5 Ways to Tell If You’re Building Muscle or Just Gaining Fat

A large and growing proportion of the population is overweight – 43% according to the World Health Organization (1). Consequently, dietary and exercise advice is often aimed at those looking to lose their excess baggage.

However, in my experience as a veteran personal trainer, there are still plenty of people for whom losing weight would be a disaster. Instead, they want to gain weight.

Some of these folks want to gain weight because they are underweight, while others want to be stronger and more muscular. Some want to gain weight for sport, e.g., to move up a weight category.

Regardless of why you want to increase your weight, it’s generally best to gain muscle without accumulating too much extra body fat. This invariably requires a careful combination of diet and strength training.

Related: Gain Weight, Not Belly Fat: The Ultimate Guide to Healthy Bulking

But, how can you tell if your workout and nutrition program are building muscle, or if you are just getting fat? After all, the numbers on the scale will increase even if you are doing everything wrong.

In this article, I reveal five simple but effective ways to differentiate muscle from fat gain.

How to Tell If You Are Gaining Muscle or Fat

So, you’re training hard and eating well, and the reading on the scale is going up…

Great!

But, before you start celebrating, it’s important to remember that while increases in body weight can mean you are building muscle, they can also indicate that you are gaining body fat.

While it’s perfectly normal to gain a little fat as you build muscle, it’s usually best to avoid gaining too much. After all, a high level of body fat is unhealthy. But how can you tell the difference between muscle and fat gain?

Here are five simple but reliable indicators you can use to assess your progress.

1. You Are Gaining Weight Too Quickly

Everyone wants to achieve their fitness goals as quickly as possible. It seems that patience is a lost virtue, which helps to explain the popularity of 30-day fitness transformations and crash diets!

However, when it comes to building muscle, trying to do it too fast usually results in fat gain.

While results vary, it’s generally accepted that exercisers should expect to gain 0.5 to 2.0 pounds of muscle per month (2). While this might seem like depressingly slow progress, keep that up for a year or more and you’ll add a significant amount of lean muscle to your frame, transforming how you look, feel, and perform.

Related: How Much Muscle Can You Gain in A Month?

In contrast, it’s very easy to gain 5, 10, or even 20 pounds of fat in a month – just eat like it’s Thanksgiving every day and stop working out!

So, don’t try to rush the muscle-building process if you want to minimize fat gain. A small kilocalorie surplus combined with an intensive strength training program will ensure that most of the weight you gain is muscle and not fat. Increases in body weight of more than a pound or two a month strongly suggest that you are gaining more fat than muscle.

2. Your Waist is Growing Faster than your Biceps

Building muscle invariably means eating more so you have the energy and nutrients necessary to fuel the hypertrophic (muscle-building) process. You must also stimulate your muscles to get bigger and stronger by exposing them to intensive, progressive, consistent workouts.

Get this balance right, and muscle growth is all but guaranteed.

However, if you eat more food than your body needs, the excess kilocalories will be converted to and stored as body fat. The relationship between lean tissue and fat mass is known as your body composition and is often expressed as a body fat percentage.  

While you can determine your body fat percentage using skinfold calipers, bioelectrical impedance, hydrostatic (underwater) weighing, or MRI scans, there is a much easier way to tell if you are gaining more fat than muscle: your waist measurement.

Related: Average Waist Size for Women and Men

The waist is a common fat storage site. Most people notice if they are gaining or losing weight because their waist measurement changes. For example, you might notice that your clothes are starting to feel tighter or looser, or that you need to use a different hole on your belt to hold up your pants.

In contrast, while there are muscles around the waist, they tend not to thicken and grow as much as things like your biceps, quads, or pecs. This is because of their anatomical structure and the way that most people train this region.

So, if your waist is growing faster than your arms, legs, or chest, there is a good chance that you are gaining more fat than muscle. Reduce your food intake to prevent further fat gain.

3. Bodyweight Exercises Are Getting Harder

Building muscle invariably means you’re going to get stronger. After all, there is an established relationship between the cross-sectional area and strength of any given muscle (3).

In simpler terms, bigger muscles are stronger muscles.

As a result, your gym weights should go up as your body weight increases. This is especially true for the big compound lifts, e.g., squats, leg press, bench press, lat pulldown, etc., which are all reliable indicators of strength.

