Lean Body Mass Calculator (Cunningham Equation)

Calculate Your LBM

Estimates only. Talk to a doctor or registered dietitian before acting on these numbers for medical reasons.

The sections below explain what each number means and how the calculator chose them.

What is lean body mass?

Lean body mass (LBM) is everything in your body that isn't fat: muscle, bone, organs, water, and stored glycogen. It's the active tissue that burns calories at rest, which is why it's the right number to anchor a cutting plan to, instead of your total scale weight.

LBM vs muscle mass

Muscle mass is a subset of LBM, typically the largest one. When people say "I gained 2 kg of muscle in 12 weeks," they usually mean LBM, since it's nearly impossible to measure pure muscle outside a research lab. For practical purposes, treat LBM as your best available proxy for muscle change during a cut.

LBM vs fat-free mass (FFM)

Same thing, different name. "Fat-free mass" is the academic term; "lean body mass" is the gym term. Both equal Weight × (1 − BF%/100). Where they diverge: FFMI (fat-free mass index) normalizes FFM by height squared, the way BMI does for total weight. FFMI is more useful for comparing across heights ("am I muscular for my height"). LBM is more useful as your daily training and nutrition anchor.

LBM vs total body weight (and why this matters for tracking)

Two people, same scale weight, different LBM. The 80 kg lifter at 12% body fat has 70.4 kg of lean mass. The 80 kg sedentary office worker at 28% body fat has 57.6 kg of lean mass. That's 12.8 kg of difference in active tissue, and it's the entire reason their nutritional needs are different.

Anchoring on total weight ignores that gap. Anchoring on LBM doesn't.

LBM vs BMI

BMI doesn't see body composition at all. A 90 kg lifter at 14% body fat and a 90 kg sedentary person at 35% body fat have identical BMI but 24 kg of LBM separating them. BMI is a population-level screening tool. LBM is an individual-level training tool.

Should you anchor protein on LBM or on total body weight?

Anchor on LBM. The reasons follow.

Why does anchoring protein on bodyweight break at the extremes?

Most calorie apps default to g protein per kg total body weight. That works for a person at average body composition. It breaks predictably in two directions:

• Above about 22% body fat (men) or 32% (women). Total-weight targets push protein higher than the body can usefully metabolize. You're paying for protein your kidneys filter out.
• Below about 10% body fat (men) or 16% (women). Total-weight targets undershoot what's needed to preserve hard-won lean mass. Cuts at this body composition fail when protein drops too low.

Worked example. Two men, same 80 kg weight:

The body-weight anchor gives both men the same target. The LBM anchor gives Person A more (he has more muscle to feed) and Person B less (he doesn't need protein for fat tissue). That's the right shape.

Why is LBM a better protein anchor than bodyweight?

Anchoring on LBM is supported by the 2017 ISSN position stand on protein and exercise, the 2014 Helms review on protein for natural bodybuilders, and the broader sports-nutrition consensus. The 2.2 g/kg LBM ceiling sits at the upper end of what studies show useful in a deficit:

• Below 1.8 g/kg LBM: lean mass retention starts to suffer measurably in a deficit
• 1.8 to 2.2 g/kg LBM: the protective range
• Above 2.4 g/kg LBM: no additional benefit shows in any controlled study

Deficit uses 2.2 because the cost of being slightly over is zero (extra protein won't hurt), while the cost of being under is real (LBM you can't get back without a second bulk).

What does a protein target look like by lean mass?

That number is non-negotiable in a Deficit cycle. Other macros flex; protein doesn't. For the bodyweight versus lean-mass debate in full, including the "1 gram per pound" rule and where it breaks, see how much protein on a cut.

What does your LBM mean for a cut?

Three things flow directly from your LBM number: a protein target, a calorie deficit, and a cycle length. Together they define an entire cut.

Your protein target: 2.2 g per kg of LBM

Covered in detail in the section above. Round to the nearest 5 g. Hit it every day. Track it as hard as you track calories.

Your calorie deficit: chosen by body fat category

A 15 to 25% deficit is the working range for most lifters. Lower if you're already lean (you can't afford LBM losses). Higher if you have more fat to lose (the body can sustain a steeper deficit without breaking into muscle for fuel).

Deficit's calculator picks the recommended deficit by interpolating between body fat anchor points, with different anchors for men and women. See the next section for the full table. The slider above lets you adjust within sensible bounds; the recommendation is the midpoint of a defensible range.

