Judo rules and scoring have changed over the years, so I am providing an update for 2016. Differences from older answers are in pin times and treatment of penalties.
There are three positive scores:
- ippon - a match ends immediately if when an ippon is scored. Referee signals with arm pointing to ceiling.
- wazari - if a player scores two wazari, the match immediately ends (wazari awasete ippon). Referee signals with arm out to side, parallel or higher to ground.
- yuko - these only count if match time expires. Referee signals with arm out to side and angled down.
There are four ways to score:
- pin (osaekomi)
- choke (shime)
- arm lock (kansetsu)
Chokes and arm locks are all or nothing; they can only score ippon. Throws and pins have their own criteria for determining scores.
The criteria for judging a throw score are:
- opponent lands largely on the back
If a throw meets all four criteria, it is an ippon. It if it missing one, it is a wazari, including if the player being thrown rolls across their back instead of landing on it. A yuko is a throw where the player lands on their side. There is no score if someone lands on their front.
In order to score, a player must perform a throwing action. It is not sufficient for an opponent to land on their back, as they would, for example, while attempting a sacrifice throw. In situations where one player is attacking and the other counters, either player may score. It may be very difficult in these situations to determine who has scored, as in the 2000 Sydney Olympics heavyweight final.
Pins require control of the opponent with the back of at least one of their shoulders on the mat and the requirement that the legs of the pinning player must be free of the player being pinned.
The duration of the pin determines score:
- 10-14 seconds - yuko
- 15-19 seconds - wazari
- 20 seconds - ippon
Matches can continue on the mat before a pin while the referee considers that progress is being made, which is subjective. If the situation stalls, the referee restarts the competitors in a standing position.
Sadly, penalties are a very important component of competitive judo. There are two types of penalties:
- major (hansoku make) Major penalties result in automatic disqualification and are relatively rare. These are for grabbing the legs from a standing position, punching, or safety violations.
- minor (shido) Minor penalties include for stalling, false attacks (which are a form of stalling), not taking a grip, pulling off a grip with two hands, holding onto one side of the opponents gi with two hands without attacking, stepping out of bounds, or pushing your opponent out of bounds. Four shidos in a match result in disqualification.
Penalties do not accumulate into positive scores before disqualification. They used to, but this was changed to encourage "positive judo" so that positive scores count above penalties. Towards the end of matches, you will often see the player in the lead stalling and accumulating penalties to preserve their lead.
The referee signals a penalty by stopping the match, making a motion to indicate the violation, and pointing a finger at the offending player.
End of time
When time expires, scores and penalties are compared to determine who wins. If one player has a wazari and the other does not, the player with the wazari wins. If the number of wazari is the same, yukos are compared. If one player has more yukos, that player wins. If the number of yukos is also the same, the shidos are compared. If one player has fewer shidos, that player wins.
When time expires and the players are tied (same scores and penalties), the match continues in golden score. The players compete until one player either scores or exactly one receives a shido, and a winner is declared.
The score format is now
Leading zeros may be omitted.
- 100-0s1 Player 1 has an ippon, and player 2 has a shido. Player 1 wins
- 1s1-0s2 Player 1 a yuko and a shido, and player 2 has 2 shidos. Player 1 wins.
- 2-10s3 Player 1 has 2 yukos. Player 2 has a wazari and 3 shidos. Player 2 wins.
- 0H-100s1 Player 1 has a hansoku make. Player 2 has an ippon (because of player 1's hansoku make) and a shido. Player 2 wins.
- 0s1-0s2 Player 1 has a shido. Player 2 has 2 shidos. Player 1 wins.