Overs are counted by valid balls, and bowlers are limited by both Law and format.

What counts as an over

An over is a group of six valid balls bowled from one end of the pitch. After the over, the next over is bowled from the opposite end. The batters stay at their ends unless the last delivery, or runs completed from it, leaves them crossed.

The key word is valid. A delivery can happen without counting as one of the six balls of the over. That is why an over can take seven, eight, or more actual deliveries if the bowler sends down wides, no-balls, or certain dead-ball deliveries.

No-balls and wides make the over longer

No-balls and wides do not count as valid balls of the over. Each normally adds one extra run, and the bowler must still complete the six valid balls unless the innings ends first or the bowler is replaced under the Laws.

Some dead-ball situations also do not count, especially where the ball becomes dead before the striker has had an opportunity to play it. Other dead-ball calls can still leave the delivery counting, depending on why the ball became dead. The umpire's signal and the scorer's count decide the official over count.

A bowler cannot bowl back-to-back overs

A bowler may change ends during an innings, but cannot bowl two overs consecutively. The restriction also prevents a bowler from bowling parts of each of two consecutive overs in the same innings.

This rule matters when an over is interrupted, resumed after a break, or completed by a replacement bowler. A captain cannot use an interval, change of ends, or partial over to let the same bowler effectively bowl both sides of a break in the normal sequence.

The bowler normally has to finish it

Other than at the end of an innings, the bowler who starts an over normally finishes it. If an interval or interruption stops play before the over is complete, the same over is completed when play resumes.

The main exceptions are incapacity and suspension. If a bowler is injured, becomes unable to continue, or is suspended from bowling under the Laws, another bowler completes the over from the same end. That replacement cannot have bowled the previous over or part of it, and cannot bowl any part of the next over.

Limited-overs cricket adds bowler quotas

In Test cricket and most multi-day cricket, there is no fixed per-innings quota such as "10 overs per bowler." Captains can use a bowler for long spells as long as the bowler does not bowl consecutive overs and remains eligible to bowl.

Limited-overs cricket is different. The match playing conditions normally limit each bowler to a share of the innings: in a standard 50-over innings, that is commonly 10 overs per bowler; in a standard 20-over innings, it is commonly 4 overs per bowler. Domestic competitions, junior cricket, and special formats can set different limits.

Rain changes the quota calculation

When a limited-overs innings is reduced, the bowling quota is usually reduced with it. Playing conditions commonly work from one-fifth of the available overs, with extra overs distributed only as needed when the innings length is not divisible by five.

For example, a 17-over T20 innings will not let every bowler bowl 4 overs. The playing conditions allocate the maximums so the fielding side can complete the innings without giving one bowler an unfairly large share. Exact reduced-match formulas are competition rules, so the umpire and scorers use the playing conditions for that match.

Quota limits are not the only restrictions

"Bowling restrictions" can mean several different things. Some are over limits: how many overs a bowler may bowl, and whether that bowler can bowl the next over. Others are delivery rules: legal arm action, legal feet, dangerous short balls, high full tosses, and unfair bowling.

Fielding restrictions can also affect a delivery in limited-overs cricket. If the field is illegal when the ball is delivered, the umpire may call no-ball under the relevant playing conditions. That is a fielding breach, not a bowler quota breach, but it still affects the ball and the over.

A bowler can be removed from the attack

A bowler can be suspended from bowling for the rest of the innings for certain serious or repeated offences. Examples include a further thrown delivery after a first and final warning, a deliberate front-foot no-ball, repeated dangerous bowling after warning, or other unfair-play procedures that require suspension.

When that happens during an over, another eligible bowler completes the over. The suspended bowler cannot bowl again in that innings, even if they still had overs left under a limited-overs quota.

How officials enforce the limits

1. Track valid balls until six have been bowled, excluding no-balls, wides, and any delivery that the Law says does not count.
2. Call Over only when six valid balls have been bowled and the ball is dead.
3. Check that the next over is from the opposite end and is not bowled by the same bowler or by someone who bowled part of the previous over.
4. In limited-overs cricket, track each bowler's quota under the match playing conditions.
5. If a bowler is incapacitated or suspended, require an eligible replacement to complete the over and prevent the replacement from bowling the next over.

The umpire's count stands

If the umpire miscounts the number of valid balls, the over as counted by the umpire stands. That can mean an over officially ends after five valid balls or continues to a seventh valid ball.

Scorers, players, and broadcasters may notice a mismatch, but the Laws give the official count to the umpire. Once the umpire calls Over, the match proceeds from that count unless a correction is made within the officials' normal match-management process.

Misunderstandings to avoid

• "Six deliveries always ends the over" is wrong. It is six valid balls, so no-balls and wides extend the over.
• "A bowler can switch ends and keep bowling" is wrong if it creates consecutive overs or parts of consecutive overs.
• "Test bowlers have a 10-over cap" is wrong. Fixed bowler quotas are limited-overs playing-condition rules, not the ordinary Law for all cricket.
• "Rain just reduces the team total overs" is incomplete. It usually changes each bowler's maximum overs too.
• "A suspended bowler can return after a break" is wrong when the Law suspends that bowler for the rest of the innings.

Source material

• MCC Law 17: The over
• MCC Law 21: No ball
• MCC Law 41: Unfair play
• ICC Playing Conditions