Body composition
What should my body fat be?
The scale cannot tell you whether you are losing fat or muscle. Set a target body fat percentage, see the goal weight it implies, and find out how much lean mass you need to protect to get there.
To reach ~21% while keeping your 115 lb of lean mass, pair your GLP-1 with about 145 g of protein a day and resistance training. Losing fat — not muscle — is what actually changes how you look and feel.
Body fat % is estimated with the Deurenberg formula when no measured value is entered. The timeline assumes roughly 0.75% of body weight lost per week with lean mass preserved — a best case, not a guarantee. A DEXA or InBody scan is far more accurate than any formula.
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How this calculator works
Weight on its own is a poor target. Two people at the same weight can look and feel completely different depending on how much of that weight is fat and how much is muscle. This calculator works in body composition instead.
- Your current body fat comes from a measured value if you have one, or a Deurenberg estimate from BMI, age and sex if you don't.
- Lean mass is your weight minus fat mass — the part you want to keep.
- Goal weight is your lean mass divided by (1 − your target body fat %), assuming lean mass is preserved.
- Maintenance calories use Mifflin-St Jeor multiplied by your activity factor.
- The protein target is roughly 1 g per pound of goal body weight, the level most associated with muscle retention during weight loss.
The goal weight assumes you keep every pound of lean mass. That is the best case, and it is exactly why protein and resistance training matter on a GLP-1 — without them, a meaningful share of what you lose is muscle, and your goal weight moves against you.
Body fat ranges, explained
| Category | Men | Women | What it means |
|---|---|---|---|
| Essential | 2–5% | 10–13% | The minimum needed for basic physiological function |
| Athletic | 6–13% | 14–20% | Competitive athlete territory; demanding to maintain |
| Fitness | 14–17% | 21–24% | Lean and visibly fit; realistic for committed training |
| Average | 18–24% | 25–31% | Typical range for the general population |
| High | 25%+ | 32%+ | Associated with higher metabolic risk |
Why lean mass is the number to watch
Rapid weight loss of any kind costs some lean mass, and GLP-1 medications make rapid loss common because appetite drops so sharply. Muscle is what keeps your metabolic rate up, your strength intact and your body looking the way you hoped it would at your goal weight. Losing it makes maintenance harder later and is a major reason weight comes back after stopping.
The two levers that actually work are protein and resistance training. Hitting your protein target even on days when you have no appetite, and lifting two or three times a week, is what turns weight loss into fat loss.
Read the full guide to maintaining muscleGuides on how your body changes
The questions people actually ask once the weight starts coming off — about muscle, skin and the way their face changes.
Watch fat and muscle move, week by week
You have a goal body fat percentage. Calqulate Vitals tracks whether you are actually moving toward it — how much of each pound lost was fat, when lean mass starts slipping, and how far you have left to go.
- Fat vs. muscle on every weigh-in
- Lean-mass loss flagged early
- Protein and goal weight in one place
- Plateau risk before it lands (Premium)
Related calculators and trackers
Sources and references
This page is built from public clinical guidance and peer-reviewed research. Always confirm decisions with a licensed clinician.
- 1.American Council on Exercise — body fat percentage categories
- 2.Deurenberg P, et al. Body mass index as a measure of body fatness. Br J Nutr.
- 3.Mifflin MD, et al. A new predictive equation for resting energy expenditure. Am J Clin Nutr.
- 4.National Institute of Diabetes and Digestive and Kidney Diseases (NIH)
Frequently Asked Questions
Common questions and answers about this calculator.
What is a healthy body fat percentage?
For men, roughly 10–20% is generally considered healthy, and for women roughly 18–28%. The American Council on Exercise splits it further into essential, athletic, fitness, average and high ranges, which is what the spectrum on this page shows. These are population guidelines, not personal targets — your clinician can help you set one that fits your health history.
How is my goal weight calculated?
It works backwards from your lean mass. Your lean mass is your weight minus your fat mass, and the calculator assumes you keep that lean mass while losing fat. Goal weight is then lean mass divided by (1 − target body fat %). That is why protecting muscle matters so much: if you lose lean mass along the way, your goal weight changes.
How accurate is the body fat estimate?
When you do not enter a measured value, the calculator estimates body fat from your BMI, age and sex using the Deurenberg formula. That is a reasonable population-level estimate but it can be off by several points for any one person, especially if you are very muscular or very lean. A DEXA or InBody scan is far more accurate — enter that number instead if you have one.
Why does the calculator push protein so hard?
Because on a GLP-1 your appetite drops sharply, and low intake is the main driver of muscle loss during rapid weight loss. Roughly 1 g of protein per pound of goal body weight, paired with resistance training, is the most reliable way to make sure the weight you lose is fat rather than muscle.
Is the timeline realistic?
It assumes about 0.75% of body weight lost per week, which is a sustainable rate that supports muscle retention. It is a best case that also assumes lean mass is fully preserved. Real progress is rarely linear — plateaus, dose changes and life all shift it, so treat the number as a planning aid rather than a deadline.
Should I aim for the athletic range?
Not necessarily. Athletic body fat levels take significant training and dietary discipline to reach and maintain, and very low body fat carries its own risks, particularly for women. For most people on a GLP-1, the healthy or fit range is a better and far more maintainable target.
Medical Disclaimer: This calculator provides general, educational estimates of body composition and is not a clinical body scan, a diagnosis, or medical advice. Body fat formulas are population estimates and can be inaccurate for any individual. Set weight and body composition goals with a licensed healthcare professional, particularly if you are taking a GLP-1 medication.
Tool Information
August 2026
Dr. Jaydeep Sanghani
Meet Akabari
Dr. Jaydeep Sanghani
MBBS, MD, DNB(Anaesth.), PDCC(CCM), DrNB(CCM)
AIIMS Bhubaneswar · AIIMS Rishikesh
Critical care specialist and anesthesiologist with advanced training from AIIMS. Reviews health calculators at Calqulate to ensure medical accuracy and evidence-based standards.
About the authors
