Body Fat Calculator
Body fat percentage is the portion of your body weight made up of fat tissue. Unlike scale weight alone, it helps you understand what’s changing when you diet, strength train, or improve cardio—because you can lose fat, gain muscle, or both. Use this Body Fat Calculator to estimate your body fat % with the US Navy tape method (popular and practical), or a BMI-based estimate when you don’t have a tape measure. For broader goals, you might pair this with a Basal Metabolic Rate Calculator to estimate daily energy needs, and a waist to hip ratio calculator to track fat distribution. Explore more tools in Health & Fitness.
Calculate your body fat percentage
Choose a method, set units, then enter your measurements. Tape measurements work best when taken consistently (same time of day, same posture).
How this Body Fat Calculator works
This calculator supports two ways to estimate body fat percentage: the US Navy (Tape) method and a BMI-based estimate. Both are estimations. The tape method uses body circumferences because where your body stores fat changes those measurements in measurable ways. The BMI method uses height and weight (plus age and sex) and is useful when you don’t have a tape, but it can be less accurate for athletes or anyone with high muscle mass.
Method 1: US Navy (Tape) — primary method
The US Navy equations use logarithms because the relationship between circumferences and body density is not linear. In plain terms, the formula “compresses” large differences in measurements so the estimate tracks real-world changes more smoothly. The calculator converts metric measurements into inches internally (and shows that conversion in the breakdown), then applies the standard equations:
Male (inches):
Female (inches):
Because logarithms require positive values, the calculator checks that the “inside” of each log term is greater than zero. If you enter measurements that would cause log10 of a zero or negative number, you’ll see a friendly inline error and the page won’t break.
Method 2: BMI-based estimate — secondary (less precise)
BMI is computed from your height and weight, then body fat % is estimated using age and sex. This page uses a widely cited adult estimate: Body Fat % ≈ 1.20 × BMI + 0.23 × Age − 10.8 × Sex − 5.4, where Sex = 1 for male and 0 for female. It’s convenient, but because BMI can’t distinguish muscle from fat, two people with the same BMI can have different body fat levels. If you’re actively training, pairing BMI with measurements (or tools like a Heart Rate Calculator) can give a clearer picture of progress.
Variable definitions (what each input means)
Use cases: when body fat percentage is especially useful
- Cutting vs bulking decisions: Body fat % helps decide whether to focus on fat loss, muscle gain, or maintenance.
- Progress tracking beyond scale weight: If weight stalls but body fat % drops, you may be recomping (losing fat while gaining muscle).
- Health screening context: High body fat % can signal elevated cardiometabolic risk even when BMI is “normal.”
- Athletic performance goals: Certain sports favor specific ranges; tracking helps avoid under-fueling or overly aggressive dieting.
- Clothing fit and waist management: Waist changes often show up earlier than weight changes; tape-based estimates reflect that.
- Calorie planning: Combine your estimate with the BMR calculator to set a realistic deficit or surplus.
Worked examples (step-by-step)
Example 1: Male, US units, US Navy (Tape)
Inputs: Height = 70 in, Neck = 15.5 in, Waist = 34 in, Weight = 180 lb. Compute: waist − neck = 34 − 15.5 = 18.5.
Fat mass ≈ 180×0.168 = 30.2 lb; Lean mass ≈ 149.8 lb. Category (male): Fitness (14–17%)—near the top of the “fitness” band.
Example 2: Female, metric, US Navy (Tape) with conversion shown
Inputs: Height = 165 cm, Neck = 34 cm, Waist = 78 cm, Hip = 98 cm, Weight = 62 kg. Convert cm → in (divide by 2.54): Height ≈ 64.96 in, Neck ≈ 13.39 in, Waist ≈ 30.71 in, Hip ≈ 38.58 in.
Fat mass ≈ 62×0.311 = 19.3 kg; Lean mass ≈ 42.7 kg. Category (female): Average (25–31%)—right at the upper end, so small measurement differences can move the label.
Example 3: BMI-based estimate (metric)
Inputs: Sex = Male, Age = 35, Height = 180 cm, Weight = 85 kg. Convert height to meters: 1.80 m. BMI = 85 / (1.8²) = 85 / 3.24 ≈ 26.23.
This estimate may read higher for muscular builds. If strength training is part of the routine, tape-based results usually match visible changes better.
Common mistakes (and how to avoid them)
- Measuring too tight or too loose: Tape should be snug and level, without compressing skin.
- Inconsistent measurement location: Waist at navel level (as instructed) each time; changing the spot changes the outcome.
- Holding your breath or “sucking in”: Exhale normally and relax the abdomen for waist measurements.
- Mixing units mentally: Enter values in the units shown next to each field (cm/in, kg/lb). The calculator converts when needed.
- Entering measurements that don’t make physical sense: For the Navy method, waist must be larger than neck (and for female, waist + hip must exceed neck), otherwise the log term becomes invalid.
- Comparing across devices/methods: Don’t compare a smart scale estimate to tape results as if they’re identical—pick one method and track the trend.
Quick tips for more consistent readings
- Measure at the same time of day (morning is common) and under similar hydration and meal conditions.
- Take 2–3 readings per site and use the average; tiny tape shifts can change the estimate.
- Keep the tape parallel to the floor—especially at the waist and hips.
- Retest every 2–4 weeks for meaningful trends; day-to-day noise is normal.
- Use body fat % alongside performance markers (strength, steps, cardio) for a fuller picture.
