Ever stepped on a scale, seen a number, and thought "but what does that actually mean?" You're not alone. Total weight is a pretty useless metric on its own—it doesn't tell you whether you're carrying muscle or fat, and it definitely doesn't tell you if that 3 kg you gained last month was progress or a problem.

That's where lean body mass comes in. This calculator shows you the weight of everything in your body except fat: your muscles, bones, organs, and water. It's the number that actually matters when you're trying to understand what's happening with your body.

Enter your sex, weight, and height, and you'll get your estimated lean body mass in seconds. More importantly, you'll finally have a metric worth tracking.

Lean body mass is simple: take your total weight, subtract all the fat, and what's left is your LBM. That includes skeletal muscle (the stuff you build in the gym), but also your bones, organs, skin, and the water throughout your body.

Here's why this matters more than most people realize.

Two people can weigh exactly 80 kg. One looks lean and athletic. The other... doesn't. The scale shows the same number for both, but their lean body mass tells completely different stories. Person A might have 64 kg of lean mass (80%), while Person B has 54 kg (67.5%). Same weight, totally different bodies.

Once you understand this, a lot of fitness confusion starts to clear up:

• Dieting but the scale won't budge? Check your LBM. If it's holding steady or increasing while your weight stays flat, you're losing fat and gaining muscle simultaneously. That's actually ideal—even if the scale is being annoying about it.
• Gaining weight during a bulk? Your LBM tells you whether that weight is muscle (good) or fat (less good). The scale can't make that distinction.
• Over 30 and worried about "losing it"? Tracking LBM helps you catch muscle loss early. We naturally lose muscle mass as we age—about 3-8% per decade after 30—but strength training can slow or reverse this. Your LBM is the scoreboard.

The good news? Once you know your number, you can actually do something with it.

Most LBM calculators give you a number and leave you hanging. Great, your lean body mass is 57 kg. Is that... good? Bad? Should you be concerned?

Let's fix that.

Women naturally have lower LBM percentages than men—and before anyone gets worked up about that, it's not a disadvantage. Women carry more essential fat for hormone production and reproductive health. Biology, not judgment.

Let me make this concrete.

A 75 kg man standing 180 cm tall has approximately 58.4 kg of lean body mass. That's about 78% lean—solidly in the "fit" category. If he's been hitting the gym and gains 3 kg over two months, he needs to check whether that weight is muscle or fat. If his LBM rises to 60.5 kg, he's added over 2 kg of muscle. That's excellent progress. If his LBM stays at 58.4 kg? He's gained 3 kg of fat. Time to reassess the bulk.

Now take a 65 kg woman at 165 cm with roughly 45.2 kg of lean mass—about 70%, right at the fit/average line. She starts a cut, and six weeks later she's down to 60 kg. If her LBM held steady at 45 kg, she's lost 5 kg of pure fat. That's exactly what successful dieting looks like. If her LBM dropped to 42 kg? She lost muscle too, probably from cutting calories too aggressively or skipping the weights.

See how this changes everything?

This calculator uses the Boer formula—not because it has a cool name, but because research has consistently shown it's one of the more accurate methods for estimating lean body mass without expensive equipment.

For men:

LBM = (0.407 × weight in kg) + (0.267 × height in cm) − 19.2

For women:

LBM = (0.252 × weight in kg) + (0.473 × height in cm) − 48.3

70 kg man, 180 cm tall:

LBM = (0.407 × 70) + (0.267 × 180) − 19.2 LBM = 28.49 + 48.06 − 19.2 LBM = 57.35 kg

So roughly 57 kg of his 70 kg is lean mass, leaving about 13 kg of fat. That's an 82% lean body mass—athletic range.

Same stats for a woman:

LBM = (0.252 × 70) + (0.473 × 180) − 48.3 LBM = 54.48 kg

Her 78% lean mass is also athletic for women. The different formulas account for natural differences in how men and women carry mass.

Pretty straightforward:

1. Pick your sex — The formula is different for men and women, so this actually matters.
2. Enter your weight in kg — For consistency, weigh yourself at the same time each day. Morning, after the bathroom, before breakfast works best. Trying to compare your morning weight to last week's post-dinner weight is just going to frustrate you.
3. Enter your height in cm — This one doesn't change (hopefully).
4. Read your results — You'll see your lean body mass in kg and as a percentage of total weight.

