Experience point

Experience Points to Level
Experience after True Reincarnation is increased, view the chart here.

Note: A character can earn up to one point less than enough XP for 2 levels. For Example: A Level 8 Character can obtain up to 439,999XP (1 point less than the amount necessary for the character to attain level 10) Some players use this process (called “Banking”) to extend the time that certain quests provide XP (ex.  A level 18 player can attain 1,899,999 XP.  This allows the character to run all of the Vale of Twilight quests on Hard with no XP loss except from repetition.) it also extends the length of time characters can spend with leveling groups. (A level 18 can run quests with capped characters with no XP loss and can run quests with characters as low as level 16 without penalizing the XP of the level 16 character.)

Gaining/Earning Experience Points(XP)
There are several methods to earn experience points, in two different places: quests and Wilderness adventure areas. Some XP adjustments apply equally to all members of the party, others are calculated individually. XP awards are not split among party members and are not affected by party size.

Quests

 * Complete optional objectives inside a quest. Common optional objectives include slaying a boss or named creature, exploring certain areas of a map, or clearing side areas of threats.  Some optional objectives are shown from the start of a mission, while others remain hidden until triggered.  Optional objective XP is granted instantly.
 * Complete all main objectives inside a quest. Some main objectives remain hidden until triggered.  Main objective XP is usually higher than that of optional objectives.  The main XP is bestowed upon completion of the quest and can be modified by the following conditions.  Changes to these conditions that occur after the main XP is awarded don't affect the amount (though they may incorrectly change the amount reported in the quest log - see below).
 * First-time completion: (individual) Completing a quest for the first time on Solo/Casual, on Normal, and on Hard difficulty each grants a +25% bonus. Completing a quest for the first time on Elite difficulty grants a +50% bonus.
 * Repetition: (individual) The first three completions have no adjustment. Completing a quest for the fourth time (i.e., the third repetition) carries a -10% penalty, with an additional -10% penalty for each further completion, for maximum penalty of -90% from the 12th completion on.  Also, beginning with the 13th completion, the main XP reward is capped at a maximum of 10% of its base value regardless of bonuses.  All completions at all difficulty levels count toward this penalty.  This penalty (including the 10% cap) is waived if there is a "first time" difficulty bonus, though that completion still counts toward increasing the penalty on later attempts.
 * Level of quest vs level of highest character in quest: (party) Compare the quest's level to the level of the highest-level character to have entered the quest (even if they later left). If that character's level is below, equal to, or one level above the quest's level, there is no adjustment. If it's +2, there's a -10% penalty. +3 = -25%, +4 = -50%, +5 = -75%, +6 = -99%, and at +7 or higher there is no XP regardless of bonuses. Casual difficulty subtracts 1 from the quest's level (if possible) for purposes of this adjustment, Hard difficulty adds 1, and Elite adds 2. ✅
 * Power-leveling penalty: (individual) If you are exactly 4 levels below the highest-level character in the party, you receive a -50% penalty. If you are 5 levels below, you receive a -75% penalty. If you are 6 levels below, you receive a -99% penalty. If you are 7 or more levels below, you receive no XP regardless of bonuses.
 * Death: (party) There is a +10% Flawless Victory Bonus  if there are no deaths. (Unconsciousness is not death unless the player releases.) There is a +5% Survival Bonus  if there are one or more Hireling deaths but no player deaths.
 * Re-entry:
 * (party) There is a +10% Persistence Bonus  if no one leaves the quest and re-enters (whether through death, recall, or exit).
 * (individual) Each of your re-entries confers a -20% penalty to you, to a maximum of -90% for the fifth. Re-entry penalties do not apply on Solo or Casual difficulty, although the Persistence Bonus can still be lost.
 * Late entry:
 * (individual) If you first entered the dungeon ten or more minutes after it spawned, you'll have a -80% penalty if the time between your entry and completion is less than a quarter of the dungeon's lifetime at that moment. If it's at least a quarter but less than half, you'll have a -50% penalty instead. (Explained by dev Phax here)
 * Killing monsters: Killing a large portion of the monsters present can confer
 * +10% Aggression  bonus
 * +15% Onslaught  bonus
 * +25% **Conquest**  bonus
 * Not killing monsters: On the flip side, completing a mission while slaying very few enemies can grant
 * +5% Discreet  bonus
 * +10% Devious  bonus
 * +15% **Insidious Cunning**  bonus
 * Disarming traps: Disarming a large portion of the traps present can confer
 * +8% Tamper  bonus
 * +10% Neutralization  bonus
 * +15% **Ingenious Debilitation**  bonus
 * Finding secret doors: Finding a large portion of the secret doors present can confer
 * +8% Observance  bonus
 * +10% Perception  bonus
 * +15% **Vigilant Sight**  bonus
 * Destroying breakable objects: Destroying a large portion of the breakable objects (barrels, vases, caskets, etc.) present can confer
 * +8% Mischief  bonus
 * <font color='blue'>+10% Vandal  bonus
 * <font color='green'>+15% **Ransack**  bonus

