Byo-yomi rules, explained

The one rule everyone gets wrong

Newcomers almost always assume that playing a move uses up a period. It does not. This is the single most important thing to understand about byo-yomi:

• Complete your move inside the period and the period resets to full. It is not consumed. You could play two hundred moves at 28 seconds each with a 30-second period and still have every one of your periods intact.
• Overrun the period and it is consumed for good, with the next period immediately counting down for the same move. Spend 65 seconds on one move with 30-second periods and you burn two periods, finishing the move in the third.
• Overrun your last period and the game is lost on time.

Leftover seconds never accumulate. A move played in 3 seconds does not bank 27 seconds for later — the next period simply starts at 30 again. This is the opposite of Fischer bonus time, where unused time does build up.

Walkthrough: 20 minutes + 5 × 30 seconds

This is the most common club and tournament setting in the world. Here is how a real game moves through it:

• Opening and middle game. You think freely and spend your 20 minutes of main time. When it hits zero you are not out — you enter byo-yomi with all five periods intact. The clock switches to showing a 30-second period and "× 5".
• Settling into rhythm. You play each move in 10 to 25 seconds. Every move resets the period to 30. You still have five periods. This can continue indefinitely.
• A hard position. You need to read out a life-and-death problem and spend 50 seconds. The first period is consumed, and you complete the move inside the second, which then resets. Four periods remain.
• The endgame. Your remaining periods are effectively four extra deep thinks for the rest of the game. Routine moves inside 30 seconds cost nothing. You budget the periods like lives.

That is the character of byo-yomi: constant, steady pressure, with a countable allowance of long thinks. It guarantees that every move for the rest of the game — even move 250 — gets at least 30 seconds. This is precisely what long Go endgames need.

Why Go uses this and chess does not

A Go game can easily run past 250 moves, and the endgame is where points are won and lost in ones and twos. A single pool of time — the chess model — runs out and ends the game arbitrarily, often in a position that is nearly finished. Byo-yomi replaces the pool with a per-move deadline, so the game ends because someone could not find a move in 30 seconds, not because a clock expired mid-endgame.

The name is literal. In professional Go a timekeeper sat beside the board and read the closing seconds aloud — "…twenty, …twenty-five, …twenty-six, twenty-seven…" — which is why a spoken countdown, rather than a beep, is what Go players actually expect from a clock. Byoyomi Timer reads those seconds in Japanese, Korean, Mandarin, English or German.

Notation and common settings

Byo-yomi settings are usually written as main time + periods × period length:

A setting with no main time is also valid — "3 × 30 s" means you are in byo-yomi from move one. Online servers including OGS and IGS default to byo-yomi for most game types, which is why it is the system most players meet first.

Byo-yomi on a phone versus on hardware

Dedicated Go clocks that handle byo-yomi properly exist but are a niche product, and cheap digital chess clocks generally offer only increment and delay — neither of which behaves like a period that resets. A phone has the advantage here: the screen can show the running period and the number of periods left at the same time, in digits large enough to read across a board, and it can speak the countdown.

In Byoyomi Timer byo-yomi is the default system, not an extra mode. Main time, period length and period count are all free to set, the transition into byo-yomi is announced out loud, and a referee can correct the period count mid-game if something goes wrong. Both players can also be given different period counts, which is the natural way to handicap a teaching game.