How the body fat calculator works
The calculator gives you two estimates.
1. US Navy tape-measure method (main result)
The US Navy method was developed by Hodgdon and Beckett at the Naval Health Research Center in 1984. They measured body fat in hundreds of Navy personnel by underwater weighing, then found the body measurements that predicted it best: height, neck and waist for men, plus hips for women. The US Department of Defense later adopted the same equations for military body composition standards.
Men: %BF = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76 Women: %BF = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387 (all measurements in inches; the calculator converts centimetres for you)
2. BMI method (rougher estimate)
The second figure uses an equation by Deurenberg, Weststrate and Seidell (British Journal of Nutrition, 1991), built from body density measurements in 1,229 adults. It needs only height, weight, age and sex, so it works when you have no tape measure:
%BF = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4 (sex = 1 for men, 0 for women; BMI = weight (kg) ÷ height (m)²)
It cannot tell muscle from fat, so it is less accurate for individuals, especially muscular people.
From your body fat percentage the calculator also works out your fat mass (weight × body fat %) and lean mass (everything else: muscle, bone, organs and water). Lean mass is what the protein calculator and the Katch-McArdle option in the TDEE calculator use.
How to measure
You need a flexible tape measure. Measure against bare skin, keep the tape level, and take each measurement twice.
- Height: without shoes, standing straight against a wall.
- Neck: just below the larynx (Adam’s apple), with the tape sloping slightly down towards the front.
- Waist: men at the navel, women at the narrowest point. Breathe out normally and don’t hold your stomach in.
- Hips (women only): around the widest part of the buttocks, feet together.
Worked example
A 30-year-old man, 5 ft 10 in (70 in) tall, 180 lb, with a 15 in neck and a 34 in waist:
- %BF = 86.010 × log10(34 − 15) − 70.041 × log10(70) + 36.76
- = 86.010 × 1.27875 − 70.041 × 1.84510 + 36.76
- = 109.99 − 129.23 + 36.76 = 17.5% body fat
That is in the Fitness band on the ACE chart and inside the healthy range for men aged 20–39. His fat mass is 0.175 × 180 = 31.5 lb and his lean mass is 148.5 lb. His BMI is 25.8, so the BMI method gives 1.20 × 25.8 + 0.23 × 30 − 10.8 − 5.4 = 21.7%, about 4 points higher. That gap is typical for someone carrying more muscle than average.
A 30-year-old woman, 5 ft 5 in (65 in), 140 lb, with a 13.5 in neck, 30 in waist and 40 in hips gets 30.5% from the Navy method (Average on the ACE chart, and healthy for her age) and 29.5% from the BMI method.
Body fat categories
The American Council on Exercise (ACE) chart is the most widely used set of categories:
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25% and over | 32% and over |
Healthy body fat by age
ACE categories ignore age. Gallagher and colleagues (American Journal of Clinical Nutrition, 2000) linked body fat to BMI cut-offs in a large multi-ethnic sample and produced healthy ranges that rise with age. The calculator uses these to place you in an age-adjusted band:
| Age | Sex | Underfat | Healthy | Overweight | Obese |
|---|---|---|---|---|---|
| 20–39 | Women | under 21% | 21–33% | 33–39% | over 39% |
| 40–59 | Women | under 23% | 23–34% | 34–40% | over 40% |
| 60–79 | Women | under 24% | 24–36% | 36–42% | over 42% |
| 20–39 | Men | under 8% | 8–20% | 20–25% | over 25% |
| 40–59 | Men | under 11% | 11–22% | 22–28% | over 28% |
| 60–79 | Men | under 13% | 13–25% | 25–30% | over 30% |
Using your result
- To track progress, re-measure every 2–4 weeks under the same conditions. A falling waist measurement with a steady weight usually means you are losing fat and gaining muscle.
- To lose fat, a moderate calorie deficit with enough protein and strength training works best. The macro calculator sets this up for you.
- Very low body fat is not a goal for most people. Going below the essential fat range can disrupt hormones, and for women menstrual cycles.
The result is an estimate. If you need a precise figure, for example for medical reasons, ask about a DEXA scan.
Frequently asked questions
How accurate is the US Navy body fat calculator?
For most people it lands within a few percentage points of lab methods such as underwater weighing, which makes it one of the better tape-measure methods. Accuracy depends heavily on measuring carefully. It is less reliable for very muscular people and for people with an unusual fat distribution.
What is a healthy body fat percentage?
It depends on sex and age. Using the ranges from Gallagher et al. (2000), a healthy range for men aged 20 to 39 is about 8 to 20%, and for women of the same age about 21 to 33%. The healthy range rises a few points with each decade.
Why do women have higher body fat than men?
Women carry more essential fat, linked to hormones and reproduction. The American Council on Exercise puts essential fat at about 10 to 13% for women and 2 to 5% for men, so every category starts higher for women.
Where do I measure my waist for the Navy method?
Men measure around the waist at the level of the navel. Women measure at the narrowest point of the waist. Stand relaxed, do not pull your stomach in, and keep the tape level and snug without pressing into the skin.
Is BMI a good way to estimate body fat?
It is a rough guide. BMI does not know how much of your weight is muscle, so it overestimates body fat in muscular people and can underestimate it in people who have lost muscle. That is why the calculator shows the tape-measure result first.
How can I measure body fat more accurately?
DEXA scans, underwater weighing and air displacement (Bod Pod) are the most accurate options, but they need specialist equipment. Skinfold callipers used by a trained person are also good. Home smart scales use bioimpedance and can swing several points with hydration.
How often should I measure my body fat?
Every 2 to 4 weeks is enough. Measure at the same time of day, ideally in the morning before eating, and take each measurement two or three times to check it.