How Long Should You Stay in a Calorie Deficit?

You’re about to start a weight loss diet. 

You know you need to eat fewer calories than you burn. 

You know roughly how much you need to eat every day to lose weight. 

Maybe you even have a meal plan mapped out. 

But there’s still one question you haven’t been able to answer: how long should you stay in a calorie deficit? 

If you poke around online you’ll see different theories about the “ideal” length of time you should stay in a deficit before giving your body a break.

Some say you should diet no longer than 12 weeks. 

Others say you can diet as long as you want. 

And others say you should only stay in a deficit for a few weeks at a time before taking a diet break, giving your body and mind a breather, and then enduring another bout of dieting.

Who’s right? 

Well, here’s the truth of the matter:

How long you should spend in a calorie deficit boils down to how fast you can lose fat without losing muscle, and this depends on how much fat you have to lose and how lean you want to get. 

So, instead of giving you a one-size-fits-all answer like “12 weeks,” I’m going to show you how to decide exactly how long you should stay in a calorie deficit to reach your goal weight. 

Let’s start by looking at how fast you can lose fat without losing muscle. 

Table of Contents

+
• How Fast Can You Lose Weight without Losing Muscle? 
• So, How Long Should You Stay in a Calorie Deficit? 
• The Bottom Line on How Long You Should Diet

How Fast Can You Lose Weight without Losing Muscle? 

Before we look at how long you should stay in a calorie deficit, you first need to understand the real goal of a weight loss diet: 

Your goal isn’t to lose weight, but to lose fat and preserve as much muscle as possible. 

In other words, the goal is to improve your body composition.

That is, while you might be able to lose 10 pounds of body weight per week following a crash diet, you’re also going to lose a fair amount of muscle. This is counterproductive, and over time leads to the dreaded “skinny fat” look most of us want to avoid.

Why does crash dieting chew up muscle? 

There’s a limit to how much fat your body can metabolize (burn) per day before it starts breaking down greater and greater amounts of muscle for energy. 

This is something bodybuilders have known for decades, but thanks to a study conducted by Seymour Alpert, a professor at the University of New Mexico, we can now pinpoint how fast you can lose fat before you start losing appreciable amounts of muscle.

Alpert parsed through data from previous studies that involved people in a calorie deficit whose body composition was also measured. Using several different mathematical models, he then compared how much fat and muscle the people lost compared to the size of their calorie deficit.

Based on his analysis, he found what he believes to be a maximal threshold of calories that the body can extract from body fat per day: 30 calories per pound of body fat per day.

We can use this formula to find how many pounds of fat you can lose per week before you start losing muscle.

First, find your body fat percentage. I’ll use myself as an example.

I’m 175 pounds and based on the guidelines in this article, I’m about 10% body fat.

Next, find how much total body fat you have.

To find this number, multiply your weight by your body fat percentage.

175 x 10% = 17.5 total pounds of body fat.

Next, multiply your total fat mass in pounds by 30 (Alpert’s equation) to find how many calories of fat you can conceivably lose per day before you start to lose muscle.

17.5 x 30 = 525

That means I can realistically maintain a daily calorie deficit of about 525 calories per day without losing muscle.

To find how many pounds of fat I could realistically lose per week, I would multiply the daily calorie deficit by 7.

525 x 7 = 3,675

Then I would divide 3,675 by 3,500 (roughly the number of calories in a pound of body fat) to find how many pounds of body fat I could lose per week.

3,675 / 3,500 = 1.05 (which we can round to 1 pound of body fat).

This means that I could expect to lose at most 1 pound of body fat per week on average before I started losing significant amounts of muscle. And sure enough, this jives with most research on competitive bodybuilders: Once you’re around 10% body fat or so, you should aim to lose no more than 0.5 to 1% of your body weight per week.

In my case, losing one pound of body fat per week is about 0.6% of my total body weight per week—right where it should be to avoid losing muscle.  

So, that’s how fast a lean person can hope to lose fat without losing muscle. 

Now let’s run the numbers for someone with a much higher body fat percentage—someone who’s 250 pounds and 40% body fat. 

First, find how much total body fat they have by multiplying their weight by their body fat percentage.

250 x 40% = 100 total pounds of body fat.

Next, multiply their total fat mass in pounds by 30 (Alpert’s equation) to find how many calories of fat they can conceivably lose per day before they start to lose muscle.

100 x 30 = 3,000

That means they can realistically maintain a daily calorie deficit of about 3,000 calories per day without losing muscle.

To find how many pounds of fat they could realistically lose per week, you’d multiply the daily calorie deficit by 7.

3,000 x 7 = 21,000

Then you’d divide 21,000 by 3,500 (roughly the number of calories in a pound of body fat) to find how many pounds of body fat they could lose per week.

21,000 / 3,500 = 6 

This means that they could expect to lose at most 6 pounds of body fat per week before they started losing significant amounts of muscle. 

In other words, someone with about six times more body fat than me (in terms of total pounds of fat) could lose fat about six times faster before they started losing muscle. And sure enough, studies show this is possible at least for short periods of time. 

