3,600 words. Reading time: ~13 minutes.

3 calculators included:
Training status calculator
Bulking calculator
Strength progress calculator

Important definitions:
p-ratio: Proportion of weight gain that is muscle vs. fat

Notable contributions of Nieck Detillon, MSc Human Movement Sciences

The never-ending mental battle of deciding whether to cut or to bulk. The sleeve of your shirt is not as tight as it should be: some extra muscle would be great, so let’s bulk.

But does your gut agree?

Don’t you have to slim down first?

Am I too “skinny-fat”?

If so, which fat percentage to aim for?

Beauty is – of course – in the eye of the beholder, and I encourage you to hold your personal standards when it comes to an ideal, ‘aesthetically pleasing’, fat percentage. For some this means seeing the outline of some abdominals, for others this means being able to count the veins on their chest.

However, some fat percentages have actual physiological advantages for bulking. In this article I’ll deep-dive the scientific literature to find out which body fat level is ideal to maximize muscle gain and minimize fat gain on a (lean) bulk.

First I’ll debunk the insulin sensitivity myth that states being lean increases insulin sensitivity and therefore lets you gain more lean muscle. Then I’ll look at the factors that do matter for a successful ‘lean’ bulk, chronic inflammation and testosterone / estrogen levels, and how they depend on body fat percentage.

I’ll end with some practical implications and a calculator that estimates how much muscle and fat you’re going to gain during a hypothetical bulk, based on your starting body fat percentage and how advanced you are.



How is this article different?

So what’s different about this article and the others on this topic? Glad you asked! Most of them refer to studies that investigated men between 20 and 35% body fat to come to their conclusions, like this study, this study, and this study.

Since most of my readers probably are at 20% body fat or below, looking at these studies is like studying how horses respond to weight training and use these insights to give humans advice. Different study subjects, different outcomes.


In this article I want to look at what happens in the more relevant range of 4-25% body fat instead of 20-35%.

Note: at the same level of leanness, a woman’s body fat percentage is generally 10% higher than a man’s. So a woman at 15% body fat is about as lean as a man at 5% body fat. Therefore, the female range we’ll investigate is 14-35% body fat. To get an idea of what these body fat levels look like, you can look at the body fat charts.

[Body fat charts dropdown]

Your body fat level affects multiple physiological factors, which have an impact on the amount of muscle and fat you’ll gain during a bulk. Let’s tackle them one by one.

Another way in which this article is different is the way I present the evidence: hover on laptop or tap on mobile to see a supporting citation or research image behind the claim.


Leading with the Conclusion

Before starting a bulk, it’s important to assess where you’re starting from. In particular, your starting body fat percentage can greatly influence the proportion of muscle and fat you will gain. Researchers call this the p-ratio.

Body fat is thought to influence the p-ratio in 3 ways: insulin sensitivity, chronic inflammation, and anabolic hormone (testosterone and estrogen) levels.

1. Insulin sensitivity: Contrary to popular belief, no data shows getting any leaner than 20% body fat (30% female) further improves your insulin sensitivity. And even if it did, higher insulin sensitivity probably doesn’t improve p-ratio.

2. Chronic inflammation: However, chronic inflammation is in fact shown to worsen p-ratio (less muscle and more fat gained). But likewise, there aren’t data showing chronic inflammation further improves if you get any leaner than 20% body fat.

3. Anabolic hormones: The only way a lower body fat percentage is shown to help p-ratio is through improved anabolic hormone levels. 8 to 12 percent body fat in men and 22 to 28 percent in women is when testosterone and estrogen are highest, respectively. Research indicates p-ratio is also highest at these body fat levels.

However, going too low in body fat can be detrimental to your testosterone and estrogen levels, and therefore your muscle gains during a bulk. If you see people online effectively building muscle at 5-7% body fat, it’s probably because they’re on anabolic steroids.

If you want to estimate what impact the start body fat level has on how much muscle and fat you gain during a bulk, you can with the Bulking calculator that is according to how advanced you are (calculators for this can also be found in this article).

This also means that if you’re skinny fat, it’s probably still best to cut first, as when you would start bulking, you’ll be gaining mostly more fat and not muscle

To assess whether your bulk is still productive, you can use the last calculator in the article.

But before you dive-bomb into a typical cut; there is an alternative: losing fat and gaining muscle at the same time. It’s not an easy endeavor, but it’s definitely possible. Even new clients who have been training for years can often make a shift towards this sought-after combination of burning the fat and piling (or at least maintaining) the brawn. I’ll talk about this more in a future article where I’ll show actual male client results, along with the methods I employed.


