You joined the gym. You bought new workout clothes. You even created the perfect playlist to help energize your workout. It’s time to get fit! With motivation welling up inside, you step out onto the gym floor and... have no idea what you’re going to do.

When you’re unsure about what to do in the gym, it’s easy to default to simply doing cardio since there’s not much of a learning curve. Learn which buttons on your machine adjust settings like speed, resistance and incline, then stay on for some reasonable amount of time.

Resistance training is different. There’s usually more to learn when you start using free weights and machines. You’ve got to pick which exercises you’re going to do, decide the order in which you’ll perform them, determine how many reps and sets you’ll do and, of course, figure out how much weight to use. Here are some tips on choosing an amount of weight that will be just right for you.

The general recommendation has long been, “just pick a weight that allows you to perform a target number of reps.” But, recent research suggests the “muscle-growth rep range” (6-15 reps) is more of a gray area than previously thought. Muscles can be made to grow as long as they are required to work hard over time. Growth is just triggered differently based on the kind of training one performs (bodybuilding, powerlifting, etc). But generally speaking, you’ll get strong lifting heavy weights for fewer reps. You’ll gain muscular endurance if you lift lighter weights for higher reps. And muscles will grow as long as you’re working hard enough.

The Right Weight Will Require the Right Amount of Effort

Muscle growth can be triggered by training at virtually any number of reps as sets are taken to a reasonable level of fatigue. What’s a reasonable level of fatigue? A good rule of thumb is to finish a set with just 1-3 reps left “in the tank.” In other words, you want to be within 1-3 reps of failure (not being able to complete another rep with good technique). As soon as you feel like you can’t perform 1-3 more good reps, the set is over. This approach can help you work with sufficient intensity no matter what weight you use while keeping you safe in the long run.

 So... How Much Weight Should I Use?

You will likely experience best results over time if you choose a weight that causes you to reach a reasonable level of fatigue (1-3 reps shy of failure) by the time you’ve performed your target number of reps. So, if you’re aiming to lift some weight for 10 reps and you can only get five before your technique goes to pieces, it’s likely too heavy. But if you’re aiming for 10 reps and you get there feeling like you could easily do five more, the weight is probably a bit on the light side.

Since there’s nothing magical about any given rep range or amount of weight, that means only one thing matters – and that’s your effort. Effort tells you everything you need to know about your exercise intensity. So working “hard enough” is a factor of how you feel while you’re doing the work.

So, let’s review what we now know about weights and rep ranges. Heavy weights lifted for fewer reps help develop more strength. Lighter weights are better suited for performing more reps which increases muscular endurance. So, if you’re looking for overall improvements in muscular tone, strength and endurance, it’s probably a good idea to work across a variety of weights and rep ranges while making sure your workouts are challenging but not impossible.