Twenty-one minutes is three sevens or a seven and a fourteen. Use the split if the work has two speeds. It is a solid first draft window: ugly sentences allowed, editing saved for the next timer.
Drink water before you start. Getting up at minute nineteen throws the block away. 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 21 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 21 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Drink water before you start. Getting up at minute nineteen throws the block away.
What is 21 minutes actually good for?
Twenty-one minutes is three sevens or a seven and a fourteen. Use the split if the work has two speeds.
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.