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Free Body Fat Calculator
Enter your height, weight, and a few tape measurements to estimate body fat percentage, fat mass, and lean mass. Four peer-reviewed formulas, metric or imperial, nothing stored.
Short answer
The most practical method without a lab is the US Navy circumference formula, which estimates body fat percentage from height, neck, and waist for men, plus hips for women. For a man: %BF = 495 / (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450, using centimetres. Multiply your body fat percentage by your weight to get fat mass; whatever is left is lean mass. The US Navy method has a published standard error of about 3.5 percentage points against hydrostatic weighing, so treat any single reading as a range rather than an exact figure.
Body fat percentage answers the question the scale cannot: how much of your weight is fat, and how much is everything else. Two people at an identical weight and height can differ by fifteen points of body fat, and every plan that follows should look different as a result.
The calculator runs four published equations on the same measurements — the US Navy circumference method, Relative Fat Mass, CUN-BAE, and Deurenberg — and shows all four, because agreement between them tells you more about the confidence of the estimate than any single figure does.
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Body Fat Calculator
Enter your measurements to estimate body fat percentage, fat mass, and lean mass. Nothing is uploaded and nothing is stored — the maths runs in your browser.
Just below the larynx, tape sloping slightly down at the front.
Across the navel, tape level all the way round.
Your body fat
20.7%
Average · 18 – 24%Likely range 17.2% – 24.2% · US Navy (tape)
Cross-check: four formulas, same measurements
Spread between the methods is normal. Where they disagree, trust the trend over weeks rather than any single reading.
What this means
The typical range for adult men who are not specifically training for leanness. Not a health problem on its own, but the band where waist measurement starts to matter.
A moderate deficit moves this. Work out maintenance calories first, take 300–500 off it, and keep protein at 1.6–2.4 g per kg so the loss is fat rather than muscle.
A tape measure gives you today. SlimAI gives you the trend.
SlimAI Body Assessment estimates body fat, fat mass, lean mass, and where fat is concentrated from two guided photos or the same six measurements — and keeps every reading so you can see which direction you are actually moving.
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Circumference and BMI equations estimate body fat; they do not measure it. The US Navy method’s published standard error is about ±3.5 percentage points against hydrostatic weighing, BMI-based equations are wider still for any one person, and inconsistent tape placement widens both. A general estimate for healthy adults aged 18 and over, not medical advice.
What Your Body Fat Percentage Tells You About Your Goal
Body fat percentage is more useful than weight precisely because it points somewhere different depending on where you are starting. The same 8 kg of loss is a good outcome at 32% and a bad one at 12%.
The number to protect is lean mass, not the one on the scale
In a deficit, some of what you lose will always be lean tissue. Two things reduce that share: enough protein — 1.6 to 2.4 g per kg of body weight — and resistance training two to four times a week. Take 300 to 500 calories off maintenance rather than half your intake, and re-measure every three to four weeks. If body fat percentage is falling while lean mass holds, the plan is working regardless of what the scale says.
A surplus works better from a lower starting body fat
Starting a bulk above roughly 20% body fat for men or 30% for women means a larger share of the surplus goes to fat. If you are above that, a short cut first usually produces a better result overall. From a leaner start, a surplus of 250 to 350 calories a day with progressive overload adds muscle at a rate your body can actually use.
Body fat percentage is the only number that shows recomposition at all
Eat at maintenance, train hard, keep protein high, and it is entirely possible to lose fat and gain muscle at the same weight. On the scale that looks like nothing happening for months. On a body fat estimate it looks like steady progress — which is why anyone recomposing needs this measurement rather than a weigh-in.
What Body Fat Percentage Actually Measures
Body fat percentage is the share of your total body mass that is adipose tissue. The rest — muscle, bone, organs, connective tissue, and water — is lean mass, sometimes called fat-free mass. A 80 kg person at 20% body fat carries 16 kg of fat and 64 kg of everything else.
That split is what makes it more informative than weight or BMI. Weight counts everything together. BMI adds height to the picture and still cannot separate a kilogram of muscle from a kilogram of fat. Body fat percentage is the first number in the sequence that describes what your weight is made of rather than how much of it there is.
It also has a floor. Essential fat — the fat required for hormone production, temperature regulation, nerve function, and organ protection — sits at roughly 2 to 5% in men and 10 to 13% in women. Below that, physiology starts failing rather than improving, which is why the lowest band on the chart is a warning rather than a goal.
