Skyrim:Magicka

Magicka is the magical energy that is used to cast spells. It is equivalent to the magic points (MP or mana) used by other RPGs.

Current Magicka

You may inspect current magicka in Skills menu.

Available Magicka
The amount of magicka you have depends on your character's level and race, as well as the existence of any active enchantment, potion, or blessing which conveys the Fortify Magicka effect. Each time you level up your character, you may add ten points of magicka. High elves inherently have 50 more magicka than members of all other playable races. Gear with a Fortify Magicka enchantment will increase your character's total magicka whenever that gear is worn. The Fortify Magicka effect is also available via many potions, as well as via the Blessing of Julianos and the Atronach Stone.  If you never make any skills legendary, outside of any effects increasing or reducing your magicka, this means your base available amount will be 100-950, depending on level and race.

Needed Magicka
The cost of a spell goes down as you improve the skill corresponding to its school and if you buy the discounting perk relevant to its spell level; in addition, enchanting effects can reduce this further, to a minimum of 0.  The cost goes up if you have the relevant dual casting perk and dual cast it.  The most expensive spell in the game costs 1426 magicka base; with dual casting, the most expensive spell that can be dual-cast costs 2344 (both values have been rounded up).  While normally you have to have a minimum skill to buy a spell tome for a spell, you can learn any spell tome you can find, and you can reset any skill to base by making it legendary, so it is possible to attempt any spell at starting level, but dual casting requires a minimum level of 20 in the relevant skill.  This means the most expensive spell you can attempt will cost 2010 magicka, by dual casting Dragonhide at skill 20 with the Dual Casting perk.  At skill 100 and with the cost reducing perk, this will cost 696 magicka; that 1426 magicka spell with the same skill and its perk will cost 424.

Restoring Magicka
Your available magicka is decreased each time you cast a spell. The blue bar in the bottom left of the screen shows you the current status of your magicka (although this bar will be invisible when magicka is full). There are a number of ways to restore magicka:
Magicka slowly regenerates over time. You regenerate 3.00% of your maximum magicka per second outside of combat and 0.99% in combat.
As a result, sleeping or waiting fully restores your magicka.
Your magicka regeneration rate can be increased by Regenerate Magicka effects, including potions and enchanted items.
Magicka will not regenerate while you are casting a spell.
Restore Magicka effects are available in various forms:
From potions and scrolls that can be found or purchased.
From custom potions.
From the Equilibrium spell, which restores magicka at the cost of health.
Absorb Magicka enchantments allow you to take magicka from enemies.
Spell Absorption effects are rare, but allow you to gain magicka when enemies cast spells at you.
High Elves can speed up magicka regeneration with their Highborn racial power (similar to the way Argonians can speed up health regeneration with their Histskin racial power).
Whenever your character levels up, your magicka is fully restored.

NPC Magicka
All NPCs and creatures also have magicka, which is calculated differently than the player's magicka. Their magicka is summed from three separate contributions:
The race provides a starting magicka. While the playable races all have a starting magicka of 50, other races have values ranging from 0 to 200.
Individual NPCs and creatures can have a fixed magicka adjustment. This adjustment is zero for most generic enemies, but can be substantial for quest-specific enemies (values range from -50 to 20000).
For most NPCs and creatures the class provides an additional 0-10 magicka points per level.
The rate at which enemies regenerate magicka is determined by their race.

Notes
Shock damage attacks drain their target's magicka in addition to damaging health.
When not in combat it takes about 33 seconds to regenerate your magicka from 0% to 100% without any effects.