Forty-five minutes is a class period in a lot of schools, and a deep work block that still fits before lunch. Treat it as one arc: start, the hard middle, and a last five minutes to capture what is next.
The last five are part of the timer, not a bonus. If you work through them, the next session starts cold. Press space to start or pause, R to reset, and F for fullscreen. The screen can stay awake after you start, using the wake lock your browser allows. Pick the alarm before you begin. A bell, a chime, a beep, or a short digital pulse all come from the Web Audio API, so there is no sound file to wait on.
This page is the 45 minutes countdown and nothing else is running underneath it. Related lengths are linked under the guide if you want a neighbor, and the timer itself stays above the fold.
Questions
How do I run a 45 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. The last five are part of the timer, not a bonus. If you work through them, the next session starts cold.
What is 45 minutes actually good for?
Forty-five minutes is a class period in a lot of schools, and a deep work block that still fits before lunch.
Will the countdown drift if I switch tabs?
No. The end is a timestamp. The display is the gap between that timestamp and now, so a background tab does not lose seconds the way a ticking counter would.