The ball is out only when the law says play has stopped.

How the referee checks it

1. Check whether the whole ball has wholly crossed the touchline or goal line, on the ground or in the air.
2. If it has crossed a line, identify the correct restart from where and why it left play.
3. If it has not crossed a line, check whether the referee has stopped play for a foul, injury, outside interference, defective ball, or another reason.
4. If the ball touched a match official and stayed on the field, decide whether possession changed, a promising attack started, or the ball went directly into the goal.
5. If none of those out-of-play events happened, play continues even if the ball hit a post, crossbar, corner flag, referee, assistant referee, or line marking.

What counts as out of play

The ball is out of play in three main situations. First, it has wholly passed over a boundary line: the touchline or goal line. Second, the referee has stopped play. Third, it has touched a match official, remained on the field, and produced one of the specific outcomes that Law 9 treats as unfair or too influential to continue.

The whole ball must cross the whole line

A ball touching the line is still in play. A ball partly above the line is still in play. The same idea applies whether the ball is rolling on the ground or flying through the air: if any part of the ball is still over any part of the line, play continues. This is why close goal-line and touchline decisions often depend on the assistant referee's angle.

Touchlines and goal lines decide ordinary restarts

When the ball wholly crosses a touchline, the restart is a throw-in. When it wholly crosses a goal line without a goal being scored, the restart is normally a goal kick or corner kick depending on which team last touched it. If the whole ball crosses the goal line between the posts and under the crossbar, a goal is awarded unless another law cancels it first.

The whistle can make the ball dead

Play is out once the referee stops it, even if the ball is still moving and still inside the field. That stoppage might be for a foul, offside, misconduct, injury, outside interference, a defective ball, a VAR review in competitions using VAR, or a restart problem. The next restart depends on the reason play was stopped, not simply on where the ball was.

A referee touch does not always stop play

If the ball touches the referee or another match official and stays on the field, play usually continues. The referee stops play only if the contact causes a team to start a promising attack, the ball goes directly into the goal, or possession changes team. In those cases, the restart is a dropped ball from the position where the ball touched the match official.

Rebounds are still live

The ball remains in play when it rebounds from a goalpost, crossbar, or corner flagpost and stays on the field. A shot off the post can be played by either team. A clearance that hits the corner flag and stays inside the field is not a throw-in or corner. The restart changes only when the whole ball crosses the relevant line or the referee stops play.

A goal needs the same whole-ball test

For a goal, the whole ball must wholly cross the goal line between the goalposts and under the crossbar. It is not enough for the ball to be mostly over the line. Goal-line technology, where used, is designed to answer this exact question. VAR may also check goal/no-goal incidents in competitions that use the VAR protocol, but the underlying rule is still the whole ball over the whole line.

In play is not the same on every restart

Once play has stopped, the next restart has its own rule for when the ball becomes live again. A kick-off, free kick, goal kick, corner kick, and penalty kick are in play when the ball is kicked and clearly moves. A dropped ball is in play when it touches the ground. A throw-in is in play when the ball enters the field after a legal delivery. Those details matter for second touches, encroachment, offside, and direct goals from restarts.

People off the field should not stop a live ball

If a team official, substitute, substituted or sent-off player, or a player temporarily off the field touches the ball while it is still in play, the referee judges what the contact did. When the ball was clearly leaving the field and the touch was not intended to interfere unfairly, the sanction is an indirect free kick without an automatic disciplinary sanction. More serious interference can be handled under the misconduct and outside-interference rules.

Misunderstandings to avoid

• "Most of the ball was out" is not enough. The whole ball must be over the whole line.
• "The line belongs to out of play" is wrong. The boundary line is part of the field area it marks, so a ball touching or overhanging the line remains in play.
• "It hit the referee, so it is automatically a drop ball" is wrong. Match-official contact stops play only in the specific Law 9 situations.
• "The post makes it dead" is wrong. Rebounds from the goal frame or corner flag are live if the ball stays on the field.
• "Players stopped, so play stopped" is wrong. Play stops when the ball is out under the law or the referee stops it.

Source material

• IFAB Law 9 - The Ball in and out of Play
• IFAB Law 8 - The Start and Restart of Play
• IFAB Law 10 - Determining the Outcome of a Match
• IFAB Laws of the Game