However, there is a type of training that tends to get harder when you gain fat – bodyweight exercises. Popular bodyweight exercises include push-ups, pull-ups, chin-ups, inverted rows, and pistol squats.

Gaining fat doesn’t tend to impair strength in exercises like squats and deadlifts. In fact, many of the strongest lifters are as fat as they are muscular. However, in bodyweight exercises like pull-ups, excess body fat is nothing more than dead weight that makes overcoming gravity much harder.

So, if you are gaining weight, adding plates to your squats and bench presses, but your ability to do pull-ups, push-ups, inverted rows, etc., is decreasing, there is a good chance that you are gaining more fat and less muscle than is ideal.

4. You Are Losing Muscle Definition

You don’t need to know your exact body fat percentage to tell if you are building muscle or gaining fat. Instead, all you need is the camera on your phone or your mirror.

Increases in body fat will have a significant impact on muscle definition, which is the ability to see the shape and outline of your skeletal muscles. For example, when you are lean, you will probably be able to see the separation in your quads, pecs, and abdominals. You may even have six-pack abs.

However, if you start to gain fat, the outline of those muscles will become less visible and may vanish altogether.

So, take periodic full-body photos to see how your definition changes over the coming weeks. If your muscles become less visible, it’s safe to say that your body fat percentage is increasing, even if your muscles are getting bigger.

5. Your Food Diary Contains Too Many Cheat Meals

Nutrition plays a critical role in the muscle-building process. After all, your diet provides the energy and nutrients your muscles need for growth and repair. As a result, I encourage all my weight loss AND weight gain clients to keep a detailed food diary. This allows me to see what they are eating and whether it supports their fitness journey.

In many cases, what I see in the pages of their diaries will tell me if they are gaining muscle or fat.

A lot of people trying to build muscle think that they can eat whatever they want and still achieve great results. This is sometimes known as “dirty bulking.” And while it’s true that you need to eat more than usual to build muscle and gain weight, this is not a license to treat every meal like a cheat meal.

Building muscle without gaining much body fat requires a small energy surplus – typically 300-500 extra kilocalories per day. But, if you are filling up on takeouts, candy, soda, and other junk foods, your energy surplus probably runs into the thousands.

It goes without saying that consuming a thousand or more extra kilocalories a day will lead to significant fat gain even as you increase muscle size.

So, instead of abandoning all plans to eat healthily as you try to build muscle, I suggest following a healthier, more balanced diet that provides just a small energy surplus. That way you will give your muscles everything they need to grow without the extra kilocalories that inevitably lead to fat gain.

You can still enjoy the occasional cheat meal, but junk food should not be a mainstay of your diet, even if you are trying to gain weight.

Closing Thoughts

Gaining muscle should be a relatively straightforward process. While your genetics, age, and gender will determine how much muscle you can build, a solid workout plan and appropriate diet should be all you need to produce noticeable results.

Unfortunately, a lot of exercisers undermine their progress by making easy-to-avoid training or nutritional mistakes. They see their body weight increasing but fail to realize they are gaining more fat than muscle mass.

The good news is that you don’t need a body composition analysis to differentiate between muscle and fat gain. In fact, I rely almost exclusively on the five signs outlined in this article to determine if my clients are building muscle or just getting fat.

Use these methods to monitor your progress and avoid the fat-gain trap. After all, above and beyond some essential body fat, having too much adipose tissue does nothing for your performance or appearance and could actually harm your health.

Related: 5 Ways to Tell If You’re Losing Weight Healthily or Unhealthily

Questions or comments? Please drop me a line below and get back to you ASAP!

References:

1. World Health Organization (2024): Obesity and Overweight. Retrieved from https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight
2. Arden NK, Spector TD. Genetic influences on muscle strength, lean body mass, and bone mineral density: a twin study. J Bone Miner Res. 1997 Dec;12(12):2076-81. Doi: 10.1359/jbmr.1997.12.12.2076. PMID: 9421240.
3. Maughan RJ, Watson JS, Weir J. Muscle strength and cross-sectional area in man: a comparison of strength-trained and untrained subjects. Br J Sports Med. 1984 Sep;18(3):149-57. doi: 10.1136/bjsm.18.3.149. PMID: 6487941; PMCID: PMC1859378.

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