Combat level

A player's, monster's, or NPC's combat level indicates the difficulty in defeating them in combat. In the very early days of RuneScape classic, players started out as Level 1 Combat, and then as Level 3 later; but this was changed and now all new players start out at Level 4, with the maximum level for RuneScape being level 200. Combat level is calculated by taking into account the following skills:
 * Attack or Strength or Ranged or Magic or Summoning
 * Defence

The formula for determining a player's combat level is: $$X + Defence + 2$$, where $$X$$ is the highest out of Attack, Strength, Magic, Summoning or Ranged.

This will give a maximum combat level of 200, for any player with 99 Defence and a single 99 in any combat stat. The minimum combat level is 4 (1+1+2). NPCs do not have the bonus 2 in their combat level.

It was announced that Constitution and Prayer do not affect combat level. When talking to Jasper to prevent combat XP he reveals that the blocked skills are Attack, Defence, Magic, Ranged, Strength, and Summoning.

As stated by Mod Rathe, the new formula is designed to streamline the calculation of combat level. Levels that are significantly lower than the player display shades of green , levels that are slightly lower than one's display yellow-green , levels that are equal to one's are displayed as yellow , and levels that are higher than yours display shades of orange  to red . In the Wilderness, levels outside the combat range will appear white. The difference in colours can be seen up to 10 levels higher or lower than your current combat level after which it will either show the brightest shade of green or red depending on whether the combat level is lower or higher.

Effects of combat level
Normally, aggressive NPC's will stop attacking a player if the player has double the NPC's combat level +1. There are exceptions, however - for example, any monsters in the wilderness that are normally not aggressive will be aggressive to players no matter what their combat level is. Also, monsters that are normally aggressive will always attack a player if the monster is level 100 or higher, since the maximum combat level is 200. Monsters with a higher combat level than a player usually cannot be attacked by using a left mouse click. Instead, players need to right click and select the attack monster option. So, to be able to attack a Blue dragon with a simple left click, a player needs a combat level of at least 111. An exception to this is with most boss monsters, such as the God Wars Dungeon generals, as well as popular high level monsters such as tormented demons, dark beasts, and metal dragons. You can also left-click a monster in Daemonheim, even if their combat level is higher than yours.

Prior to the Evolution of Combat Update, hovering your mouse over a player would display their combat level in the top-left of the screen. This was viewed as Jagex stressing combat as the primary means of achievement in the game. After the update, however, mousing over a player will display their total level (if their weapon is sheathed) instead, which is the cumulative total of all of their skill levels. In some situations, such as in the Wilderness, you still see a player's combat level. A player can toggle between showing their combat level or skill total to other players by right-clicking the [-] button on the top-right of their keybind bar and selecting Toggle-sheathe.

There are some requirements in game that require a certain combat level. There are three quests requiring a certain combat level, Smoking Kills and Dream Mentor (requiring 130 and 120 combat respectively) and World Wakes (requires 140 Combat). Training with certain Slayer Masters, and also the Void Knight activities Pest Control and Conquest, also have combat level requirements. Also many quests require players to kill somewhat difficult monsters, so having a higher combat level would be very helpful.

The combat level system applies to monsters as well. After the EoC, most low level monsters have an increase in combat level. For example, the weakest goblins are level 4 instead of 2, and guards are level 50 instead of being in the 20s. Many high level monsters have their combat levels decreased to being deceptively low. The level 702 TzTok-Jad, for instance, became a level 140, while still being a challenge to higher levelled players.

NPCs do not have an extra 2 added to their combat level, hence why enemies wearing tier 11 armour in Dungeoneering have a combat level of 198.

Combat levels prior to the Evolution of Combat
Before the Evolution of Combat, a player's combat level was calculated with a totally different approach. The formula included one's Constitution, Prayer, and Summoning alongside his or her other combat skills.

With the older formula, a player with 99 Attack and Strength would have a higher combat level than another with 99 Ranged or Magic, allowing warriors of far higher level than those of rangers and mages. This has been remedied with the newer formula, so that players of all combat styles can obtain equal combat levels.

Standard formula
In the Combat Triangle Wallpaper update, Jagex revealed the basic formula used for calculating the basic combat level (excludes Summoning) on the top right of the wallpaper.

Please note: The basic formula revealed in the wallpaper, however, is at an early stage and using the formula as provided does not in all cases result in the combat level players would have observed in game. This is particularly true for Mages and Rangers, as also here even and odd levels did seem to play a role in calculating the correct combat level. On the RuneScape Forums (Combat skill section) a more accurate calculation method has been found by a group of adventurers - see calculations below.

Just as with the current formula, the previous combat levels would be calculated for each combat type (Melee, Magic and Ranged), and the highest number was displayed as the combat level.

The character's Prayer level used in the formula was the character's lowest even level – for example, if you had level 43 Prayer, the formula would act as if you had 42.

Note: Combat levels were rounded down in-game.

Note that the floor function, $$\lfloor x \rfloor$$, means round down to the nearest integer (e.g. $$\lfloor 1.5 \rfloor = 1$$).

Summoning extension
In member worlds, a player's Summoning level would be taken into account for the formula in the following cases:


 * In non-Wilderness areas
 * In the Wilderness when the player either:
 * had a summoned familiar.
 * was carrying a summoning pouch or summoning scroll.

Note that the floor function, $$\lfloor x \rfloor$$, means round down to the nearest integer (e.g. $$\lfloor 1.5 \rfloor = 1$$).

When it was not included, the potential Summoning increase was displayed separately – for example, 79+6 instead of 85.

Summarised
(X = your highest stat level from Magic, Ranged, Attack or Strength).

The following formula summarises the above:

Where $$max(x,y,z)$$ returns the highest value of $$x$$, $$y$$ and $$z$$. On free-to-play worlds, the Summoning fraction would be left out.

Trivia

 * In DeviousMUD the highest an account ever reached was 52.
 * Past maximum combat levels include level 123 in RuneScape Classic, 126 before Summoning was added, and 138 before the Evolution of Combat was introduced.
 * In Runescape Classic, players started at 1 combat. This was later changed to 3.
 * According to the new formula, it is possible to obtain 99 Constitution and Prayer while staying at level 4.