Your suggested cycle length: 8 weeks

8 weeks is the Deficit default. Long enough to see meaningful body composition change (3 to 5% body fat reduction at a 17.5 to 20% deficit), short enough that adherence holds. The app supports 4 to 12 week cycles for users with shorter goals or longer phases, but most lifters land at 8.

The data on diet length is consistent: cuts beyond 12 weeks see a sharp drop in compliance and a sharp rise in metabolic adaptation. Cuts under 4 weeks barely show measurable composition change. 8 weeks is roughly where the bell curve peaks.

After the cycle, you stop. You don't extend "just two more weeks." You move into a maintenance phase, at minimum equal in length to the cut. Then, if you want, another cycle. A cut has an end date.

How much should you cut by? Use your body fat category.

The right deficit depends on how much fat you have to lose. Anchored to body fat % rather than total weight, the recommendations look like this:

For men

For women

Why does the recommended deficit differ by sex?

Essential body fat is structurally higher in females (about 10 to 13%) than in males (about 3 to 5%). The same category, "lean, average, overweight," lands at different absolute body fat percentages because the floor is different. The recommendation tracks the structure rather than the absolute number.

Deficit's calculator interpolates linearly between these anchors rather than using hard category cutoffs. A user at 18% body fat (between the 15% and 25% male anchors) gets a recommendation partway between 20% and 25% deficit, not a sudden jump at a category boundary.

Why Cunningham beats Mifflin-St Jeor for athletes

Almost every calorie calculator on the internet uses the Mifflin-St Jeor equation. It's the default in MyFitnessPal, Cronometer, MacroFactor, and Calculator.net. It's a fine equation for the population it was developed on, which was mostly sedentary. For the full methodology and sources, see how Deficit works, and the Cunningham equation for athletes for how it compares to the other BMR formulas.

If you've been training for two years or more, your body composition is not average. Mifflin-St Jeor will systematically underestimate your metabolic rate. Cunningham fixes that.

How does Cunningham differ from Mifflin-St Jeor?

Mifflin-St Jeor uses total body weight, with adjustments for height, age, and sex:

RMR (men)   = 10×weight(kg) + 6.25×height(cm) − 5×age + 5
RMR (women) = 10×weight(kg) + 6.25×height(cm) − 5×age − 161

Cunningham uses lean body mass directly. Height, age, and sex aren't needed:

RMR = 500 + 22 × LBM(kg)

The simplicity is the point. Cunningham assumes that what burns calories at rest is lean tissue, and it does, almost entirely. Two people of the same height, age, and sex can have substantially different metabolic rates if their LBM differs. Mifflin-St Jeor can't capture that. Cunningham does.

Why does lean mass matter more than total weight?

A worked example. Two men:

Mifflin-St Jeor gives them the same RMR: about 1,768 kcal/day. Cunningham gives different RMRs:

• Person A: 500 + 22 × 60.8 = 1,838 kcal/day
• Person B: 500 + 22 × 70.4 = 2,049 kcal/day

A roughly 280 kcal/day gap. Stretched over an 8-week cut, that's about 16,000 kcal, or roughly 2 kg (4.4 lb) of LBM that didn't need to be lost, because Person B was eating in too steep a deficit the whole time.

How accurate is the Cunningham equation?

For trained populations, Cunningham predicts RMR within ±10% of measured values in most studies. Mifflin-St Jeor is similar in accuracy for the general population but consistently underestimates trained subjects by 5 to 15%.

For practical use: Cunningham gets you closer to your real RMR than any other equation that doesn't require an indirect calorimetry chamber.

Why are Deficit's PAL values lower than other calculators?

Most calculators pair Mifflin-St Jeor BMR with generic PAL (Physical Activity Level) multipliers in the 1.2 to 1.9 range. Deficit's calculator above uses a tighter 1.10 to 1.40 range. That's intentional.

Cunningham gives a BMR roughly 10% higher than Mifflin-St Jeor for typical body comp, so the activity multiplier has to come down proportionally. Otherwise TDEE inflates by about 10% and you eat too much in maintenance, too little in deficit (because the deficit slides off a too-high base).

The math works out. The TDEE prediction lines up with the standard Mifflin × generic-PAL combo, but built on a more accurate body-comp-aware foundation.

Why this matters: if you compare Deficit's TDEE output to a generic calculator's output and pick the higher one, you're not getting a more accurate number. You're getting a calibration mismatch. Pick one calculator and trust it end to end.

How do you measure body fat at home?

The Cunningham equation needs body fat percentage. Most people don't have a number. The methods below get you one without a clinic visit, ranked from cheapest to most accurate. For a dedicated tool that runs both the Navy and skinfold methods, use the body fat percentage calculator.

