Do I need to train to failure?

Direct answer

No. You don't need to take every set to failure to make progress.

What does matter is that your working sets are challenging enough to provide a reason for your body to adapt.

Explanation

One useful way of judging this is reps in reserve, or RIR. This simply means estimating how many more good repetitions you could have completed at the end of a set.

For example, if you finish eight reps on a leg press and feel confident you could have completed another two or three before you couldn't perform another good rep, you've finished with roughly 2–3 reps in reserve.

The number of reps you leave in reserve matters because the set still needs to be challenging enough to create a useful training stimulus. If you finish a set with eight or more repetitions still comfortably available, for example, you've probably stopped well before the muscle has been meaningfully challenged.

That doesn't mean every set needs to finish one rep away from failure. The goal is to make sure your working sets are genuinely demanding enough to contribute towards progression, without assuming that taking every set to absolute failure will automatically produce better results.

There is one problem, though: people are often further from failure than they think, particularly when they're newer to resistance training.

Someone might finish a set believing they only had another two reps available when, if they actually continued, they could have completed another six.

That's why occasionally taking an appropriate exercise to failure, once you're confident with the movement and your technique, can be useful. Not because failure itself is required, but because it can help you learn what genuinely hard training feels like.