The best example of this is a study conducted by scientists at the University of Las Palmas de Gran Canaria. In this case, the researchers had 15 overweight men aged 18 to 55 follow an absolutely backbreaking diet and workout program for four days. 

The participants were limited to eating just 10% of their total daily energy expenditure per day. That is, they maintained a 90% calorie deficit, and all of those calories came from either whey protein or sucrose (sugar).

If that wasn’t bad enough, they also walked for 8 hours per day and did another 45 minutes of a hand-cranking machine, so that they maintained a 5,000 calorie deficit per day. 

After four days, they lost 4.4 pounds of pure body fat on average. Although they technically lost about 6.6 pounds of lean mass, most of this was due to losing water weight. After they were given a few days for their body’s hydration levels to return to normal, they found they only lost about two pounds of actual muscle. 

And if they’d been lifting weights and eating sufficient protein, chances are good they wouldn’t have lost much of any muscle. 

Of course, just because you can lose fat this fast doesn’t mean you should. Many people who attempt crash diets like this end up binging afterward and gaining back all of the weight they lost and then some. The people in this study were closely monitored by a team of researchers, but if you don’t have that level of accountability, your chances of falling off the wagon are much higher.

That said, you also don’t necessarily want to take things “slow and steady” when you have a lot of weight to lose, either. Research shows that the more weight people lose in the first few weeks of a diet, the more successfully they stick to their diet, the more weight they lose, and the more weight they keep off over the long-term.

(And if you’d like specific advice about how many calories, how much of each macronutrient, and which foods you should eat to reach your weight-loss goals, take the Legion Diet Quiz.)

So, circling back to the original question: how fast can you lose weight without losing muscle? 

Regardless of your body fat percentage, a good rule of thumb is to aim to lose around 0.5 to 1% of your body weight per week. 

It’s a good idea to base your weight loss goals on a percentage of your body weight, because this method works for both lean and overweight people.

Using the example from earlier, a man who’s 250 pounds would aim to lose 1.25 to 2.5 pounds per week—an aggressive but very achievable weight loss goal. This strikes a good balance between rapid short-term results and sustainable long-term progress.

Likewise, a leaner person like me would aim to lose around 0.9 to 1.8 pounds of body fat per week, which is about as fast as I could hope to lose fat without losing muscle according to Alpert’s analysis.

I also recommend you aim for slightly different rates of weight loss depending on your body fat percentage. If you’re >15% body fat as a man or >25% body fat as a woman, aim to lose 0.75 to 1% of your body weight per week on average. You can lose more than this without losing muscle, but 0.75 to 1% per week is enough to see rapid weight loss without running into the many negative side effects of crash dieting. 

If you’re <15% body fat as a man or <25% body fat as a woman, aim to lose 0.5 to 0.75% of your body weight per week on average. This reduces your chances of losing muscle while dieting. 

Summary: Aim to lose 0.5 to 1% of your body weight per week. If you’re under 15% (men) or 25% (women) body fat, aim to lose 0.5 to 0.75% of your body weight per week. If you’re over 15% (men) or 25% (women) body fat, aim to lose at least 0.75 to 1% of your body weight per week.

So, How Long Should You Stay in a Calorie Deficit? 

Now that you know roughly how much fat you should aim to lose per week, you can map out how long you need to stay in a calorie deficit to reach your goal. 

First, you need to decide how lean you want to get and how much fat you’ll need to lose to get there. 

Once again, there’s no one-size-fits-all answer here, but here are a few guidelines depending on what you want to do with your body: 

1. If you’re overweight and just want to significantly improve your health, your first milestone should be to lose at least 10% of your body weight. This is enough to improve your insulin sensitivity, reduce your blood glucose and blood pressure, and produce a number of other health benefits, and is very achievable in a relatively short amount of time.
2. If you’re overweight and want to significantly improve your health and your looks, aim to get down to at least 15% as a man or 25% body fat as a woman. This is enough to get almost all of the health benefits of being very lean, but is much easier to achieve and maintain than lower body fat percentages.
3. If you want to look like an athlete or even get a six-pack, aim to get down to at least 10 to 12% body fat as a man or 20 to 22% body fat as a woman. This is more difficult to achieve but still relatively easy to maintain if you know what you’re doing, and is lean enough to significantly boost your muscle definition, vascularity, and overall attractiveness.
4. If you want to get “shredded,” aim to get down to at least 6 to 8% body fat as a man or 18 to 20% body fat as a woman. This is difficult to achieve, more or less impossible to maintain for more than a few months at a time (at least without compromising your strength, quality of life, and energy levels), and really offers no health benefits that you can’t get at slightly higher body fat percentages. But, it does look cool. 🙂

After deciding how lean you want to get, you just need to determine how many pounds of fat you need to lose to reach your desired body fat percentage.

For example, let’s say you’re currently 220 pounds and 30% body fat, and you want to get down to a healthy 15% body fat (as a man). 

Thus, you need to lose about 15% body fat in total (30 – 15 = 15).

First, figure out how many pounds of fat you need to lose by multiplying your current body weight (220 pounds) by how many percentage points you need to lose (15%).