Navy method (free, tape measure)

The U.S. Navy method estimates body fat from circumference measurements: neck and waist for men, plus hips for women. It's free, takes two minutes, and is accurate to within ±3% for most people.

You need a flexible tape measure. Wrap it level, don't compress the skin, take three readings and average them. The calculator above has this method built in. Click "Don't know your body fat %?" to use it. The sex setting determines whether a hip measurement is needed.

The Navy method is least accurate at extremes (under 8% body fat or over 30%), because the equation was fitted on people in between. If you're at the edges, use calipers, a smart scale trend, or DEXA instead.

Smart scales (cheap, use with caution)

Smart scales use bio-electrical impedance analysis (BIA). A tiny current passes through your body, and the scale infers fat percentage from how much resistance it meets.

The number BIA gives you is wrong. Often by 5+ percentage points.

But the trend is useful. Weigh in at the same time every morning, in the same hydration state (after using the bathroom, before drinking), and the day-to-day error becomes consistent. Don't trust the number; trust the change. A scale that says you're 18% body fat might mean 22%, but if it says 18% today and 16% in three weeks, you've probably lost about 2%.

Skinfold calipers (cheap, accurate with practice)

A pair of skinfold calipers measures subcutaneous fat at specific body sites. Plastic models cost $15 to $30 (Slim Guide, Accu-Measure). Professional metal calipers (Harpenden, Lange) cost $200+ and are more consistent at low body fat percentages.

Accuracy: ±3 to 4% when technique is good. Worse than DEXA, better than smart scales, and more accurate than the Navy method in trained populations, because it measures fat directly rather than inferring it from circumferences.

The protocol that matters: the 3-site Jackson-Pollock equation. Three pinches in three specific locations.

The catch: you need a partner. Self-measuring chest, triceps, and back sites accurately is nearly impossible. The other catch: the first 5 to 10 measurements you take will be wrong because technique takes practice. After that, repeat measurements drift within ±1%, which is enough for cycle-tracking even without absolute precision.

DEXA scan (gold standard, worth it once per cycle)

A DEXA (dual-energy X-ray absorptiometry) scan is the home gym-goer's gold standard. Accurate to within ±1%. Costs $50 to $100 in most US and EU cities. Takes 10 minutes.

You don't need one weekly. You need one at the start of a cycle and one at the end. That's two data points, two real numbers you can put into a Cunningham calculation, and direct evidence of how much body fat you lost vs how much LBM you kept.

What's a "good" lean body mass?

There's no universal answer. LBM only makes sense relative to your height, sex, and training history. Some rough orientation:

LBM by sex and height (rough ranges)

For trained populations, typical LBM ranges land roughly here:

These are athletic ranges. Average untrained populations sit about 5 to 10 kg lower. Elite physique competitors and dedicated bodybuilders can sit about 5 to 15 kg higher. FFMI normalizes for height and is a more useful "am I muscular for my frame" metric than raw LBM at the extremes.

LBM by age

LBM peaks around age 25 to 30 and naturally declines roughly 0.5 to 1% per year after 40 unless actively trained. Resistance training preserves and adds LBM at any age. This isn't about prevention, it's about continued stimulus.

When LBM stops being a useful metric

Two cases. For severely obese individuals, LBM includes a lot of structural mass (bone density, fluid) that isn't meaningful for athletic performance. Body fat % matters more than absolute LBM. For competitive bodybuilders, FFMI and segmental composition (lean mass per body region) matter more, because total LBM has plateaued.

Frequently Asked Questions

Sources

• A reanalysis of the factors influencing basal metabolic rate in normal adults. Cunningham JJ. Am J Clin Nutr. 1980. (PubMed)
• A new predictive equation for resting energy expenditure in healthy individuals. Mifflin MD, St Jeor ST, et al. Am J Clin Nutr. 1990. (PubMed)
• International Society of Sports Nutrition Position Stand: protein and exercise. Jäger R, et al. J Int Soc Sports Nutr. 2017. (free full text, PMC)
• Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. Helms ER, Aragon AA, Fitschen PJ. J Int Soc Sports Nutr. 2014. (free full text, PMC)
• Protein supplementation and resistance training-induced gains in muscle mass and strength: a systematic review and meta-analysis. Morton RW, et al. Br J Sports Med. 2018. (PubMed)
• Prediction of percent body fat for U.S. Navy men from body circumferences and height. Hodgdon JA, Beckett MB. Naval Health Research Center, Report 84-11. 1984. (DTIC)