How do you know how heavy to lift?

Walking into a gym and picking up a dumbbell can feel like a test you didn't revise for. Too light and you wonder if you're wasting your time; too heavy and you're doing something that looks like interpretive dance. There is a simple, practical way to choose, and it doesn't require knowing your maximum or doing any maths.

Reps in reserve, in plain English

Reps in reserve, often shortened to RIR, is a way of judging how hard a set is. A 2024 scoping review in Perceptual and Motor Skills describes an RIR scale as a measure of how close you are to muscle failure during a set: in other words, how many more repetitions you could have done with good form before you couldn't do another.

So if you do eight reps and feel you could have managed about two more, that set was "two reps in reserve".

For everyday strength training, finishing most sets with a few reps still in the tank is a comfortable and sensible place to be. You are working, not suffering.

Estimating it is a skill

The same review notes that people with more lifting experience may judge their reps in reserve more accurately than beginners, though the evidence is mixed, and it suggests some caution for newcomers. That is a good reason to err on the light side at first. If you think you had two in reserve, you may well have had more, and that is fine.

How to find your starting weight

1. Pick a number of reps. Something like eight to twelve is a friendly range for most exercises.
2. Start light. Lighter than your ego suggests. Do a set and notice how it feels.
3. Ask the question. At the end, how many more could you have done with the same tidy form? Lots more: go up a little next set. About two or three: that's your weight. None, or your form fell apart: go down.
4. Write it down. A note on your phone saves guessing next time.

Form is the non-negotiable bit. If the last reps involve swinging, arching your back or holding your breath and turning purple, the weight is too heavy for now.

Going up gradually

Once your chosen weight starts to feel easy, with more than a few reps left over at the end of every set, it's time for a small step up. A useful approach:

• Add reps first. If you've been doing eight, work up to twelve with the same weight.
• Then add a little weight and drop back to eight.
• Use the smallest jump available. Small plates, the next dumbbell, the next pin on the machine.

Progress will be uneven. Some weeks things feel heavy for no reason at all, because of sleep, stress or simply Tuesday. On those days, keep the weight the same or go lighter. That's not going backwards; it's sensible.

Different exercises, different weights

You'll probably find you can lift more on some exercises than others; for most people, a weight that feels fine for legs is far too much for shoulders. Machines and free weights feel different too; our guide to machines or free weights covers that choice. Find the right load for each exercise separately.

If you're just starting

If you're new or coming back after a long break, spend the first few sessions learning the movements with light weights, or with just your body weight. Many gyms offer an induction, and a qualified trainer can check your form. Our guide to starting strength training if gyms intimidate you helps with the nerves.