Hero and summon scaling is level-based. Every time a Hero gains a level most of their statistics are increased by a percentage. Basic Attack damage, Health, Health regen, ability statistics (Damage, Healing and Shield amount), and summons (health and damage) scale as a percentage increase. Hero's maximum Mana and Mana regen instead have a level-based linear scaling. Most percentage-based scaling is a 4% increase, with variance after balance adjustments. Talents that increase an ability's damage by a flat amount, such as Kael'thas' Convection, are not subjective to scaling. Talents that grant an active ability, such as Xul's Bone Spear, or deal a separate instance of damage, such as Alarak's Show of Force, do scale.
Minions', Mercenaries', Monsters' and Vehicles' statistics are increased every minute by a unit-specific flat amount, which is multiplied by a scaling factor that increases every 5 minutes. Most structures (except the Core and Bell Towers on Towers of Doom) do not scale as the game goes on. They do have a flat amount of health and do a flat amount of damage that has less of an impact as the game goes on. This means that in the early game diving towers is dangerous, but as the game goes on structures provide less protection. Unlike most other buildings, the Core's HP and Shields scale from minute 1 to minute 20, capping out at 22,000 life and 11,000 shields for a total effective HP of 33,000. After the 20 minute mark no time-based scaling happens, meaning that level becomes more important.
Heroes' and Summons' Statistics at a certain level can be calculated using the following formula:
BaseValue × (1 + ScalingPercent/100)^Level
- Note that "base value" indicates a "level 0" value, not a level 1 value.
Kael'thas' "Flamestrike" has 345 base damage with 4% scaling and 200 bonus damage from completed "Convection" quest Talent. At level 10 the damage of single Flamestrike is:
345 × (1 + 4/100)^10 + 200 = ~710 damage