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This question originally appeared on Quora. Answer by Darren Beattie.

Essentially, gaining weight comes down to a caloric surplus, which in today’s modern era generally isn’t all that difficult for a lot of people as there are plenty of food companies trying to get us to eat more. That’s why 2/3’s of the population in the western world are considered overweight and about half are considered ‘obese.’ All you have to do is eat more than you expend on a continual basis and you’ll gain weight.

But for some — a misnomer but namely the ‘ectomorph’, or more accurately the NEAT freak, which I’ll explain below — and usually a small percentage of the population — typically young skinny men, but sometimes young skinny girls too — gaining weight can actually be incredibly difficult.

To gain weight in ‘healthy’ way is even more difficult because we’d want to preferentially gain lean tissues, versus fat (adipose) tissue. Most people when they gain weight in a conventional way (the 2/3’s of the population) gain considerably more fat tissue than lean tissue, driving them up into medically concerning levels of body fat. Few gain just one or the other, even people who are formally considered overweight and obese individuals carry more muscle than their skinny counterparts. For people in this population, the weight they gain is largely assumed to be about 75% fat and 25% lean tissue. A better case scenario generally agreed upon is the reverse of 75% muscle to 25% fat, but that’s only possible via a training stimulation that causes increases in muscle, bone and other structural tissues (i.e. you will probably need to strength/resistance/weight train or exercise).

Generally <25% body fat for men and <32% body fat for women is considered ‘normal’ or ‘healthy.’ 2–4% is considered essential for men and 12–14% is considered essential for women, meaning you get health complications being ‘too skinny’ if you’re at these levels consistently, which few people are but sometimes eating disorders and other issues can result in an unhealthy level of very low body fat — which is though to be an endocrine organ these days, so it does a bunch of ‘healthy’ things when you have the right amounts.

If you want to use the less accurate method of BMI (Body Mass Index) you’re trying to be under 25 and above 18, but it doesn’t take into account what kind of mass you hold. You don’t have to be ripped or jacked at these levels and as a male at about 25 on the BMI scale but relatively low in body fat, BMI can be deceiving in some individuals, particularly young fit males. It’s easier to calculate and monitor though.

Either method can be a starting point and also provides a person with some flexibility. You’ve got a range of about 20% body fat to work with or 7 BMI points and still be considered ‘healthy.’ This gives you a lot of wiggle room for gaining weight (even if a lot of it isn’t lean tissue) in a ‘healthy’ manner, even if it seems small, it makes the fact that anyone deliberately trying to gain weight will also gain some fat a little less daunting, right?

Most people know the old adage of you have to eat 3500 kcal above your needs to gain a pound of fat. That’s certainly an option, so long as you don’t drift up beyond those numbers I mention above. It’s not exactly true (water weight is deceiving) but close enough. Yet few know that it appears to take a minimum least 400–700 kcal to build a pound of muscle (muscle is mostly water). Assuming that it didn’t require any calories to do the work of building these tissues.

However, that’s only the small end of the theoretical number, because muscle is more metabolically active and has a high energy cost of building (no one can gain pure muscle either, you get other tissues/fat along with it…), when you look at the practical amount of food a person needs to eat to get a pound of muscle it’s probably more like 2200–3500 kcal in excess; With a great deal of that energy being lost during the process of building.

Assuming you’re doing everything right and eating a ton of extra food, lets say you eat 100–500 extra kcal a day you’re lucky to get a pound of extra weight a week, unless of course you’re retaining water, which is a completely separate issue. Keep in mind that weight and lean weight are not the same things.

I just want to provide perspective because the reason people often struggle with weight gain is simply because they underestimate how many calories they burn in a day (Typically because of NEAT - Non-Exercise Activity Thermogenesis) and/or completely underestimate how much food they actually eat.

Hint: You probably have to eat a lot more than you think, or you’d gain the weight you’re trying to gain, right? You don’t have to go reaching for all these obtuse rationalizations like you have a fast metabolism or whatever, fundamentally you’re just not getting enough energy into your body to convert to fat/bone/muscle/etc…

In my answer to What should I eat for clean bulking? I laid out what’s called the Cunningham Equation.

I didn’t detail it in that answer but if you can’t really figure out your body fat, you should use the Navy Body Composition Tool, it’s not incredibly accurate but it’s better than guessing: US Navy Calculator - Body Fat Calculator

I’d also note that people who have difficulty gaining weight should assume their NEAT is higher than they believe it to be. Little things like fidgeting, bopping, bouncing, etc… can use up more energy than you think. The reason I said NEAT freak above is because research into Non Exercise Activity Thermogenesis, or the amount of energy people burn inadvertently via unconscious movement, reveals that some people burn more calories (ultimately have a ‘higher’ metabolism - Read my answer to How can I boost my metabolism? and also You Don’t Need to “Recover” or “Improve” Your Metabolism to understand this concept better), simply because they move without thinking, more than they assume.

