120 magery and 120 chivalry, you won't be a focused mage. If you are not eligible to be focused, then your SDI pvp cap will be 20%. For PvE there is no cap on spell damage, so pack it on.

There is no formula needed for Spell Damage Increase because it is a true conversion, a 10% increase means your spell damage is increased 10% and so on. The intensity range of the Spell Damage Increase item property is 1% to 18% for found rings and bracelets. The apparent maximum is 50% for special items (e.g., Shattered Obelisk Invasion Spellbook).

Qualifiers

As with most code over time, Spell Damage Increase is not as straight-forward as it seems. It does affect Poison Strike and Wither, but it does not affect Pain Spike or Holy Light. It works with Word of Death, as the spell is not dependent on a secondary ability check, e.g. like Pain Spike, which involves Spirit Speak, or Holy Light, which involves Karma.

Calculating Spell Damage

A character's skill level of evaluating intelligence is the first thing that comes into play in calculating the magery base spell damage. The formula for how much spell damage your getting from your Evaluating Intelligence is this: ((evaluating intelligence * 3) / 100) + 1.
Similar with Spirit Speak for Necromancy Spells and Focus or Imbuing for Mysticism Spells, Arcane Focus for Spellweaving Spells.
The next factor involved in spell damage is intelligence. The formula for this is 1% spell damage increase for every 10 points of intelligence, or (intelligence / 10) and drop the decimals.
Spellweaving spells Arcane Empowerment and Reaper Form increase spell damage.
A bards Inspire (giving SDI) & Invigorate (giving intelligence) buffs the spell damage and too.
Berserk ability of the gargoyle race may increase SDI (up to 12%).
Getting the Guild of Arcane Arts trade deal city buff increases the SDI value by 5%
Inscription Skill bonus. This is a 10% increase in the base damage for Magery Spells. (This bonus only takes effect when a character has achieved the level of Grandmaster or higher in Inscription.)
Being cursed by a target's Blood Oath.
Factor in any SDI imparted from items.
Factor the use of a Slayer spellbook property matching the target.
Don't forget the well timed usage of an Spell Focusing Sash.
Last but not least, the Summer Dragonfish Pie gives a straight +5 spell damage and the Grapes of Wrath gives +15 spell damage.

On the other hand factor in resist lowering and additional damage received issues on the target like:

SDI Items

Heads

The Scholar’s Halo 15%
Wizard's Crystal Reading Glasses 15%
Kasa of the Raj-In 12%
Dr. Spector's Lenses 12%
Treasures and Trinkets Reading Glasses 10%
Hat of the Magi 10%
Crown of Tal'Keesh 10%
Crown of Arcane Temperament 8%
Obsidian Earrings 8%

Necks

Pendant of the Magi 5%

Chests

Legs

Arms

Cuffs of the Archmage 20%
Midnight Bracers 10%

Robes

Cloaks

Sets

Elven Leafweave 15% (total from 4 pieces)
Greymist Armor 8% (2% per from 4 pieces)

Rings

Compassion's Eye 20%
Crystalline Ring 20%
A ring out of the Random Magic Item Generation System max. 18%
An imbued ring max. 12%
Ring of the Soulbinder 10%
Djinni's Ring 10%
Ecru Citrine Ring (random, max seen 8%)

Bracelets

A bracelet out of the Random Magic Item Generation System max. 18%
An imbued bracelet max. 12%
Enchanted Coral Bracelet 10%
Venom 10%

Talismans

Earrings

Yukio's Glass Earrings 15%

Books

The relative effective %Damage output is in parentheses to set the order.

(330) A target specific Lesser Slayer Shattered Obelisk Invasion Spellbook max. 30%, triple damage
(325) A target specific Lesser Slayer Scrapper's Compendium 25%, triple damage
(315) Anon's Spellbook 15% (Dinosaur, Myrmidex or Eodon Tribe Slayer, triple damage)
(309) A target specific Lesser Slayer Inscribed Spellbook max. 9%, triple damage
(230) A target specific Super Slayer Shattered Obelisk Invasion Spellbook max. 30%, double damage
(225) A target specific Super Slayer Scrapper's Compendium 25%, double damage
(215) Juo’nar’s Grimoire 15% (Super Slayer, double damage)
(215) Conjurer's Grimoire 15%, Undead Slayer, double damage
(210) Fallen Mystic's Spellbook 10% (Fey Slayer, double damage)
(209) A target specific Super Slayer Inscribed Spellbook max. 9%, double damage
(150) Maximum vs creatures with no slayer vulnerability Shattered Obelisk Invasion Spellbook max. 50%
(130) War of Shadows Invasion Spellbooks max. 30%
(125) Scrapper's Compendium 25%
(115) Tome of Lost Knowledge 15%
(110) Tome of Enlightenment 10%
(109) An Inscribed Spellbook max. 9%