It separates fat loss from weight loss
Losing 4 kg can mean 4 kg of fat, or 2 kg of fat and 2 kg of muscle. Only a composition estimate can tell the two apart.
It has a healthy floor as well as a ceiling
Very low body fat is a health risk, not an achievement. Women in particular face menstrual and bone-density consequences below the essential band.
It reframes plateaus
A stalled scale during a recomposition phase is normal and often means fat is falling while muscle rises.
It sets protein and calorie targets more precisely
The Katch-McArdle equation calculates basal metabolic rate from lean mass, which is more accurate than weight-based formulas once you know your composition.
One tape measurement is a data point. A trend is progress.
SlimAI Body Assessment estimates body fat, fat mass, lean mass, and regional fat distribution from two guided photos or the same six measurements — and keeps every reading so you can see the direction, not just today.
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The US Navy Body Fat Calculator, and Three Cross-Checks
No formula on a web page measures body fat. Each one is a regression: a statistical relationship fitted to a population whose composition was measured properly, then applied to your numbers. That is why the calculator shows four results instead of pretending one is definitive.
The US Navy circumference method is the primary estimate whenever you supply tape measurements, because it reads the body directly. Developed by Hodgdon and Beckett in 1984 for US Navy body-composition standards, it uses the ratio between neck and waist girth against height, and adds hip circumference for women. Its published standard error is around 3.5 percentage points against hydrostatic weighing.
Relative Fat Mass, published by Woolcott and Bergman in 2018, is deliberately simpler: height divided by waist, scaled and offset by sex. It was validated against DEXA in a large NHANES sample and tends to outperform BMI-based estimates, particularly at higher body fat levels.
CUN-BAE and the Deurenberg equation both work from BMI, age, and sex, with no tape required. They are the fallback when you have no measuring tape to hand — less precise for any individual, but useful as a sanity check on the circumference result.
Agreement is a confidence signal
When all four land within a few points of each other, the estimate is probably close. A wide spread usually means an unusual build — very muscular, very tall, or carrying fat atypically.
Consistency beats precision
A method that reads 2% high every time still tracks change perfectly. Pick one, measure the same way, and read the direction.
BMI-based formulas struggle with muscle
CUN-BAE and Deurenberg infer fat from weight-for-height, so they overestimate for trained people in exactly the way BMI does.
Nothing here is a diagnosis
These are fitness estimates for healthy adults aged 18 and over, not clinical measurements and not medical advice.
How to Measure Yourself So the Number Means Something
The formulas are fixed; the measurements are where the error creeps in. A centimetre of inconsistency at the waist can move the result by more than a point of body fat, which is enough to erase a month of real progress or invent progress that never happened.
Use a flexible tape, not a metal one. Measure first thing in the morning, before eating or drinking, after using the bathroom. Stand relaxed with your arms at your sides and breathe out normally — do not suck in, and do not push out. Keep the tape snug enough to stay in place without compressing the skin, and level all the way around; check the back in a mirror.
Take each measurement twice and use the average. If the two readings differ by more than half a centimetre, take a third. Then write down exactly where you placed the tape, because the next measurement in four weeks needs to be taken in the same place to be comparable.
Neck
Just below the larynx, with the tape sloping very slightly downward at the front. Do not flare your shoulders.
Waist (men)
Horizontally across the navel. This is the standard Navy site and is not the narrowest point.
Waist (women)
At the narrowest point of the torso, usually just above the navel.
Hips (women)
At the widest point of the buttocks, with feet together and the tape level.
From a Tape Measure to a Body Fat App That Remembers
This page gives you one estimate. It does not know what you measured last month, and it will not know what you measure next — nothing here is stored, and nothing is uploaded.
SlimAI Body Assessment is the same job done continuously and without the tape. It estimates body fat percentage, fat mass, lean mass, and regional fat distribution from two guided smartphone photos: the app reads six circumference measurements from the images, then the same validated formulas used here — US Navy, Relative Fat Mass, and CUN-BAE — produce a result presented as a likely range with a confidence rating. A manual mode takes the six measurements typed in, with no photos at all.
Because the readings are kept, the app shows the trend rather than the reading: body fat over time next to weight over time, so you can see the two lines diverge during a recomposition phase. That is the part a calculator structurally cannot do, and it is the difference between knowing your body fat percentage and actually using it.
The rest of the app is what makes the number move. SlimAI sets a daily calorie goal and gram-level protein, carbohydrate, and fat targets from your profile, counts every meal you photograph or log against them, and recalculates as your composition changes.
