Why is ninety seconds the common default?

It is long enough for moderate compound work to recover most of the way, and short enough that a session of five exercises still fits in an hour.

When should I rest longer?

On heavy sets of five or fewer, and whenever the following set drops by more than a rep or two.

Does the rest need to be exact?

No, but it needs to be consistent. Rest drifts upward through a session, and timing it is mostly a defense against that.

Timer · preset of rest timer

90 second rest timer

A minute and a half between sets. The default rest for most accessory and moderate compound work.

Open the full rest timer to change anything this preset fixes

The timer

Rest: 90s

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: 1:30
Working: 1:30
Periods: 1

01WorkRest1:30

How it works

Ninety seconds is the rest most training programs assume when they do not say. It recovers a moderate compound set almost fully, and it keeps a five exercise session inside an hour, which is the practical constraint most people are actually working under.

It suits sets of six to twelve at a weight that is hard but not near maximal. That covers the majority of what most people do, which is why it ends up as the default.

The reason to time it rather than guess is drift. Rest is the one part of a session nobody tracks, and it stretches as the session goes on and the phone comes out. A fifty minute session becomes seventy without anyone deciding to make it longer.

Check it against the next set. Losing a rep or two is normal. Losing four means the rest was too short, and the honest response is more rest rather than more effort.

Questions

Why is ninety seconds the common default?: It is long enough for moderate compound work to recover most of the way, and short enough that a session of five exercises still fits in an hour.
When should I rest longer?: On heavy sets of five or fewer, and whenever the following set drops by more than a rep or two.
Does the rest need to be exact?: No, but it needs to be consistent. Rest drifts upward through a session, and timing it is mostly a defense against that.