If you want to build muscle and don’t want to spend a ton of time to make it happen, you need to know how to bulk.

What is bulking? Put simply, it’s taking in more calories that you need per day, so that—with the right training program, of course—those excess calories go to build muscle, according to Minnesota-based exercise physiologist Mike T. Nelson, Ph.D., C.S.C.S.

The Math Behind Bulking

Commonly considered the most efficient way to grow muscle mass quickly, bulking is all about pure calorie math. If your calories consumed each day are equal to or less than the number of calories that you burn per day, your body just doesn’t have the fuel to build much muscle. It’s too busy using whatever calories it does have to keep you alive to bother with tacking on muscle. However, if you’re in a caloric surplus, meaning you are putting away more daily calories than you’re burning, then you have calories left over after each strength session to build, build, build.

But here’s the thing: As much as we’d like, the body won’t store 100 percent of those extra calories as rock-hard muscle. Some of them will stick around as fat—hence the whole “bulk” nomenclature. That’s why bulking first became popular as a way for bodybuilders to add mass during the off season when their body fat percentage didn’t have to be near non-existent. They could pack on some muscle, some fat, and then go into a “cut” phase, drastically cutting calories to lose weight right before competition, he says.

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With this bulk-cut protocol, they could step on stage with more muscles and less fat than they would be able to if they had tried to grow muscle and lose fat without swinging their calories

“Let’s say, over a course of a year spent bulking, you gain 24 pounds total, 12 from muscle and 12 from fat,” Nelson explains. “With an intelligent plan, you can probably lose those 12 pounds of fat in a couple of months without losing much muscle. So 14 or so months to increase muscle mass by 10 or 12 pounds without any overall increase in fat mass? That’s a lot faster than it would take if you tried to gain 10 to 12 pounds of muscle, but without letting yourself gain some fat along the way.”

The obvious downside here is that bulking does mean that you’ll carry some extra fat for a certain amount of time. And close examination of the practice does call into question the long-term effects of gaining and losing; one 2018 study from the Stanford University School of Medicine IDed 318 separate genes that work differently when the body makes only small fluctuations in fat mass.

“When bulking, guys have to be ok with being fatter than they are used to being for a while, and if they aren’t, that’s OK,” Nelson says. “If they want to stay leaner while gaining muscle, they can do it, it will just take longer.”

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Clean Bulking

Nelson says that longer, slower bulking periods—involving small caloric surpluses as opposed to turning into a human garbage disposal—tend to pack on far less fat than short, aggressive ones.

“Traditionally, bulking meant, ‘I’m just going to put on a lot of mass and eat shitty food and I’ll gain what I gain,” says Abbie Smith-Ryan, Ph.D., C.S.C.S., director of the Applied Physiology Laboratory at the University of North Carolina at Chapel Hill. “But you can be much more strategic and healthy about it.”

Commonly referred to as “clean bulks,” more strategic bulking protocols prioritize whole, minimally processed foods along with protein. That way, extra calories come packaged alongside vitamins and minerals, and the body has a steady supply of amino acids for muscle building.

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How to Bulk

When determining exactly how to bulk, you first need to consider how much muscle mass you want to gain, how much time you have to do it, and what ratio of muscle-to-fat gain is acceptable to you. “If I can gain a pound of muscle for every pound of fat gained, I’m ecstatic, but that might freak out someone else,” he says, noting that bulks often last anywhere from eight to 12 weeks to a year.

After figuring out how many calories someone can eat per day without gaining or losing (tracking your calorie intake for a week and then taking an average is one easy way to do it), bumping up daily calories by 250 to 500 each week, depending on how aggressive the bulker wants to be.

However, as Smith-Ryan notes, nutrition on its own won’t end in muscle mass gained. You also have to perform the right kind of training. She recommends that heavy strength training combined with traditional bodybuilding workouts (think: body-part split workouts with set-rep schemes of about 3X10) make up the majority of your training, with high-intensity interval training to help lower the amount of weight you gain from fat while bulking.

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“Research is beginning to show that interval training also stimulates muscle protein synthesis and can therefore help reduce body fat while increasing muscle as part of a bulking program,” Smith-Ryan says.

She recommends spending little, if any, time performing low-intensity cardio (like jogging and cycling) during a bulk since it will likely not encourage muscle growth, but it will downsize whatever caloric surplus you have that could go to building muscle. No calories wasted.