1. Body fat percentage and insulin sensitivity: debunking the myth

Jump back to Conclusion

Insulin sensitivity describes how sensitive certain tissues – like muscle, liver and fat – are to the insulin signal that tells them to ‘open their doors’ and let blood glucose enter their cells. Basically, being very sensitive to insulin is the reverse of having diabetes type 2, when you’re resistant to insulin.

You often hear that being very lean increases insulin sensitivity, which would then enable you to build more muscle and less fat during a bulk.

Indeed, when you lean down from obese to overweight, your insulin sensitivity is bound to go up (Tap). But remember the study subject analogy about horses vs. humans: this is not necessarily true for non-overweight people, as your insulin sensitivity doesn’t seem to further improve when you get leaner than about 20% body fat (30% for females) (Tap).

This is probably because visceral fat – located between your organs instead of under the skin – often starts to accumulate when body fat exceeds 20% in males. And gaining visceral fat is closely related to lower insulin sensitivity (Tap) [2]. But visceral fat has more shitty side-effects, as I will discuss soon.

So 20% body fat (~30% for women) seems to be the ‘magical’ cut-off point. If you get fatter than that you’ll likely start to accumulate more visceral fat and become less sensitive to insulin.

Since many serious trainees (and I suppose you, the reader) are around or leaner than 20% body fat, they probably don’t carry that much visceral fat around, so shouldn’t worry about it too much.

But let’s imagine that insulin sensitivity does improve all to way until you can count your ribs. Will it help you gain more muscle and less fat during a bulk?

Insulin sensitivity doesn’t directly affect fat loss or – more important during a bulk – fat gain.

When it comes to muscle gains, higher insulin sensitivity does not contribute to a better muscle-to-fat gaining ratio during a bulk, (Tap) not even in identical twins. (Tap)

Take-home messages:

  • When you’re more sensitive to insulin, your body more easily stores glucose from the blood into liver, muscle, and fat cells.
  • When your insulin sensitivity is low, you’re ‘insulin resistant’.
  • If you’re under 20% body fat (30% as woman) your insulin sensitivity is likely optimal.
  • Above 20% body fat – as visceral fat starts to accumulate – insulin sensitivity decreases.
  • Even if insulin sensitivity would get better if you approach ~10% body fat, it does nothing for your lean mass gains during a subsequent bulk.



2. Chronic inflammation kills muscle growth: relevant to us?

Jump back to Conclusion

Now that we busted the insulin sensitivity myth, let’s get to the next prey and see whether it improves with lower body fat and increases lean gains during a bulk: chronic inflammation.

Research shows that, again, when you’re overweight, losing weight will probably lower your chronic inflammation levels (Tap) [2], meaning your body spends less time producing inflammatory reactions to harmful things like infections, injuries and toxins.

But remember the humans vs. horses analogy? To date, I have found no scientific data about chronic inflammation in leaner individuals, so we can’t say with certainty whether someone with 12% body fat has lower inflammation levels – and can therefore grow more muscle – than someone with 20% body fat.

To further confirm this, by releasing more of the inflammatory Interleukin-6 (IL-6) visceral fat in particular enhances chronic inflammation, which is in turn related to muscle loss (Tap) [2].

There is a possible explanation for this: when chronic inflammatory levels are lower, the acute inflammatory signal for muscle repair and growth after a workout is much clearer, because there’s less chronic inflammation noise to wash it out. To pose an analogy: you can clearly see a water drop create a ripple in a still pond (low chronic inflammation), this is less clear when it’s raining and the water is not still (high chronic inflammation). Similarly, your body can more clearly see the signal of a stimulated muscle that needs repair in a body with low inflammation.

Yeah I love analogies. Let’s visualize it while we’re at it.

And research supports this rather cool analogy:

Adding insult to injury: higher chronic inflammation levels are also linked to lower testosterone levels. (Tap)

So as a man you probably don’t want to bulk at body fat percentages over 20% (30% women). Visceral fat likely increases, together with chronic inflammation, which in turn kills muscle growth.

Take-home messages

  • Unlike insulin sensitivity, chronic inflammation does matter when it comes to lean bulking: it decreases muscle growth.
  • Chronic inflammation probably does this by ‘washing out’ the clearness of the inflammatory signal to repair muscle after a workout.
  • From 20% body fat (30% in women), once visceral fat start accumulating, chronic inflammation (IL-6 levels) starts to increase.
  • There is no data to say chronic inflammation is different at 8% or 12% or 16% body fat, so while it hurts muscle gains in the overweight, it shouldn’t concern leaner people much.




3. Anabolic hormone levels: the crucial factor

Jump back to Conclusion

Anabolic (muscle building) hormones. Finally. This is where things get interesting for leaner people.

