Why is 40/20 so common?

The round lands on a whole minute, which makes a class easy to run and a session easy to count. Twenty seconds is also long enough to change stations, so it works as a circuit split as well as an interval one.

How does it compare to 45/15?

Five seconds less work and five more rest, which sounds trivial and is not. Twenty seconds lets you move between exercises; fifteen does not.

Can I use it as a circuit?

Yes. Four movements over ten rounds gives two and a half laps, though a dedicated circuit timer will name the stations for you.

Timer · preset of interval timer

40/20 interval timer

Forty seconds of work and twenty of rest, ten rounds. The standard conditioning split, and the one most group classes are built on.

Open the full interval timer to change anything this preset fixes

The timer

Rounds

Lead-in

Work: 40s
Rest: 20s

The clock runs fullscreen and keeps the screen awake. It needs JavaScript, so with scripting off the plan below is the whole session written out.

Total: 9:50
Working: 6:40
Periods: 20

01ReadyGet ready0:10
02WorkWork0:40
03RestRest0:20
04WorkWork0:40
05RestRest0:20
06WorkWork0:40
07RestRest0:20
08WorkWork0:40
09RestRest0:20
10WorkWork0:40
11RestRest0:20
12WorkWork0:40
13RestRest0:20
14WorkWork0:40
15RestRest0:20
16WorkWork0:40
17RestRest0:20
18WorkWork0:40
19RestRest0:20
20WorkWork0:40

How it works

Forty seconds on and twenty off is the default conditioning interval, and its popularity comes down to arithmetic. One round is exactly one minute, so ten rounds is ten minutes and nobody has to work out where the session ends.

The twenty second rest is the other reason it spread. It is the shortest gap that still lets you walk to another station, pick up a different weight and start on time. That makes the same split work for a solo interval session and for a four station circuit.

At two parts work to one part rest, the effort sits between 30/30 and 45/15. You can hold a strong pace but not a maximal one, and by round seven the rest starts feeling short.

Use it as the middle rung. When ten rounds of 30/30 stops being hard, move here rather than to 45/15. The jump is smaller and the twenty seconds keeps your options open on what you can run in it.

Questions

Why is 40/20 so common?: The round lands on a whole minute, which makes a class easy to run and a session easy to count. Twenty seconds is also long enough to change stations, so it works as a circuit split as well as an interval one.
How does it compare to 45/15?: Five seconds less work and five more rest, which sounds trivial and is not. Twenty seconds lets you move between exercises; fifteen does not.
Can I use it as a circuit?: Yes. Four movements over ten rounds gives two and a half laps, though a dedicated circuit timer will name the stations for you.