Body composition measurement exists because body weight is a poor description of a human body. Two people weighing 85 kg can be carrying completely different amounts of fat, muscle and bone, and their energy requirements — the calories required just to stay alive — can differ accordingly. This guide covers how the estimates are actually made, why they disagree, and how to turn a body fat figure into a defensible energy target.
Lean body mass, and why it drives the arithmetic
Lean body mass (LBM) is body weight minus fat mass. It includes skeletal muscle, bone, organs and connective tissue. It matters for metabolism because metabolically active tissue is mostly lean: muscle is far more metabolically costly per kilogram than fat, and organ mass burns more per kilogram than skeletal muscle. So two people of the same weight have different energy requirements, and the difference is almost entirely in how much of that weight is muscle.
The relationship is straightforward:
Fat mass = body weight × body fat %
Lean body mass = body weight × (1 − body fat fraction)
Using the worked example below: a person of 85 kg at 22% body fat has fat mass = 85 × 0.22 = 18.7 kg and LBM = 85 × 0.78 = 66.3 kg. That 66.3 kg is the number that goes into a lean-mass-based metabolic formula, and it is also the figure that moves during a cut — which is why tracking it shows whether weight loss is coming from fat or from muscle. The percent change calculator is useful for comparing how quickly body weight and lean mass move relative to their starting values.
The four methods, and why they disagree
Skinfold calipers (DRI method). Pinching measured sites — typically chest, abdomen, thigh, triceps and subscapular — and reading a table keyed to age and sex. Cheap and portable, but heavily dependent on the operator: the same site pinched differently can shift the estimate several percentage points. Treat any result as carrying at least ±3–4 points of uncertainty.
Tape measures (US Navy method). Circumferences at the neck, waist, hip and, for the substituted variant, chest. The body fat calculator can run this from four numbers. It is more reproducible than skinfold because it is less dependent on finger technique, and it is free, which makes it the best cost-to-value option for trend tracking.
Bioimpedance analysis (BIA). A small current is passed through the body and the resistance is read, since fat conducts poorly relative to water-rich lean tissue. It takes seconds and is built into many consumer scales. The problem is that the reading depends heavily on hydration, and hydration varies by several litres across a day with meals, sweat and salt intake. If you use one of these, standardise the conditions — same time of day, similar meal timing, after urinating — or accept that 1–2 points of day-to-day movement is measurement noise rather than real change.
DEXA scan. Dual-energy X-ray absorptiometry separates bone, fat and lean tissue with far better precision and also reports bone density. It is the clinical reference and the closest thing to a gold standard. It is not cheap, it is radiation-based so it is not meant for frequent repeat measurement, and results can still shift with hydration and gut contents.
Hydrostatic underwater weighing remains a genuine reference method too, but it is inconvenient enough to be used in research rather than self-tracking. The practical resolution to the disagreement: pick one method and never switch, because tracking a trend with a consistent method is far more informative than comparing one BIA reading against a DEXA scan.
Basal metabolic rate, and which formula to trust
BMR is the energy needed to maintain basic life functions at rest. Two formulas dominate:
Katch-McArdle (uses lean body mass):
BMR = 370 + 21.6 × LBM(kg)
Mifflin-St Jeor (uses total weight):
Men: BMR = 10W + 6.25H − 5A + 5
Women: BMR = 10W + 6.25H − 5A − 161
where W is weight in kg, H is height in cm and A is age in years. Mifflin-St Jeor replaced the older Harris-Benedict formula as the general standard because it was derived from a larger modern sample and fits measured energy expenditure better.
The choice between them is not obvious. Katch-McArdle has the advantage of responding to body composition: as you lose fat and preserve muscle, its estimate falls less than a total-weight formula would, which is more accurate during a cut. It is unusable, though, when you do not have a trustworthy body fat estimate — feeding a BIA figure into it compounds one measurement error into another. Mifflin-St Jeor needs nothing but height, weight and age, which makes it the practical default when body fat is unknown.
From BMR to TDEE: the activity multiplier
BMR is not a daily calorie target. TDEE (total daily energy expenditure) adds the energy used for movement through a multiplier applied to BMR:
- 1.2 — sedentary: desk work, little deliberate exercise
- 1.375 — lightly active: exercise 1–3 days per week
- 1.55 — moderately active: exercise 3–5 days per week
- 1.725 — very active: hard exercise 6–7 days per week
- 1.9 — extremely active: physical job plus training, or elite endurance work
These are the standard published factors, and they carry the same honest caveat as every other number here: they are population averages describing a whole category, not an individual. Two people both at "moderately active" can differ by 20% in actual expenditure. The multiplier also does less work than it appears to, because the categories are broad — a 20 km run and a gentle walk can both land in "moderately active" at very different costs. This is why tracking scale weight over several weeks and comparing against the prediction is the only real validation available, and it is the approach the calorie calculator is built around.
Worked example, end to end
A 32-year-old man weighing 85 kg with a measured body fat of 22%.
Step 1 — lean body mass. LBM = 85 × (1 − 0.22) = 85 × 0.78 = 66.3 kg. Fat mass = 85 × 0.22 = 18.7 kg.
Step 2 — BMR via Katch-McArdle. BMR = 370 + 21.6 × 66.3 = 370 + 1,432.08 = 1,802.08 kcal, so approximately 1,802 kcal.
Step 3 — activity factor. At moderately active, multiply by 1.55: TDEE = 1,802.08 × 1.55 = 2,793.22 kcal, so approximately 2,793 kcal per day.
Step 4 — the deficit. A common sustainable starting point is a 500 kcal daily deficit, giving a predicted intake of 2,793 − 500 = 2,293 kcal.
