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 working out one'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 will be 4 (1+1+2).

It was announced that Constitution, Summoning and Prayer do not affect combat level. Allegedly, Summoning is nonetheless counted as a combat stat.

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 level 200 for members. 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.

Combat level is the only in-game visible parameter that players have to evaluate another player without using Stat Spy or the Inspect option in Dungeoneering. High combat levels do not always accurately describe a player's capabilities in other skills. There are players with maximum combat that have very low levels in other skills and there are pure skillers with no combat levels and very high non-combat skills. Yet, there is a reasonable chance that a high combat level also implies a decent skill level and a reasonable knowledge of the game.

There are some requirements in game that require a certain combat level. There are two quests requiring a certain combat level, Smoking Kills and Dream Mentor (both requiring 85 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 is supposed to apply 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. TzTok-Jad, for instance, becomes a level 140 while still being a challenge to higher levelled players.

The new system also does not explain how Forgotten mages could be level 198 while they wear celestial robes that requires 99 Magic and Defence, which should give a combat level of 200.

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.
 * Before the combat update the minimum combat level was 3.
 * According to the formula, it is possible to obtain 99 Constitution while staying at level 4.