Body Fat Percentage Chart for Men and Women
These are the American Council on Exercise categories, the most widely cited descriptive bands for adults. Women sit higher than men at every level because essential fat requirements differ — a woman at 22% and a man at 15% are in comparable condition.
| Category | Men | Women | What it usually means |
|---|---|---|---|
| Essential fat | 2 – 5% | 10 – 13% | The physiological minimum. Competition-stage only, and not sustainable |
| Athletic | 6 – 13% | 14 – 20% | Trained athletes. Visible definition with recovery still intact |
| Fitness | 14 – 17% | 21 – 24% | Lean, healthy, and realistic to maintain year-round |
| Average | 18 – 24% | 25 – 31% | Typical for adults who are not training for leanness |
| Obese | 25% and above | 32% and above | Associated with raised metabolic risk; a steady deficit is the usual start |
These bands describe populations, not individuals, and they shift with age — body fat naturally rises across the lifespan as lean mass declines. Where fat is stored matters too: abdominal and visceral fat carry more metabolic risk than the same amount stored elsewhere. These bands apply to adults aged 18 and over. They are not validated for children and teenagers, who need age-and-sex-specific percentile charts, and they are not appropriate during pregnancy.
How to Calculate Your Body Fat Percentage
Five minutes with a tape measure gets you a usable estimate. The calculator above handles the arithmetic and the unit conversion.
- 1
Measure your height and weight
Barefoot against a wall for height. Weigh yourself first thing in the morning, after the bathroom, before eating or drinking, in the same clothing each time.
- 2
Measure your neck
Wrap a flexible tape just below the larynx, sloping slightly downward at the front. Keep your shoulders relaxed and do not flare them out.
- 3
Measure your waist
Men: horizontally across the navel. Women: at the narrowest point of the torso. Breathe out normally and keep the tape level all the way around.
- 4
Measure your hips if you are female
At the widest point of the buttocks, feet together, tape level. The Navy formula for women needs this third measurement.
- 5
Enter the numbers and read all four results
The calculator returns the US Navy estimate as the primary figure and shows RFM, CUN-BAE, and Deurenberg alongside it. A tight cluster means a confident estimate; a wide spread means treat the number as a starting point.
- 6
Re-measure in three to four weeks
Same time of day, same tape placement, same conditions. The change between two measurements is far more useful than either measurement on its own.
The Formulas Behind This Calculator
Everything on this page is computed in your browser from these published equations. All lengths are in centimetres. No data is uploaded, and nothing is stored between visits.
US Navy — men
%BF = 495 / (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450Hodgdon & Beckett (1984). Standard error around 3.5 percentage points against hydrostatic weighing.
US Navy — women
%BF = 495 / (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450The female form of the same method, which adds hip circumference to the girth term.
Relative Fat Mass (RFM)
%BF = 64 − (20 × height / waist) · women: 76 − (20 × height / waist)Woolcott & Bergman (2018), validated against DEXA in an NHANES sample.
CUN-BAE
%BF = −44.988 + (0.503 × age) + (10.689 × S) + (3.172 × BMI) − (0.026 × BMI²) + (0.181 × BMI × S) − (0.02 × BMI × age) − (0.005 × BMI² × S) + (0.00021 × BMI² × age)Gómez-Ambrosi et al. (2012), where S = 1 for women and 0 for men. Needs no tape measure.
Deurenberg (BMI-based)
%BF = (1.20 × BMI) + (0.23 × age) − (10.8 × sex) − 5.4Deurenberg et al. (1991), where sex = 1 for men and 0 for women. The original BMI-to-body-fat regression.
Lean body mass and fat mass
fat = weight × (%BF / 100) · lean = weight − fatLean mass is the figure a calorie deficit should be built to protect.
Sources and method
- Hodgdon, J.A. & Beckett, M.B. (1984) — Prediction of percent body fat for US Navy men and women from body circumferences and height — the circumference method used as the primary estimate on this page.
- Woolcott, O.O. & Bergman, R.N. (2018) — Relative Fat Mass as a new estimator of whole-body fat percentage, validated against DEXA in NHANES data.
- Gómez-Ambrosi, J. et al. (2012) — Clinical usefulness of a new equation for estimating body fat (CUN-BAE), the BMI-based estimate shown as a cross-check.
- Deurenberg, P. et al. (1991) — Body mass index as a measure of body fatness — the age-and-sex-adjusted BMI regression included for comparison.
- American Council on Exercise body-fat categories — The descriptive bands (essential, athletic, fitness, average, obese) used in the chart and in the result labels.