- If BMI is your only option, consider adding a waist metric with the waist-to-hip ratio calculator for better health context.
Accuracy & Method
Privacy-first: This Body Fat Calculator runs entirely in your browser. Your inputs are not sent to a server, stored, or shared. The math is computed locally on your device.
Rounding policy (implemented): Body fat % is displayed to 1 decimal. Fat mass and lean body mass are displayed to 0.1 unit (kg or lb). Intermediate steps use higher precision internally, then round at the end for clarity.
Method difference: The US Navy tape method is generally more informative for tracking body composition changes because it directly uses body circumferences. The BMI-based estimate is best treated as a “ballpark” number—useful when you lack tape measurements, but less reliable for muscular builds, older adults with sarcopenia, or anyone whose body type differs from population averages.
Last Updated: January 16, 2026
Sources & References
- US Navy body fat estimation equations (tape circumference method with log10 terms)
- BMI concept and BMI calculation formulas (metric and US customary)
- Adult BMI-based body fat estimation approach incorporating age and sex
- General body composition measurement principles (circumference consistency, trend tracking)
FAQ
These answers match the calculator’s methods and the category ranges shown above.
Which method is more accurate: US Navy tape or BMI estimate? ⌄
In most everyday situations, the US Navy tape method is the better choice because it uses measurements that change with body fat distribution (waist, neck, and for females, hips). It’s especially helpful for tracking progress over time. The BMI-based estimate is convenient and fast, but BMI can’t separate muscle from fat—so it may overestimate body fat for athletic, muscular people and sometimes underestimate it when muscle mass is low. If you can, use tape measurements as your “primary trend” and keep BMI as a backup reference.
How do I measure neck, waist, and hips correctly for the tape method? ⌄
Use a flexible tape measure and keep it level (parallel to the floor). For the neck, measure just below the Adam’s apple, snug but not compressing the skin. For the waist, measure at navel level after a relaxed exhale—avoid sucking in or holding your breath. For hips (female equation), measure around the widest part of the hips/glutes. Take two or three readings per site and use the average. Consistency matters more than perfection, because you’re usually looking for trends across weeks.
How often should I remeasure body fat percentage? ⌄
For most people, every 2–4 weeks is a sensible cadence. Body composition changes are real but usually gradual, and measuring too frequently can amplify normal day-to-day variability (hydration, meal timing, and posture can all nudge tape measurements). If you’re in a structured plan—like a cut or a training block—choose a consistent day and time (many people use morning) and repeat under similar conditions. Track the trend rather than fixating on a single reading.
What body fat percentage ranges are considered healthy? ⌄
“Healthy” depends on sex, age, performance goals, and medical context. The category ranges shown in the results are a friendly reference: men tend to have lower essential fat than women, and athletic ranges can be lower than average without being unhealthy when nutrition and recovery are adequate. For general fitness, many adults aim for a “fitness” or “average” band that supports energy, hormones, and performance. If you’re unsure what’s appropriate for you, use these ranges as guidance and discuss goals with a qualified professional.
Why can athletes have a “high BMI” but a lower body fat percentage? ⌄
BMI uses only height and weight, so it doesn’t know whether your weight comes from muscle, fat, bone, or water. Athletes—especially strength and power athletes—often carry more lean mass, which raises weight and can push BMI into “overweight” territory even when body fat is relatively low. That’s why BMI-based body fat estimates can read higher than tape-based methods for muscular people. If your training includes strength work, measurements (tape) and performance markers often reflect progress better than BMI alone.
Do smart scales measure body fat better than tape methods? ⌄
Many smart scales use bioelectrical impedance (BIA), which estimates body composition by sending a small electrical signal through the body. BIA can be influenced by hydration, skin temperature, recent exercise, and even sodium intake. Tape methods are not perfect, but they’re often more stable for trend tracking when you measure consistently. If you use a smart scale, try to weigh under the same conditions each time and compare trends within the same device—not across different devices or methods. For a more rounded view, combine results with waist measurements.
Does hydration or eating change body fat calculator results? ⌄
Hydration and food intake can affect the look and feel of your waist (bloating, water retention) and can influence BIA smart scale readings even more dramatically. For tape measurements, small changes in abdominal fullness or posture can shift the measurement by a few millimeters, which can slightly change the estimate. That’s why the best practice is to measure under similar conditions—often in the morning, before a large meal—and use averages. Over weeks, genuine fat loss typically produces a clearer downward trend than short-term hydration noise.
Should teens use body fat calculators, and are age formulas valid? ⌄
Caution is recommended for teens and children. Growth and development change body composition rapidly, and adult-based formulas (including BMI-to-body-fat estimates) may not reflect adolescent norms. If a teen is tracking health, the priority should be overall wellbeing, nutrition, and activity rather than chasing a specific number. If measurement is needed for sports or medical reasons, it’s best done with guidance from a healthcare professional who can interpret results in an age-appropriate way. For adults, the age term in the BMI estimate helps approximate typical population patterns.
What if the calculator shows an error about the log term being invalid? ⌄
For the US Navy method, the formula includes log10(waist − neck) for males and log10(waist + hip − neck) for females. Logarithms require a positive input, so the quantity inside log10 must be greater than zero. If you enter a neck value that’s larger than the waist (or larger than waist + hip for females), the math becomes invalid and the calculator will show a friendly inline message instead of a broken result. Recheck the measurement locations and units, then try again with corrected values.