A Body Fat Calculator computes body fat from the inputs you provide. It applies the standard formula to the values you enter and returns the result instantly, without sending any data to a server. Free Body Fat Calculator. The tool.
Estimates body fat percent from tape measurements.
US Navy circumference method: men use waist - neck and height; women add hip.
US Navy method using neck, waist (and hip for women) circumference.
The Body Fat Calculator estimates your body fat percentage using the US Navy circumference method - a formula developed by Hodgdon and Beckett for the US Department of Defense. Unlike BMI, this method accounts for body shape by measuring neck, waist, and (for women) hip circumferences, providing a much better estimate of actual body composition.
The calculator also shows your fat mass and lean mass in kilograms, and classifies your result using the American Council on Exercise (ACE) body fat categories - from essential fat through athlete, fitness, average, to obese levels.
You only need a flexible tape measure to take measurements at home. No calipers, no special equipment.
BF% = 86.010 × log₁₀(85-38) - 70.041 × log₁₀(170) + 36.76
= 86.010 × log₁₀(47) - 70.041 × log₁₀(170) + 36.76
= 86.010 × 1.672 - 70.041 × 2.230 + 36.76
= 143.81 - 156.19 + 36.76 = 24.4% (Average category).
Fat mass = 70 × 0.244 = 17.1 kg. Lean mass = 52.9 kg.
Three numbers often confused; they answer different questions:
| Metric | Formula | What it tells you |
|---|---|---|
| BMI (Body Mass Index) | weight kg / height m^2 | Rough screen for under/normal/over weight. Bad for athletes. |
| BMR (Basal Metabolic Rate) | Mifflin-St Jeor equation | Calories your body burns AT REST in 24 hours |
| TDEE (Total Daily Energy Expenditure) | BMR x activity multiplier | Total calories burned per day, including activity |
| Body fat % | DEXA / calipers / bioimpedance | More accurate than BMI for athletes and very lean people |
| Category | BMI range | Notes |
|---|---|---|
| Underweight | < 18.5 | Increased risk of malnutrition, osteoporosis |
| Normal | 18.5 - 24.9 | Lowest health risk for most adults |
| Overweight | 25.0 - 29.9 | Increased risk of cardiovascular disease, diabetes |
| Obese class I | 30.0 - 34.9 | High risk; lifestyle change recommended |
| Obese class II | 35.0 - 39.9 | Very high risk; clinical intervention often warranted |
| Obese class III | >= 40.0 | Extreme risk; bariatric options often discussed |
South Asians have higher cardiovascular risk at lower BMI. India, Singapore, and many Asian countries use:
The simplified energy balance:
To lose 0.5 kg / 1 lb per week, eat ~500 kcal/day below TDEE. To gain muscle, eat 200-500 kcal/day above TDEE with adequate protein (~1.6g per kg body weight per day) and resistance training.
Caveats:
| Macronutrient | Daily target | Energy density |
|---|---|---|
| Protein | 0.8 g/kg sedentary; 1.6-2.2 g/kg if exercising | 4 kcal/g |
| Fat | 20-35% of total calories; minimum ~0.5 g/kg | 9 kcal/g |
| Carbohydrate | Fills the rest; minimum 130 g/day for brain function | 4 kcal/g |
| Fiber | 25-38 g/day | ~2 kcal/g (variable) |
| Water | 30-40 ml/kg body weight | 0 kcal/g |
It's accurate to within ±3-4% of DEXA scan results for most people. More accurate than BMI and nearly as good as skinfold calipers, while being easier to do at home.
For men: 14-17% is fitness level, 18-24% is average. For women: 21-24% is fitness, 25-31% is average. Essential fat minimums are 2-5% (men) and 10-13% (women).
Men: at the navel level. Women: at the narrowest point of the natural waist. Tape should be snug but not compressing skin.
BMI is a population screening tool, not an individual diagnosis. It overestimates body fat in muscular athletes and underestimates it in people with low muscle mass. Body fat percentage (DEXA, calipers) is more accurate. Waist-to-height ratio under 0.5 is another useful single number.
0.5-1 kg (1-2 lb) per week is sustainable for most adults. Faster loss is possible but typically comes from water, glycogen, and muscle - not just fat. Crash diets also slow metabolism and rebound when normal eating resumes.
BMR estimates are within ±10% of measured rates for most people. TDEE depends on self-reported activity, which is usually overestimated by 20-50%. Track actual weight change over 2-4 weeks and adjust intake to match.
Almost all daily fluctuation is water (sodium, carbohydrate storage as glycogen which holds 3-4x its weight in water) and digestive contents. Fat loss is on the order of 100-150g per day at most. Weigh weekly at the same time for trend, not daily.
For weight loss, the two are equivalent if total calories are the same. Some people find fasting easier to comply with (skipping breakfast = automatic deficit). Others find it harder. The best diet is the one you can sustain.
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.