I'm posting this because it appears that most people do not understand the extent to which various things affect damage. I might make this into a guide later but honestly it's a nightmare to properly explain in detail.
I will be talking about damage from abilities only, not auto attacks.
Base ability damage formula (This is the number shown on your stats screen; you generally don't need to know the formula):
Single wep: WeaponDamage*SpeedMod + DamageBoost
2h weapon: WeaponDamage*SpeedMod + 1.5*DamageBoost
Dual wield: MainhandWeaponDamage*SpeedMod + OffhandWeaponDamage*SpeedMod + 1.5*DamageBoost
WeaponDamage is the displayed damage stat on your weapon when you hover over it.
For ammunition-using weapons with a displayed damage stat, use only the damage stat on the ammo and ignore the damage stat on the weapon.
All ammunition has an effective speed of "Fastest" for calculating damage.
For spells, the damage is 9.6*EffectiveSpellLevel (*1.5 if using staff). See below for an explanation of EffectiveSpellLevel.
SpeedMod is equal to:
-> 1 for weapon speed "Fastest"
-> 0.7875 for weapon speed "Fast"
-> 0.65 for weapon speed "Average"
DamageBoost is your current Strength/Range/Magic level (includes boosts or debuffs to your skill level) plus the sum of damage boosts from equipped armour of the same combat style as your weapon.
EffectiveSpellLevel is the minimum between your magic weapon level and the level cap of your spell. Level caps of spells are:
Air: 16 Strike - 40 Bolt - 62 Blast - 80 Wave - 90 Surge
Water: 22 Strike - 45 Bolt - 64 Blast - 80 Wave - 90 Surge
Earth: 28 Strike - 52 Bolt - 69 Blast - 80 Wave - 90 Surge
Fire: 34 Strike - 58 Bolt - 74 Blast - 80 Wave - 90 Surge
Smoke: 60 Rush - 73 Burst - 80 Blitz - 90 Barrage
Shadow: 63 Rush - 75 Burst - 80 Blitz - 90 Barrage
Blood: 67 Rush - 79 Burst - 90 Blitz - 90 Barrage
Ice: 69 Rush - 80 Burst - 90 Blitz - 90 Barrage
Magic dart: 65
Divine storm: 70
Note that due to this, magic damage is capped at level 90 in dungeoneering even with a lv99 weapon, while melee and range are not capped.
Final ability damage = Base ability damage * Prayers + Potions
Prayers: 1.06 for normal prayers, 1.08 for piety, 1.1 for turmoil, 1.16 for zealot-boosted prayers
Potions: +8 damage for every level in Str/Range/Mage ABOVE your base level
Practical examples:
Relative base damage of weapons:
Assuming lv99 stats, 100% accuracy, no potions, no prayers:
Lv80 weapons are 12.5% more damage than lv70 weapons.
Lv90 weapons are 24.9% more damage than lv70 weapons.
Lv90 weapons are 11.1% more damage than lv80 weapons.
Dual drygores are 6.4% more damage than Dry mainhand Enhanced Excalibur offhand
Dual drygores are 3.4% more damage than Dry mainhand Chaotic offhand
Dual drygores are 7.1% more damage than Chaotic mainhand Dry offhand
Virtus wand + Seismic singularity is 3.7% more damage than Dual Virtus
Even without considering accuracy, weapon tier makes a significant difference.
Effectiveness of stats and potions:
Assuming 100% accuracy, no potions, no prayers:
Barrows weapons are 3.9% more damage with 99 Str than 70 Str
Chaotics are 2.2% more damage with 99 Str than 80 Str
Drygores are 0.9% more damage with 99 Str than 90 Str
Assuming level 99 stats, 100% accuracy, no prayers:
Lv80 weapons are 13.5% more damage with Super potions than with no potion
Lv80 weapons are 17.6% more damage with Extreme potions than with no potion
Lv80 weapons are 3.7% more damage with Extreme potions than with Super potions
Lv90 weapons are 12.1% more damage with Super potions than with no potion
Lv90 weapons are 15.9% more damage with Extreme potions than with no potion
Lv90 weapons are 3.3% more damage with Extreme potions than with Super potions
Super potions give more of a boost to damage than Turmoil.
The difference between Extremes and Supers is noticable, but relatively low.
Effectiveness of power armour:
Assuming level 99 stats, no potions, no prayers:
Lv80 weapons are 7.4% more damage with Lv70 power armour than with no power armour
Lv80 weapons are 8.5% more damage with Lv80 power armour than with no power armour
Lv80 weapons are 1.01% more damage with Lv80 power armour than with Lv70 power armour
Assuming 10 hour degrade, you need to make 19m gp per hour to justify using Lv80 power armour instead of Lv70 power armour for dps.
Assuming level 99 stats, Extreme potions, and Turmoil:
Lv90 weapons are 6.7% more damage with Lv80 power armour than with no power armour
Lv90 weapons are 7.6% more damage with Lv90 power armour than with no power armour
Lv90 weapons are 0.85% more damage with Lv90 power armour than with Lv80 power armour
I have no clue how fast lv90 power armour degrades.
Lv70 power armour is a significant improvement over not using power armour, but generally it is not worth it to upgrade to lv80 or lv90 power armour unless you need the defence.
Drygore Mace vs Drygore Longsword/Rapier:
At lv99 Str with no potions/prayers/power armour:
Drygore Longsword/Rapier is 0.394% more damage than Drygore Mace
At lv99 Str with extremes/turmoil/Lv70 power armour"
Drygore Longsword/Rapier is 0.327% more damage than Drygore Mace
Drygore Long/Rap is very slightly stronger than Mace, but the difference is fairly negligible.
Edited by -Leaf-