Combat level

A player's, monster's, or non-player character's combat level indicates the difficulty in defeating him, her, or it in combat. Levels that are lower than yours display shades of green, levels that are slightly lower than one's display yellow-ish , levels that are equal to one's are displayed as yellow and levels that are higher than yours display shades of orange to red.

Normally, aggressive non-player characters stop attacking you when you reach higher than double their combat level. There are exceptions, however - for example, any monsters in the Wilderness will be aggressive no matter what your combat level, which is extremely annoying to player killers fighting in a single-way area.

Monsters with a higher combat level than a player cannot be attacked by using a left mouse click. Instead, players need to right click and select the attack monster option (you can change this setting in the Options tab). So, to be able to attack a Blue dragon with combat level 111 with a simple left click, a player needs a combat level of at least 111. It is possible to edit this in the settings under Controls by changing Attack option priority (default is "Depends on combat levels").

All new players start out at combat level 3, and the maximum level for RuneScape 2 is level 126, while in RuneScape Classic, it is level 123.

Simply

 * 1) Take your Prayer level and divide it by two and round down (for example, if your Prayer level is 43, dividing by two and rounding down gives you 21)
 * 2) Add this to your Hitpoints and Defence levels and divide the result by 4. This is your base combat level. (For example, if your Hitpoints is 60, Defence is 70 and prayer is 43 you should end up with 37.75)
 * 3) Add your Strength and Attack levels together and multiply by 0.325. Add this to your base combat level and you should have your melee combat level.
 * 4) If your Magic or Ranged level is exceptionally higher than your Attack and Strength, carry on - in the calculation noted below Magic is used, but if your Ranged is exceptionally higher, use that instead in all cases
 * 5) Divide your Magic level by 2 and round down, and then add your Magic level again to this (for example, if your Magic level is 83, you should end up with 124)
 * 6) Multiply this by 0.325 and add the result to your base combat level calculated above, and you should have your magic combat level.

Wiki combat calculator
This calculator can be used to experiment how combat skills affect combat level.

Comments
Outside of raids and using Stat Spy, combat level is the only in-game visible parameter that players have to evaluate another player. High combat levels do not always accurately describe a players' 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.

A more complicated assessment is in the trustworthiness of a player in trades. Some players place higher trust in higher combat level players, as they have more to lose when they get banned after being reported for illegal trading practices.

Helpful knowledge
Since pures use precise levels to take advantage of their combat level, it is useful to know what levels for certain skills will increase their combat level. Previously shown, combat level will gradually increase as your Hitpoints level increases; to be more precise (with level 1 Defence and Prayer since this is mainly for pures), your combat level will go up by 1 the next time your Hitpoints reaches a level which is an integer when you subtract 5 and divide by 4. For example (ignoring the minimum hitpoints level being 10, for simplicity), your combat level will increase when your Hitpoints level reaches 5 (because 5-5 = 0, 0/4 = 0), 9 (9-5 = 4, 4/4 = 1), 13 (13-5 = 8, 8/4 = 2), 17 (17-5 = 12, 12/4 = 3), and so on (keep adding 4). However, your combat level will not increase if you advance your Hitpoints level to any other level

Note: these calculations are only for Hitpoints level increases and affect your combat level in this way regardless of your levels for Attack, Strength, Ranged, and Magic (which obviously will also have additional effects on your combat level).

Exceptions
Some monsters do not have a combat level. These are:
 * All reanimated monsters
 * Kolodion (excluding his final form)
 * Solus Dellagar
 * Evil creature
 * NPCs of Chambers of Xeric (with the exception of the Great Olm)