Every new exercise starts with a guess. The trick is to make the first guess safe and cheap, then correct it with what the set tells you. Done this way, you know your working weight within three sets, and by the second session it is right.

The short answer

Start light, do a test set of the reps your program asks for, and count how many more you could have done. One to three left: that is your weight. Four or more: go heavier. Could not reach the bottom of the range: take off about 10 percent. No table of percentages can do this for you, because it needs a one-rep maximum you do not know yet.

Why a formula does not work

Most weight charts work from a percentage of your one-rep maximum: 60 to 80 percent for sets of 8 to 12, in the classic scheme. A beginner has no maximum to start from, and testing one on day one is a skill of its own. The American College of Sports Medicine’s 2009 position stand sidesteps the problem: it sets a beginner’s load as the weight you can lift for 8 to 12 reps, not as a percentage.

Machines add another reason. The number on a weight stack depends on the machine’s pulleys and levers, so 40 on one chest press is not 40 on the next one, and neither tells you what to put on a bar.

Reps in reserve

Reps in reserve (RIR) is the number of reps you could still have done with good form when you stopped. A 2016 study tied it to the familiar effort scale of 1 to 10: a 10 means nothing left, a 9 one rep left, an 8 two. Judging each set this way adjusts the weight to your day, set by set, as a 2016 guide to the scale describes.

The estimate is imperfect but usable. In a 2022 meta-analysis of 12 studies with 414 people, lifters underestimated the reps they had left by about one on average. They judged better near the end of a set and in sets of 12 reps or fewer, and experience made little difference. So use a test set of 12 reps or fewer, rate it at the end, and expect about one more rep than you think.

The test set

  1. Warm up with the empty bar, light dumbbells or a light stack setting: 8 to 10 easy reps.
  2. Pick a weight you are sure of for the top of your range, and do the set to that number.
  3. Rate it: how many more reps were there?
  4. Adjust with the table. The next set is a working set.
Reps left after the test set What it means Next set
None, or form broke early Too heavy About 10% lighter
1 to 3 Right Keep the weight
4 to 6 A little light One step heavier
7 or more Far too light Two or three steps heavier

By equipment

Equipment Start with One step Watch for
Barbell The empty bar, 20 kg (about 45 lb) 2.5 kg or 5 lb, the smallest plate each side Many gyms also have lighter bars
Dumbbells A pair you could press or curl 15 times The next pair, usually 2 kg or 5 lb more A big jump for a small muscle: widen the rep range
Machines The second or third plate of the stack One plate Note the seat setting; numbers do not carry over
Body weight An easier version, such as the Incline Push-Up A harder version The angle changes the load

The step matters more than the start. Progressive Overload explains why small steps keep you progressing and what to do with dumbbells whose next size is a big jump.

A first session, worked through

The Barbell Bench Press in Beginner Full-Body Barbell, 3 days asks for 3 sets of 6 to 10 reps.

Set Weight Reps Reps left Decision
Warm-up 20 kg 10 Many Add 10 kg
Warm-up 30 kg 6 Many Add 10 kg
Test 40 kg 10 4 or 5 One step heavier
1 42.5 kg 10 2 or 3 Keep
2 42.5 kg 9 2 Keep
3 42.5 kg 8 1 or 2 Keep; next time aim for 10, 9, 9

In pounds the weights are about 45, 65, 90 and 95. Three sets in, the working weight is found; the next session starts from it and adds reps.

What it means for your week

Every exercise in our programs comes with its rep range, so the test set always has a target: in Beginner Full-Body Barbell, 6 to 10 on the squat and the presses, 8 to 12 on the rows and pulldowns, 10 to 15 on the leg machines. Spend the first session of a new program finding weights and the second confirming them. After that, the weight changes by the rule of your program: add reps within the range, then a small step. The 2009 position stand puts that step at 2 to 10 percent once you can do one or two reps more than the target. How Many Reps Should You Do? covers the ranges themselves.

How to apply it

  1. Take the rep range from your program, or from the reps guide.
  2. In the first session, find each weight with one test set, a little lighter than you think.
  3. In the second, confirm it: the same weight, one more rep if it comes.
  4. From then on, follow the progression: reps first, then the smallest step.
  5. Write every set down. The last session’s numbers are where the next one starts.

Trenno keeps that record in front of you. The Previous column shows last session’s sets beside the empty ones, and the reps cell shows the target range until you log a number. Each exercise card has Easy, Normal and Hard: mark an exercise Easy and next time the card offers a heavier weight to try, with Apply; mark it Hard and the card reminds you that last time was hard.

Mistakes and myths

  • Starting where someone else lifts. Their weight says nothing about your first set.
  • Testing a one-rep maximum on day one. A near-maximal lift before you know the movement is the wrong first step, and you do not need the number.
  • Changing the weight every set forever. Adjust within the first session, then keep the weight until the reps say otherwise.
  • “Machines are only for beginners.” The American College of Sports Medicine’s 2026 position stand found no difference in strength gains between free weights and machines. Start where you are confident.

FAQ

How do I know how many reps I had left?

Reps slow down as a set nears failure; the 2016 study found bar speed falling as the effort rating rose. When a rep grinds, you likely have one or two left. Rate a few sets and you get better at it.

What if the dumbbells jump too much?

Stay at the lighter pair and add reps until you reach the top of a wider range, such as 8 to 15, then move up.

Do I need to know my one-rep maximum?

No. The rep range and the reps you had left set the weight without it.

I moved to another gym. Start over?

For machines and cables, yes: do a test set, because the numbers differ. For barbells and dumbbells your weights carry over.

Sources