If the goal is maximum damage, using a slayer spellbook to match your target is clearly the most important element. These are crafted spellbooks with 0 SDI just to make the damage comparison clear. No additional SDI, but slayer books:

(300) A target specific Lesser Slayer Inscribed Spellbook, triple damage
(200) A target specific Super Slayer Inscribed Spellbook, double damage

Shields

Hook's Shield 10%

Weapons

Maximum Possible SDI by items

As of June 2022, the maximum Spell Damage Increase from equipment is 185%, which is constituted from the following:

The Scholar’s Halo or Wizard's Crystal Reading Glasses 15%
Pendant of the Magi 5%
Protector of the Battle Mage 5%
Spell Woven Britches 10%
Cuffs of the Archmage 20%
Greymist Armor gloves 2%
A bracelet out of the Random Magic Item Generation System 18%
Either Compassion's Eye or Crystalline Ring 20%
Any one of the talisman slot items: Mana Phasing Orb, Necromancer's Phylactery, Mystic's Memento, Wizard's Curio 5%

(**A target specific Lesser Slayer Shattered Obelisk Invasion Spellbook 30%, triple damage)

A Shattered Obelisk Invasion Spellbook 50%
Hawkwind's Robe 5%
Jumu’s Sacred Hide 5%
Hook's Shield 10%
Yukio's Glass Earrings 15%

Imbuing

The following resources are required to imbue the highest intensity of Spell Damage Increase (12%):

Item Properties
Stat Modifiers	Dexterity Bonus • Hit Point Increase • Intelligence Bonus • Mana Increase • Stamina Increase • Strength Bonus
Regenerations	Hit Point Regeneration • Mana Regeneration • Stamina Regeneration
Hit Effects	Assassin Honed • Blood Drinker • Battle Lust • Hit Cold Area • Hit Curse • Hit Dispel • Hit Energy Area • Hit Fatigue • Hit Fire Area • Hit Fireball • Hit Harm • Hit Life Leech • Hit Lightning • Hit Lower Attack • Hit Lower Defense • Hit Mana Drain • Hit Mana Leech • Hit Physical Area • Hit Poison Area • Searing Weapon • Splintering Weapon • Hit Stamina Leech
Historic	Ruin • Force • Might • Mystic • Power • Vanquishing • Newbified
Damage Types	Physical Damage • Fire Damage • Cold Damage • Poison Damage • Energy Damage • Chaos Damage • Direct Damage
Magic	Casting Focus • Faster Casting • Faster Cast Recovery • Lower Mana Cost • Lower Reagent Cost • Mage Armor • Mage Weapon • Resonance • Spell Channeling • Spell Damage Increase • Spell Focusing • Spell Consumption
Weapon	Damage Increase • Swing Speed Increase • Use Best Weapon Skill • Weapon Speed • Hit Chance Increase
Archery	Balanced • Damage Modifier • Lower Ammo Cost • Range • Velocity
Death & Theft	Blessed • Cursed • No-Drop/No-Trade • Insured • Owned By
Damage Dealing	Damage Eater • Rage Focus • Reactive Paralyze • Reflect Physical Damage • Soul Charge
Other	Artifact Rarity • Defense Chance Increase • Durability • Elves Only • Enhance Potions • Gargoyles Only • Lifespan • Lower Requirements • Luck • Mana Burst • Mana Phase • Night Sight • Self Repair • Shard Bound • Skill Bonus • Strength Requirement • Uses Remaining • Weight
Negative	Antique • Brittle • Cursed • Ephemeral • Prized • Massive • Unwieldy • Unlucky •