All bonuses are added together and can make a quest worth the time and effort to complete even if there are penalties. However, there are two cases where a quest's main XP reward will be 0 regardless of bonuses. The first is if you meet a condition that explicitly prohibits XP. The second is if there are enough penalties to reduce the reward to a subtotal of 0 XP before bonuses are applied. Due to roundoff issues, you may or may not get XP if your penalties total exactly -100%. You definitely won't if they go below that.

XP Log
Bonuses and penalties are calculated and locked in when the main quest is completed, which may not be accurately reflected in the quest XP log. The XP log bonuses/penalties continue to be updated after the XP is locked, so you may see some bonuses/penalties listed that you didn't actually get (e.g., you may see a re-entry penalty reducing your listed XP because you finished-out and re-entered after the quest was completed: you actually got the full XP, but it now displays less than that).

The XP log is updated when: (Explained by dev Phax here)
 * 1) (individual) YOU log in
 * 2) (party-wide) Player Dies
 * 3) (party-wide) Henchman Dies
 * 4) (party-wide) Monster Dies
 * 5) (party-wide) Chest Looted
 * 6) (party-wide) Secret Door Discovered
 * 7) (party-wide) Trap Disabled
 * 8) (party-wide) Breakable Smashed

Wilderness Areas

 * Complete Slayer objectives by killing increasing numbers of enemies anywhere in the zone. Each objective requires more kills than the last, but is also worth more XP.
 * Complete Explorer objectives by visiting locations of interest scattered throughout the zone. Each location is worth XP the first time it's visited, with a one-time bonus for visiting all of them.
 * Complete Rare Encounter objectives by killing rare named bosses. Bosses are worth XP (and a loot chest) every time one is killed, with a one-time bonus when they've all been killed once. Each rare boss has only a chance of spawning every time the area is entered; an ordinary enemy often appears in its place instead.

Wilderness area objective quests are granted automatically the first time the area is entered. They do not have a difficulty setting. Their progress is tracked between visits, and they cannot be reset or abandoned.

Wilderness areas have different XP adjustment rules than dungeon quests. Each wilderness area has a range of levels it supports. If all party members are within the area's range, they all receive normal XP and objective progress. If any characters are above the area's maximum level, no one but them can make progress toward objectives. The ones above the limit can still complete objectives, but they receive reduced XP: 1/2 if they're one level over the maximum, 1/3 for two levels, 1/4 for three, 1/5 for four, and so on. Characters below the area's minimum level never make progress toward objectives and are completely ignored when determining XP for the other members in their party.

The "Art" of Gaining XP
There are many opinions on the best difficulty order to complete quests to get the maximum amount of XP. Here are a few examples of various five-run methods using "The Kobolds' New Ringleader" and a party of level 1-3 characters. This is one of the most-run quests at level 2 so it makes a good example.

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Note that the commonly-suggested method of "three times on Normal, then once on Hard and once on Elite" yields close to the maximum but not quite the absolute greatest XP possible. It tends to be quicker than any method that gives more XP, though, which often makes it the most efficient plan, and it can be done solo by all players. Methods that don't start with Normal difficulty require a V.I.P. subscription (which can start on Hard) or a party.

Note also that this very rough analysis doesn't take into account how long it takes to complete each run, assumes everyone can handle Elite difficulty with no deaths, and doesn't necessarily extend to longer repetitions.