Pro tip: Don't obsess over daily fluctuations. Water weight, food in your stomach, and a dozen other factors can swing your weight by 1-2 kg day to day. Track weekly or bi-weekly, and look for trends over 4-8 weeks. That's where the real information is.

People mix these up constantly, so let's be clear: lean body mass is NOT the same as muscle mass.

LBM includes your muscles, yes—but also bones, organs, skin, and water. Skeletal muscle (the kind you actually train) makes up roughly 40-50% of your lean mass. So if your LBM is 60 kg, your actual muscle tissue is probably somewhere around 24-30 kg. The rest is everything else keeping you alive.

Two sides of the same coin. If 78% of your weight is lean mass, then 22% is fat. Some people prefer tracking body fat percentage, others prefer LBM. Doesn't really matter—you're measuring the same thing from different angles.

BMI is... limited. It only looks at height and weight. A 100 kg bodybuilder and a 100 kg couch potato at the same height have identical BMIs, but wildly different body compositions. One is elite-level fit; the other is at serious health risk. BMI can't tell the difference. LBM can.

Alright, you've got your number. Maybe you want it higher. Here's what actually works—no gimmicks.

There's no way around this one. Building lean mass requires resistance training. Your muscles need progressive overload—meaning you gradually increase weight, reps, or volume over time—to have any reason to grow.

Aim for 2-4 strength sessions per week. Focus on compound movements: squats, deadlifts, bench press, rows, overhead press. These recruit multiple muscle groups and give you more bang for your buck than isolation exercises.

Here's where your LBM number becomes immediately useful.

Most protein recommendations are based on total body weight, which doesn't make much sense. Your fat tissue doesn't need protein—your muscles do. A better approach: 1.6-2.2 grams of protein per kilogram of lean body mass.

With an LBM of 57 kg, that's:

• Low end: 57 × 1.6 = 91g protein daily
• High end: 57 × 2.2 = 125g protein daily

Spread that across 3-5 meals. Your body can only use so much protein at once for muscle building, so 40g per meal is more effective than 120g in one sitting.

Muscles don't grow in the gym—they grow while you're resting afterward. If you're training hard but sleeping 5 hours a night, you're sabotaging yourself.

Get 7-9 hours of sleep. Take rest days seriously. Allow 48-72 hours before training the same muscles again. Overtraining is real, and it'll tank your progress faster than almost anything else.

You can't build something from nothing. If you're trying to gain lean mass while eating at a significant caloric deficit, your body will prioritize survival over muscle growth. A slight surplus—100-300 calories above maintenance—gives your body the raw materials it needs.

Yes, you might gain a tiny bit of fat along with the muscle. That's normal. You can cut later.

No formula can tell you exactly what's inside your body. If it could, DEXA machines wouldn't cost $100,000 and medical researchers would be out of work.

The Boer formula is well-validated and reasonably accurate for most adults—usually within 2-5% of clinical measurements. That's good enough for tracking changes over time and making informed decisions about your training and nutrition.

For most healthy adults with fairly typical body proportions, you'll get a useful estimate. It's great for:

• Monitoring your own progress week to week
• Getting a realistic baseline for goal-setting
• Calculating protein needs more accurately than total-weight methods
• Understanding your body composition without paying for a scan

The Boer formula can be less accurate for people at the extremes:

• Very muscular individuals (competitive bodybuilders, strength athletes)
• People with very high or very low body fat percentages
• Older adults who've experienced significant muscle loss
• Anyone with medical conditions affecting body composition

If you're in one of these categories and need precise numbers—for medical reasons, competitive athletics, or just peace of mind—DEXA scans, hydrostatic weighing, or BodPod assessments give you direct measurements instead of estimates. They cost money, but they're definitive.

For 90% of people, this calculator gives you exactly what you need: a reliable number you can track over time. Whether your LBM is truly 57.35 kg or actually 56.8 kg matters a lot less than whether it's trending up or down over the next three months.