Interval Timer

The ratio is the whole prescription, and it decides which energy system you are actually training.

1:1, such as 3 minutes on and 3 minutes off, sits in the aerobic power range. You should be able to hold roughly the same pace on the last round as the first, and if you cannot, you started too hard.

2:1, such as 4 minutes on and 2 off, is threshold work. It accumulates fatigue deliberately, and the last round is supposed to be ugly.

1:2 or 1:3, such as 30 seconds on and 90 off, is anaerobic power. Each interval should be near maximal, and the long rest exists so it can be.

This timer ends on the final work interval rather than adding a trailing rest, because a session that finishes with two minutes of standing around is just two minutes of standing around. Tabata is the deliberate exception, since a true Tabata is exactly four minutes including that last rest.

One limit worth knowing before you start: a browser tab cannot make sound once your phone locks or you switch apps, because browsers suspend background tabs to save battery. For a workout where the phone sits in your pocket or face down on the floor, use the app instead. It keeps the clock and the beeps running with the screen off.