- SlimAI Body Assessment method — The same US Navy, RFM, and CUN-BAE formulas applied to six measurements read from two guided photos, returned as a likely range with a confidence rating.
FAQ
Still Wondering About SlimAI
How do I calculate my body fat percentage?
How do I calculate my body fat percentage?The most practical method without a lab is the US Navy circumference formula. Measure your height, neck, and waist — plus hips if you are female — then apply %BF = 495 / (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450 for men, or the female form which adds hip circumference. The calculator on this page does the arithmetic and also runs Relative Fat Mass, CUN-BAE, and Deurenberg on the same inputs so you can see how closely the methods agree.
What is a healthy body fat percentage?
What is a healthy body fat percentage?For men, roughly 14 to 24% covers the fitness and average bands; for women it is roughly 21 to 31%. Below 6% for men and 14% for women approaches essential fat, which is a health risk rather than a target. Above 25% for men and 32% for women is where body fat percentage is associated with raised metabolic risk. These bands shift with age, and where fat is stored matters as much as how much there is.
How accurate is a body fat calculator?
How accurate is a body fat calculator?The US Navy circumference method has a published standard error of about 3.5 percentage points against hydrostatic weighing, so a reading of 20% means roughly 16.5 to 23.5%. BMI-based equations such as CUN-BAE and Deurenberg are less precise for individuals, particularly for muscular people. All of them are far more reliable at tracking change over time than at pinning down an absolute number, so measure consistently and read the trend.
Is this body fat calculator suitable for everyone?
Is this body fat calculator suitable for everyone?No. It is a general fitness estimate for healthy adults aged 18 and over. It is not validated for children or teenagers, it is not appropriate during pregnancy or breastfeeding, and it is not medical advice or a diagnosis. If you have a medical condition, take medication, or have a history of disordered eating, speak to a qualified health professional before using body fat percentage to set diet or training targets.
Is the Navy body fat calculator accurate?
Is the Navy body fat calculator accurate?It is one of the more accurate methods available without equipment, which is why the US Navy uses it for body-composition standards. It reads actual body dimensions rather than inferring from weight, so it handles muscular builds better than BMI-based formulas do. Its weak point is measurement error: an inconsistently placed tape moves the result more than the formula itself does.
What is the difference between body fat percentage and BMI?
What is the difference between body fat percentage and BMI?BMI is weight divided by height squared. It counts all your mass together and cannot distinguish muscle from fat, so a muscular person is routinely classified as overweight. Body fat percentage describes what your weight is made of. Two people with an identical BMI of 26 can be at 12% and 30% body fat — the same category, completely different situations.
How do I calculate lean body mass?
How do I calculate lean body mass?Lean body mass is your total weight minus your fat mass. Multiply your weight by your body fat percentage divided by 100 to get fat mass, then subtract. Someone at 80 kg and 20% body fat carries 16 kg of fat and 64 kg of lean mass. Lean mass is worth tracking during a diet because it is the number a calorie deficit tends to erode if protein and resistance training are missing.
How fast can body fat percentage change?
How fast can body fat percentage change?Realistically, about 0.5 to 1 percentage point a month for most people in a moderate deficit. Faster than that usually means part of the loss is lean mass or water. Because a good calculator has an error band of several points, a single reading four weeks apart may not show a real change clearly — three or four readings over three months will.
What is the best body fat app?
What is the best body fat app?That depends on whether you want a one-off number or a trend. Calculator apps that log tape measurements are fine for the former. SlimAI is built for the latter: AI Body Assessment estimates body fat percentage, fat mass, lean mass, and regional fat distribution from two guided photos or six manual measurements, keeps every reading so the trend is visible next to your weight, and connects it to calorie and macro tracking so there is a lever to move the number with. It runs on iOS and Android and is free to download.
Can I measure body fat from a photo?
Can I measure body fat from a photo?Yes, with the same caveats that apply to any estimate. SlimAI Body Assessment reads six circumference measurements from two guided photos and runs them through the US Navy, RFM, and CUN-BAE equations, returning a likely range and a confidence rating rather than a single hard number. It removes the tape-placement error that dominates manual measuring, but it is still an estimate, not a DEXA scan.
Is this body fat calculator free?
Is this body fat calculator free?Yes. It is free, requires no account, and runs entirely in your browser — your measurements are never sent to a server or stored between visits. SlimAI is also free to download, with a Pro tier for unlimited AI scans and AI Body Assessment photo analysis.
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