Body fat percentage is related to hormone levels. Testosterone and growth hormone (Tap) in particular are higher when a man’s body fat gets closer to about 10%.


And more testosterone will likely make you store less body fat and gain more lean mass while bulking.

For example, in identical twins those with higher levels of anabolic hormones (like testosterone) – gained proportionally more muscle and less fat on a daily 840 calorie surplus (Tap).

Other research shows testosterone levels are clearly related to how much muscle you gain on a bulk (Tap).

Women, I didn’t forget about you. Since estrogen helps muscle building in women (Tap) [2], we’d want to have it as high as possible during a bulk (from a muscle building and not necessarily a health perspective).

Research indicates 24-30% body fat is the sweet range for estrogen (Tap), especially during a bulk. (Tap)

Since estrogen peaks around 25-27% body fat during a caloric deficit (a cut) (Tap), women may want to stop their bulk before they reach this body fat level. This guarantees maximum muscle retention during the following cut.


Take-home messages

  • The only thing body fat levels impact up to lower body fat levels is testosterone.
  • As your bulking starting point is within 9-12% in men or 20-24% in women, you will probably gain more muscle and less fat over the course of the bulk.
  • And you can keep on ‘lean’ bulking like this, but only up to a point, which is about 13-17% for men and 25-27% for women. Thereafter more of your calorie surplus will likely end up in your fat cells, because of the 2 factors we discussed: higher chronic inflammation and lower testosterone (or estrogen) levels.




How low in body fat can you go?

It seems that the leaner you get, the better you’re able to gain muscle and not fat. However, don’t go overboard and set your body fat standards at the Instagram level, though. Shit will hit the fan. An Instagram profile showing more skin than clothing is often a collection of snapshots of people’s best moments. If you let those dictate your leanness standards, you’re basically comparing yourself to:

  • Someone who is probably on anabolic drugs (which saves the hormones).
  • Someone who shot pictures of the best 0.27% of their year (their photoshoot day).
  • Someone who selected the best photos out of hundreds, that were additionally touched up in photoshop.
  • Someone who has great genetics (naturally high testosterone/estrogen levels and great bone structure), about 0.01% of the population.

And if they’re not on drugs, their hormones are probably down the gutter when they try to maintain a lean physique year-round. You can’t go that low without some serious consequences.

Research shows when competitors near contest-lean levels and people start confusing their triceps muscles with hoof irons, testosterone levels plummet lower than your liking for squat rack curlers (Tap) [2]. In the same situation, women’s estrogen levels get wiped away, and sometimes their menstrual cycle with it (Tap).

Their hormone levels quickly recovered after competition, when they started eating more. But it’s probably not their increased calorie intake but mostly their regained fat that made this recovery possible (Tap).

(tap for female version)


There are some ways to alleviate these hormone valleys, such as increasing fat intake, but it’s often not enough to completely prevent them.

Either way you’re screwed if you adhere to the Instagram levels of leanness: you’re comparing yourself to either the shredded-lean but hormone-debilitated person, or someone who takes exogenous hormones.

Therefore I also recommend unfollowing any people you don’t personally know that set ridiculous standards, unless they post up cool infographics like I do. And that’s not just an intuitive suggestion.

This research-based image gives you an idea of the impact of social media standards (Tap) on your well-being.

Take-home messages

  • Going too low in body fat percentage is going to hurt your anabolic hormone levels (testosterone and estrogen).
  • Whenever you see someone online maintaining under 8-10% body fat levels year-round while growing muscle mass, it’s very likely they are on some sort of ‘anabolic assistance’.
  • Therefore you should never compare yourself with them. Even better: clear your Instagram of their profiles if you’re serious about personal well-being.


Yep, that was quite the de-tour: back to the ideal fat percentage to bulk.



A more realistic body fat ideal

So far we’ve seen that scientific research leads us to a more realistic and optimal body fat percentage standard for muscle growth. Between 8-12% seems to be ideal for men, while 18-24% seems ideal for women. These correspond to higher testosterone and estrogen, and low chronic inflammation, but also to being more fertile (Tap).

Other research demonstrates that as your body fat percentage rises during a bulk, you’ll likely gain increasingly more fat and less muscle (Tap). Further, a major review of overfeeding experiments discovered that people with higher initial fat percentage gained less muscle and more fat during their ‘bulk’ (Tap).

(note the muscle-to-fat gain rates are just an example and not based on real data; see the calculator further down to get actual estimates of muscle-to-fat gain)

Scientific researchers express the muscle-to-fat ratio in the image with the “p-ratio”, short for “partitioning ratio”. In a sense, it describes how the calories you ingest get ‘divided’ (partitioned) over muscle and fat mass.

