How to calculate body fat percentage

Body fat percentage tells you what your weight is actually made of. Two people at 170 lbs can look — and be — completely different: one at 15% body fat is a lean athlete, one at 30% is average and probably carrying more visceral fat than they want. BMI can’t distinguish them. This guide walks through the five ways to measure body fat %, why the US Navy tape method is the best free option for most people, and how to track it reliably.

Why body fat % matters more than BMI

BMI is a two-number ratio — height and weight — that was never designed for individual diagnosis. It was built in the 1830s for population statistics. A lifter at 5'10", 200 lbs, and 12% body fat shows up as “overweight” on the BMI chart despite carrying less fat than 90% of the population. Meanwhile a sedentary office worker at the same height and 160 lbs might read as “healthy” BMI but carry 30% fat and corresponding metabolic risk.

Body fat percentage is what doctors, coaches, and insurance actuaries actually care about under the hood. If you can measure only one thing beyond the scale, measure this.

The five methods, ranked by accuracy

1. DEXA scan. X-ray based, ±1% accuracy, splits fat from lean mass and bone. The research-grade standard. Runs $50–$150 per scan at a local clinic.

2. BodPod. Air-displacement plethysmography. ±2% accuracy. Similar cost to DEXA, less widely available.

3. Skinfold calipers. A trained operator pinches 3 or 7 sites and maps to a body fat %. ±3% if done by a pro; ±6% if self-measured. Cheap and repeatable but operator-dependent.

4. US Navy tape method. Free, needs only a flexible tape. ±3–4% accuracy for most adults. Based on neck and waist circumference (plus hip for women). This is the best no-cost option.

5. Bioimpedance (BIA) scales. Zap a low current through your body and estimate from resistance. The numbers swing by 3–5% with hydration alone, so day-to-day trends are noisy. Better than nothing, but the tape wins on consistency.

The US Navy tape method in practice

The Navy formula has been used by the US military since 1984 for PFA compliance. It works because abdominal fat is the dominant differentiator between body compositions at the same height.

For men: BF% = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76. For women: BF% = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387. You measure in inches. Don’t do the math by hand — the body fat calculator handles it instantly.

Measure correctly. Waist at the navel, tape level all the way around, do not suck in. Neck just below the larynx, tape flat. Hips (women) at the widest point. Measure first thing in the morning before eating or drinking — a full stomach can add half an inch to the waist and swing the result 1–2 percentage points.

Healthy ranges

The American Council on Exercise bands body fat percentages roughly as follows. Men: 2–5% essential, 6–13% athletic, 14–17% fit, 18–24% average, 25%+ high. Women carry more essential fat: 10–13% essential, 14–20% athletic, 21–24% fit, 25–31% average, 32%+ high. These shift slightly with age.

“Healthy” isn’t the same as “low.” Men below 6% and women below 14% can lose menstrual function, bone density, immune health, and testosterone. The athletic range is a better target than the essential one for almost everyone.

How often to measure

Once every 2–4 weeks is enough. Body fat doesn’t swing meaningfully day-to-day — the scale does, but most of that is water and glycogen. If you measure daily you’ll drive yourself insane chasing noise.

Track two numbers over 8 weeks: body fat % and waist circumference. If both trend down, you’re losing fat. If waist drops but body fat holds, it’s probably hydration. If body fat rises while weight holds, you’re losing muscle — slow the cut or eat more protein.

What actually moves the number

There’s no secret. A caloric deficit of 300–500/day (tracked honestly), 0.8–1.0 grams of protein per pound of target weight, resistance training 3+ days a week, and 7+ hours of sleep. Over 12 weeks that combination drops 2–4 points of body fat in most people — repeatable by almost anyone, which is why it’s not an advertised magic protocol.

Pair the body fat calculator with our BMR calculator to set the calorie target, and the calorie calculator for a goal-based intake number. See the build muscle as a beginner guide for the training side.