220 x 15% = 33 pounds

Thus, you need to lose 33 pounds to reach your goal of achieving 15% body fat. (This assumes you’re lifting weights and eating enough protein to preserve your muscle mass, so more or less all of the weight you lose is body fat.).

Next, figure out how many pounds of weight you should aim to lose per week.

Since you’re over 20% body fat, you’ll want to lose about 0.75 to 1% of your body weight per week. 

220 x 0.75% = 1.65 (which you can round to 1.5 pounds per week)

220 x 1% = 2.2 (which you can round to 2 pounds per week)

Why bother going to the trouble of making both of these calculations?

Because you’ll never lose weight exactly as fast as you expect. Holidays, weddings, parties, anniversaries, and flagging motivation and willpower can all slow things down, so it’s wise to think of weekly weight loss goals in terms of ranges rather than exact numbers. Around 1.5 to 2 pounds per week is a better goal than 1.75 pounds per week. 

Anyway, now that you know how many total pounds of weight you need to lose (33) and how many pounds of weight you should aim to lose per week (1.5 to 2), you just need to divide the first number by the second. 

First, figure out how long you’ll need to spend in a calorie deficit if you lose the minimum amount of weight per week (suboptimal progress): 

33 / 1.5 = 22

Then figure out how long you’ll need to spend in a calorie deficit if you lose the maximum amount of weight per week (optimal progress): 

33 / 2 = 16.5 (which we can round to 17)

Thus, you’d want to spend 17 to 22 weeks in a calorie deficit to reach your goal of 15% body fat without losing muscle. 

Alright, let’s recap the process one more time: 

1. Decide how lean you want to get.
2. Figure out how many total pounds of fat you’ll need to lose to get there.
3. Determine how much weight you should lose per week as a percentage of your body weight (0.5 to 1% of your body weight). Use this number to determine how many pounds of fat you should aim to lose per week.
4. Divide the total number of pounds you need to lose by your weekly rate of weight loss in pounds.
5. And viola, you’ll know how many weeks you should stay in a calorie deficit.  

Just to make things crystal clear, let’s repeat the process quickly using numbers for a leaner person. I’ll use myself as an example again:

I’m 175 pounds and 10% body fat.

First, I’d decide how lean I want to get. Let’s say I want to get down to 8% body fat.

Next, I’d figure out how many total pounds of fat I’d need to lose to get there.

I’d need to lose 2% of my total body weight to reach my goal (10% – 8% = 2%), which translates into 3.5 pounds (175 x 2% = 3.5). We can round this to 4 pounds.

Since I’m around 10% body fat, I’d want to lose about 0.5% of my body weight per week, which translates into 0.9 pounds per week (175 x 0.5% = ~0.9). We can round that to 1 pound per week.

Then, I’d divide the total number of pounds I needed to lose (4) by my weekly rate of weight loss in pounds (1) to determine how many weeks I should stay in a calorie deficit to reach my goal. 

4 / 1 = 4 weeks. 

Once again, though, this is assuming all of the weight I lose comes from body fat and that I lose weight exactly as fast as I planned, which more or less never happens. This is especially true when you’re trying to get down to very low body fat levels, where there’s very little margin for error and your body is fighting tooth and nail to hold onto every last blob of body fat.

Thus, if you’re lean and trying to get even leaner, I recommend you plan on staying in a calorie deficit 25% longer than the numbers say you need to on paper. For me, that would be about 5 weeks (4 x 125% = 5). 

That might seem like a long time “handicap,” but based on my experience with my own body and talking to natural bodybuilding coaches like Eric Helms, Lyle McDonald, and others, it’s a realistic target.

(And again, if you feel confused about how many calories, how much of each macronutrient, and which foods you should eat to reach your goals, take the Legion Diet Quiz to learn exactly what diet is right for you.)

The Bottom Line on How Long You Should Diet

How long you should spend in a calorie deficit depends on how fast you can lose fat without losing muscle, and this depends on how much fat you have to lose and how lean you want to get. 

Although you can lose weight extremely fast through crash dieting, your real goal should be to lose fat as quickly as possible without losing muscle. The best way to accomplish this is to maintain an aggressive, but not reckless, calorie deficit. 

Research shows there’s a limit to how fast your body can burn body fat before it starts to break down muscle for energy. 

The more body fat you have, the faster you can lose fat without losing muscle. As a corollary, the less body fat you have and the leaner you want to get, the slower you have to lose fat to avoid losing muscle. 

So, if you want to know how long you should spend in a calorie deficit, here’s the whole process from eggs to apples: 

1. Decide how many pounds you need to lose to reach your desired body fat percentage. 
2. Decide how many pounds of fat you can lose per week on average without losing muscle. For most people, this will be between 0.5 and 1% of their body weight per week.
3. Divide the total number of pounds you need to lose by your weekly rate of weight loss (in pounds), and this will tell you how many weeks you should spend in a calorie deficit to reach your goal. 

That’s it! 

What are your thoughts on how long you should stay in a calorie deficit? Have anything else you’d like to share? Leave your thoughts in the comments section below!