Twenty minutes is a class segment, a power nap boundary, and a kitchen window for something that needs a glance. Research on focused work often clusters near this length because attention frays after it, not because of a magic number.
Put the timer where the room can see it. A twenty minute block that only you can see becomes thirty. 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 20 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 20 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Put the timer where the room can see it. A twenty minute block that only you can see becomes thirty.
What is 20 minutes actually good for?
Twenty minutes is a class segment, a power nap boundary, and a kitchen window for something that needs a glance.
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.