Step 5 — the timeline. At roughly 7,700 kcal per kg of body fat, losing 15.4 kg requires 15.4 × 7,700 = 118,580 kcal in total. At 500 kcal per day that is 118,580 ÷ 500 = 237.2 days, or about 34 weeks.
The same answer arrives more usefully the other way round: 500 × 7 = 3,500 kcal per week, and 3,500 ÷ 7,700 = 0.4545 kg per week. At that rate 15.4 kg takes roughly 34 weeks — nearly eight months. This is worth confronting early, because a large fraction of people abandoning a cut do so over timing rather than over effort. A 15 kg loss is a project measured in seasons, not weeks, and a plan that looks reasonable on paper at 15 kg often looks unachievable in practice at 30 kg.
Why 7,700 is not a constant
The figure is a rule of thumb with real limitations, and treating it as an exact conversion produces predictable errors.
- It is an approximation of an average. Energy density of body fat is close to 7,700 kcal per kg, but reported figures range roughly 7,000–7,700 because stored fat is not pure triglyceride and is not fully mobilisable.
- Water moves with the fat. Each gram of glycogen is stored with roughly 3 g of water, and fat tissue itself is about 10–13% water. Early reductions therefore remove more water than fat, which is why the scale often drops faster in week one than the arithmetic predicts, then appears to stall.
- Energy expenditure falls with weight loss. This is the important one, and it is covered next.
- The deficit itself changes. The 500 kcal figure is relative to TDEE, and as TDEE falls the same absolute deficit becomes a larger proportional intervention.
Practically, treat 7,700 kcal per kg as a long-run average with a ±10% band, and use weeks rather than days as the tracking horizon. Why percent change lies to you covers the related point that a percentage move measured from a small starting number exaggerates the apparent rate of change — which is exactly the shape of a fast early loss.
Metabolic adaptation and the plateau
During sustained weight loss, total energy expenditure typically falls by more than the loss of body mass alone would predict. The mechanism is adaptive: falling fat stores reduce leptin signalling to the hypothalamus, which responds by lowering spontaneous movement, reducing insulin sensitivity and increasing hunger — a set of responses that historically protected against famine and now works against deliberate restriction.
The consequence is that a 500 kcal deficit is not permanent. After some months the same intake produces a smaller deficit, weight loss slows, and the honest move is to revise TDEE from current data rather than cut harder. Aggressive restriction makes this worse, and cycles of severe dieting followed by regain are strongly associated with progressively worse outcomes — a well-documented pattern rather than a failure of discipline. The BMI guide covers where the measurement assumptions behind all of this come from, and is worth reading before attaching any number to a health decision.
Protein, and protecting lean mass
Protein intake in a deficit is usually expressed in grams per kilogram of body weight, with roughly 1.6 to 2.2 g/kg the commonly used range to help preserve lean mass during weight loss. Higher intakes are sometimes used during aggressive cuts and lower intakes are adequate at maintenance, but this is a sports nutrition range rather than a medical threshold.
Two caveats matter more than the number. Resistance training matters at least as much — muscle is preserved by the stimulus to use it, not by protein alone, and a well-fed sedentary person loses more lean mass than a slightly under-fed one who lifts. And the range is not a cliff: the difference between 1.4 and 1.6 g/kg is small, while the difference between training and not training is not.
Reasonable body fat ranges, offered as the sports nutrition conventions they are rather than medical cut-offs: 15–20% for men and 20–25% for women, with athletes often lower and the lower end of any range not being automatically healthier. Essential fat — in cell membranes, hormones and nerve tissue — is around 2–5% of body weight, and sustained intake below that is dangerous. The water intake calculator covers the other frequently neglected variable, and calories and macros covers how to build the intake side of the plan.
This article is educational and is not medical advice. Body fat estimates and energy requirements carry substantial individual variation and method-dependent error, and every threshold mentioned here is drawn from published ranges that vary by authority and by population. Nothing here should be used to diagnose a condition or to design a clinical or therapeutic diet. Speak with a physician or registered dietitian before making significant changes to your eating or training, particularly if you have a medical condition, are pregnant or breastfeeding, take medication, or have a history of disordered eating.
Frequently asked questions
Which body fat method is accurate enough to track progress?
No consumer method is accurate enough on its own, so the practical answer is to pick one and keep using it, because consistency reveals the trend while absolute accuracy rarely improves. DEXA is the clinical reference for bone and fat mass. Tape measures are reliable and free, bioimpedance is convenient but strongly affected by hydration, and skinfold calipers depend heavily on the operator.
What is lean body mass and why does it matter for metabolism?
Lean body mass is your body weight minus fat mass, and it includes muscle, bone and organs. It matters because metabolically active tissue is mostly lean tissue, so two people at the same weight can have very different energy requirements. Formulas that use lean mass, such as Katch-McArdle, track changes in body composition better than those that use total weight alone.
Why has my resting metabolic rate fallen after dieting?
Because sustained weight loss lowers total body mass, and with less metabolically active tissue the resting requirement falls with it, sometimes by more than the weight lost would predict. Metabolic adaptation is a real component of the plateau and a normal physiological response, not a failure of willpower, which is why an indefinitely reduced intake is a poor long-term strategy.
How much protein is needed to preserve muscle in a deficit?
For most people in a calorie deficit, intakes around 1.6 to 2.2 g per kilogram of body weight per day are commonly used to help preserve lean mass, with resistance training mattering at least as much. Individual needs vary, and higher intakes may be appropriate during aggressive cutting, but this is a sports nutrition range rather than a medical threshold.