Almost every week someone walks up to me holding a pair of dumbbells, looks at the rack, and asks the same quiet question. "Is this too light? Is this too heavy? How do I even know?"

Most people never get a straight answer. So they grab something way too light because they're nervous, or they copy the person next to them and spend the next week walking down stairs sideways.

Here's the short answer. Pick a weight you can lift with good form for the reps you're given, and stop the set when you feel like you have about two good reps left. If you could have done five more, go a little heavier next set. If your form slips, go lighter. Then write it down and add a little over time.

I'm Matt, and I own The Detour Gym in Rochester. We've been coaching everyday adults, busy parents and athletes on the same floor since 2014, and this one rule is how we load almost everything. Here's why it works and how to use it.

Why "how heavy" is the wrong first question

There's no magic number. The same weight feels completely different depending on your sleep, your stress, your training history, and whether you ate lunch.

So instead of asking "how much weight," ask "how hard did that set feel?" Coaches call this reps in reserve: how many more good reps you could have done when you stopped. Researchers built a training scale around it in 2016, and it's now a standard tool in strength coaching (Zourdos and colleagues, 2016).

It adjusts to you every day. On a great day, two reps left might be a heavier weight. After a night shift or a sick kid, it might be lighter. Both count. Consistency beats perfection.

What does "two reps left" actually feel like?

The last couple of reps slow down. You have to focus. Your breathing picks up. But your form still looks like your first rep, and if a coach asked for two more, you could. Here's the quick check:

Way too light: you could have done five or more extra reps and you were chatting the whole time. Go up a little next set.
About right: the last couple of reps were hard but clean, and you had about two left. Stay here or nudge up next time.
Too heavy: your back rounds, your knees cave, you have to swing or bounce the weight, or you couldn't finish the reps. Go lighter. Good reps with less weight beat ugly reps with more, every time.

One honest note. Nobody guesses perfectly. A review of 12 studies found people miss by about one rep on average, usually thinking they have fewer left than they do (Halperin and colleagues, 2022). That's normal, and it gets better with practice and a coach watching.

Do you need to go until you can't lift anymore?

The American College of Sports Medicine released its new resistance training Position Stand in 2026, pulling together 137 systematic reviews and more than 30,000 people. Training to momentary failure did not consistently improve results for healthy adults (ACSM, 2026). Another review looked at 15 studies on muscle growth and found no real advantage for going all the way to failure compared with stopping a little short (Refalo and colleagues, 2023).

Effort still matters. A 2024 analysis found muscle growth tends to improve as sets end closer to failure, while strength gains looked similar across a wide range of effort (Robinson and colleagues, 2024). That's why "about two left" works. Close enough to get the benefit, far enough that your form stays good and you can train again in two days.

Light weights or heavy weights?

Both build muscle. A well known review found that lighter and heavier loads built similar amounts of muscle when people worked hard, but heavier loads built more strength (Schoenfeld and colleagues, 2017). The ACSM Position Stand says the same thing in its own way: heavier loads, around 80 percent of your max, for 2 to 3 sets, are best for strength, and about 10 or more hard sets per muscle per week helps muscle growth (ACSM, 2026).

You don't need to know your max. About 5 to 8 reps with two left is heavier work. About 12 to 15 reps with two left is lighter work. A good week has some of both.

How to add weight without getting hurt

Here's the simple plan we use with first timers at Detour.

Warm up first. A few easy sets of the same movement with a light weight.
Pick a starting weight you know you can do well. Start lighter than your pride wants. You can always go up.
Use the two reps left check on every working set.
Write it down. Weight, reps, and how it felt.
Add a little at a time. When every set hits the top of your rep range with two left, take the smallest jump next time. Small jumps add up fast over a Minnesota winter.

Muscle does the work, and a little more over time is how strong gets built.

How we coach it at The Detour Gym

In BUILT and every other class, a coach is on the floor helping you pick weights, watching your form, and telling you when it's time to go up. Every movement scales, so a beginner and a seasoned lifter can share the same workout at the right weight for each.

We're at 4921 W Frontage Rd N, about five minutes from Mayo Clinic. If you want to learn this in person, you can start with our strength classes, read how many days a week to work out, or check out strength training after 40 if that's where you are right now.

FAQ

How much weight should a beginner lift?

There isn't one right number for everyone. Pick a weight you can lift with clean form for the reps you're given, and stop when you have about two good reps left. If it feels too easy, go up a little next set. If form slips, go lighter. Write it down and build from there.

Is it bad to lift until failure?

It's not wrong, it's just usually not needed. The 2026 ACSM Position Stand found training to momentary failure did not consistently improve results for healthy adults. Stopping with one to three reps left gets most of the benefit and keeps your form and recovery in better shape.

Should I lift heavy or light to build muscle?

Both work. Research shows lighter and heavier weights build similar muscle when you push hard, while heavier weights build more strength. A mix of heavier sets of about 5 to 8 reps and lighter sets of about 12 to 15 reps, each ending with about two reps left, covers both goals well.

Where can I learn to lift safely in Rochester MN?

At The Detour Gym in Rochester, every class is coached, every movement scales, and a coach helps you pick the right weight. We're on West Frontage Road about five minutes from Mayo Clinic. A free intro is the easiest way to start: you meet a coach, talk goals, and try a few movements.

Ready to start?

You don't need a perfect plan. You need a weight that feels right today and someone to help you add a little over time. If you'd like a coach in your corner, come in for a free intro and we'll help you find your starting point.

Sources

Currier BS, et al. American College of Sports Medicine Position Stand. Resistance Training Prescription for Muscle Function, Hypertrophy, and Physical Performance in Healthy Adults: An Overview of Reviews. Medicine and Science in Sports and Exercise. 2026;58(4):851 to 872. doi.org/10.1249/mss.0000000000003897 and the ACSM summary at acsm.org
Zourdos MC, et al. Novel resistance training specific rating of perceived exertion scale measuring repetitions in reserve. Journal of Strength and Conditioning Research. 2016;30(1):267 to 275. PubMed
Halperin I, et al. Accuracy in predicting repetitions to task failure in resistance exercise: a scoping review and exploratory meta analysis. Sports Medicine. 2022;52:377 to 390. PubMed
Refalo MC, et al. Influence of resistance training proximity to failure on skeletal muscle hypertrophy: a systematic review with meta analysis. Sports Medicine. 2023;53(3):649 to 665. PubMed
Robinson ZP, et al. Exploring the dose response relationship between estimated resistance training proximity to failure, strength gain, and muscle hypertrophy: a series of meta regressions. Sports Medicine. 2024. PubMed
Schoenfeld BJ, et al. Strength and hypertrophy adaptations between low vs. high load resistance training: a systematic review and meta analysis. Journal of Strength and Conditioning Research. 2017;31(12):3508 to 3523. PubMed