So if you gain 10 lbs of bodyweight and 6 lbs of this is muscle, your p-ratio is 60%

If you want to calculate the proportion of muscle-to-fat gains for your own hypothetical bulk using this p-ratio, use the calculator below. You can try out different starting body fat percentages to see how it affects the results.

However, to use the bulking calculator properly you first need to find out how advanced you are (beginner, intermediate, advanced). You can find the calculator for that in this different tab. It’s best to take the ‘average’ training status you get over the various exercises.

If needed, you can estimate your calorie maintenance (when you’re not bulking or cutting, as then it will adjust up or down) with this calculator.

The calculator is based on:

Extra note: this calculator assumes you’re natural, meaning not on anabolic steroids or other anabolic drugs. For people on steroids p-ratios can easily stay elevated at 100% or become greater than 100%, which indicates they gain muscle and lose fat on a bulk, which is physiologically not impossible.

Tap here for a full table of the p-ratios used for the different body fat percentages.

Without further ado, let’s get to the calculators.



Female bulking calculators


Male bulking calculators


Note that the calorie surpluses are net calorie surpluses. When you start a bulk your calorie expenditure will probably increase, so a net 750 kcal surplus at the start may degrade to a net 450 kcal surplus when you expend more calories per day. Therefore you may have to keep increasing your caloric intake to stay at a 750 kcal net surplus. Read more in this post.

If you need help with a solid bulking meal plan; I’m offering an individualized meal plan service that’s rooted in a science-based and macronutrient-precise algorithm.


Take-home messages

  • Research shows the ideal fat percentage for men and women to gain muscle is 8-12% and 18-24%, respectively.
  • The p-ratio describes what proportion of body weight gain is muscle. Thereby we also know how much fat was gained.
  • You can estimate the muscle and fat gains of a hypothetical bulk with the calculator above.



Indicators of a bulk gone bad

It’s highly individual when a bulk goes from leanmassing into fat-festing. For some it happens at 11% body fat, for others it happens at 14% body fat.There are 2 assessments you can do to determine whether most of your weight gain will still be muscle.

1. When, as a man, you actually start to get a ‘pot belly’, as in, you can safely place a beer on your gut, this indicates significant visceral fat gain, and therefore high odds of chronic inflammation and bad potential for further muscle growth.

2. When your skinfold caliper measurements keep increasing (you’re getting fatter), but your strength on big exercises hardly increases, muscle gains may have stopped, as it’s probably impossible to gain muscle without gaining strength (Tap).

Men can perform the first assessment by trying to locate their little man while urinating. The second one needs more precise scrutiny. What is quick strength progress and what is slow progress?

First use this flowchart to confirm that your training volume and recovery capacity are not the cause of a lack of progress. If it’s not, then you may not be progressing because your body fat levels are too low or too high, which could have something to do with your testosterone, estrogen, or inflammation levels, and therefore a sign to stop your bulk.

The following calculator informs you how much strength you can expect to gain on an effective bulk in some compound movements.

The calculations are based on muscle gaining estimates from myself, Menno Henselmans, Greg Nuckols, Lyle McDonald, Alan Aragon, and scientific research [1, 2], which I translated to arguably the most accurate indicator of muscle gains: strength gains on the big exercises.

If you want to see the exact calculations to come to the numbers used in the calculators below, see this Sheets document. You can email any suggestions for improvement to info@stijnvanwilligen.com

If you think the strength progress from the calculator seems rather slow, then just take the rate per week and multiply it by 20 to see how much strength you would gain in ~5 months! Over longer stretches of time, sustained little strength gains add up to great accomplishments.


Bench press strength progress calculator

Squat strength progress calculator

(Romanian) Deadlift strength progress calculator

Hip thrust strength progress calculator

If your strength progress is lower than the expected, and your recovery capacity (flow diagram) is on point, it may be time for a cut.

If you’re still progressing at a moderate or great pace, then you’re highly probably still gaining muscle and your bulk is still effective.

Take-home messages

  • Because some people still have great anabolic hormones at higher or lower fat percentages, we want to look at some other indicators of a ‘bulk gone wild’.
  • Visually, having a pot belly indicates high visceral fat levels, which in turn indicates high chronic inflammation, which can hurt your muscle gains.
  • Another great way to assess whether you’re still gaining a good amount of muscle and not too much fat is the combination of a skinfold caliper and strength progress in the bigger exercises (see calculator).
  • To effectively use the calculator, make sure your recovery capacity is where it should be (see flow diagram).