If you're the kind of person that taps your feet all the time, or flips a pencil in your hand constantly, you might be a NEAT freak. This can increase your calorie needs significantly by several hundred calories a day or even more. That’s why I use the Cunningham equation; it accounts for some of the finer details of weight gain.

For those that don’t care about the details, you can find a calculator for this equation here: The Cunningham Equation Calculator

A simpler approach — though it doesn’t look nearly as impressive nor account for things like NEAT ) or TEF — is to multiply your weight in pounds by 18 kcal (fat is 9 kcal, protein is 4 and carbs are 4 simplistically speaking) or in kilograms by 39 kcal.

200 pound person needs a minimum of 3600 kcal a day (roughly) to gain weight.

A 100 kg person needs a minimum of 3900 kcal a day (roughly) to gain weight.

However, the Cunningham equation and this simplistic equation are still just general starting points or estimations. If you’re doing what these equations indicate and you’re not gaining weight, then you need to add more calories. This is actually a huge reason people don’t succeed, they assume they are doing something perfectly right based on only an estimate. You have to tweak as you go. There are a number of reasons for this:

  • Thermic Effect of the Food you Eat
  • NEAT
  • Training
  • Your GI tract could absorb better or worse
  • Genes

A big problem tends to be that too many people rely on the assumption that they only need 2000 kcal, which is incredibly general and obviously a young moderately active boy needs a lot more calories than a 40 year old soccer mom. Do not assume that your maintenance calories are that static.

Another conundrum being that as you gain weight, you actually need more energy and thus have to continue to eat more and more and more to keep gaining weight. So eating enough also gets progressively harder the more weight you gain. Keep that in mind, as you gain a pound here or there, you will actually need more to continue gaining more, it is not a static process.

In simplistic terms, you have to create a pretty big calorie surplus in order to gain weight, and to do so healthfully you want to minimize fat gain within the appropriate ranges.

A lot of people tend to do what bodybuilders might call ‘bulking’ followed by ‘cutting’ whereby you have generic attempts to gain weight, and then cut fat, rather than trying to build to exclusively muscle and other lean tissues preferentially over fat. Keeping in mind there are ways to do that.

The second biggest and most important thing that every single person trying to gain weight should do is weight train. AKA Resistance Training AKA Strength Training.

Lifting weights with a progressive hypertrophy oriented program is key to building muscle and ensuring that as much weight that you gain are lean tissues over fat tissues. It won’t be perfect typically, but it’s a very important consideration.

Some good programs for this aim that you can find online (there are plenty of others…):

Defranco’s Westside For Skinny Bastards

Chad Waterbury’s Huge in a Hurry

Nate Green’s Built For Show

Stuart McRobert’s Brawn or Beyond Brawn

Mike Mejia and John Berardi’s Scrawny to Brawny

The most important thing about any routine you are on, is that it’s something to can stick with. You might not get the best possible gains from everything, but the workout you do, will always be more effective than the one you don’t.

The third most important consideration is protein intake.

Protein is muscle sparring, and sure it’s the building block of muscle but like I said muscle is actually mostly water. Eating 0.8–1.5 grams per pound of bodyweight, or 1.8 grams - 3.3 grams of protein per kilogram of bodweight seems to generally be the sweet spot for muscle preserving and building.

A lot of people get on about macros (the ratios of macronutrients a person eats) as being vitally important here but outside of protein and achieving a calorie surplus I don’t really see any obvious advantage of getting more calories via fat or carbs, so I prefer to keep them pretty balanced.

Fundamentally (tl;dr) you need three things really:

  • A calorie surplus (typically in excess of 2200–3500 kcal a week or 18+ kcal per pound or 39+ kcal plus per kg) above your needs (which is a moving target, but I gave a bunch of different ways of finding a starting point)
  • A progressive weight lifting program that achieves metabolic stress, mechanical tension and muscle damage on a repetitive basis.
  • The necessary protein intake to mitigate muscle wasting while you build

There are other things to consider, but I’d focus here. Other things are secondary to these, things like:

  • Adequate Recovery
  • Adequate Sleep (aids in recovery and muscle builds while you sleep/rest)
  • Enough volume of training
  • Genes (some people build more muscle than others naturally and you can’t do anything about it)
  • Training experience
  • Eating experience (many people struggle to stuff this much food in, so you have to learn strategies like using liquid calories, eating more but less satiating foods and other things that people sometimes worry aren’t ‘healthy’ because they are the opposite strategies used for weight loss)
  • Cognitive Dissonance
  • Many other small influences that I’m missing that ultimately influence the first 3.

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