Like any combat sport, the complete rule-set for competition is extensive and has variances at both an international, governing body and grade (e.g. amateur/professional) level. Sometimes different promotions have their own revisions to rules, usually regarding the number and length of rounds and the permitted duration of clinching.
There are roughly 20-24 official rule categories (depending on how they are classified) and within each are several clauses/rules which detail the specific rules within each category.
Below, you'll find a basic overview of the rules, along with distinctions between amateur & professional, some typical variations and a list of governing bodies with links to their respective complete rule-sets.
There are many similarities between some of the basic rules of Muay Thai, Boxing and Kickboxing, especially in terms of the size of ring (up to 24ft wide), glove size (6-12oz typically), the use of hand wraps, wins are by TKO/KO or judge's decision and the fact that fighters compete at the same/similar skill level (although in professional bouts, this is more down to the promotion and its matchmakers) and weight class.
Besides these similarities, there are some fundamental rules in Muay Thai that make it unique, including:
Competitors enter the ring by jumping over the rope (women often go under the rope in Thailand).
Competitors pay homage and seal the ring by walking around the ring in a counter-clockwise circle, stopping at each corner and bowing.
The performance of the Wai Kru is an especially unique distinction. It's a a traditional dance which is influenced by an individual's Muay Thai coach and feelings toward their opponent.
Competitors have 8 points to strike with – fists, elbows, knees, shins.
Another unique distinction is use of the clinch. This stand-up wrestling provides competitors the opportunity to grapple, sweep, throw and strike at close quarters.
Competitors can catch kicks and sweep or dump their opponents.
At a glance, the most obvious distinctions are the use of protective equipment. In amateur bouts, fighters use the whole array of protective gear, including shin guards, head guard, elbow guards, body guard and sometimes knee guards, in addition to the standard groin guard and mouth guard. Whereas professional bouts have merely groin guards and mouth guards for protection (elbow guards on some occasions).
Furthermore, amateur bouts often exclude spinning elbow strikes and have revised round durations. For example, 5 x 1.5 or 5 x 2 instead of 5 x 3 for professionals.
The most prestigious amateur competition is the International Federation of Muay Thai Amateur Championships which is an annual event that attracts people from all over the world.
It's most often outside of Thailand that the variations occur more frequently. These can be between different promotions/leagues, at inter-club/amateur/professional level and also region specific. For example:
Some promotions restrict the time competitors can be in the clinch.
In Europe, the minimum age for official amateur and/or professional bouts is 15 – unlike Thailand where its common to see children as young as 8 compete professionally.
Some promotions and regions revise the number of rounds and their duration (e.g. 3 x 5 with 1 minute break or 5 x 2 with 1 minute break).
At inter-club level, there are usually restrictions on the power of strikes.
Not every region adheres strictly to having both fighters perform the Wai Kru before a bout.
This list isn't exhaustive and it's not to say there are always variations. Instead, this list simply contains some of the most common variations which are often imposed to help make the bouts more “entertaining” for audiences who may not fully understand or appreciate the intricacy of Muay Thai.
You will also find, especially in Europe, that there are classes of bout which have their own variations of rules. In the UK, they are as follows:
N class: This is the starting point for most fighters. 5 X 1.5 minute rounds with no knees or elbows to the head and both fighters wear shin pads.
C Class: 5 X 1.5 minute rounds with no knees or elbows to the head and no shin pads are worn.
B Class: 5 X 2 minute rounds and allows the use of knees to the head but not elbows.
A Class: 5 X 3 minute rounds and allows the use of both knees and elbows to the head.
These classes are designed to help fighters comfortably progress through the ranks.
If an event isn't officially sponsored or endorsed by any of the following governing bodies, you may need to ask for a full set of rules to be sure. Otherwise, you can see a list of the key, global governing bodies below, complete with links to their websites which contain more information about them and their complete rule-sets:
World Muay Thai Council (WMTC)