Body fat calculator (US Navy tape method)

The US Navy method estimates your body fat percentage from a few tape measurements and your height: neck and waist for men, neck, waist and hips for women. For most adults it lands within about 3 to 4 percentage points of a lab test, which is good enough to tell you roughly where you stand and, more usefully, whether you're moving in the right direction month to month.

How the Navy body fat formula works

The equations come from Hodgdon and Beckett at the Naval Health Research Center (1984), who measured US Navy personnel by underwater weighing and then found the tape measurements that predicted their results best. The idea is simple: fat collects around the belly (and in women, the hips), while neck size tracks your frame and muscle. Subtract one from the other, correct for height, and you get a decent estimate of body density, which converts to body fat.

In centimetres, the formulas this calculator uses are:

• Men: body fat % = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450
• Women: body fat % = 495 ÷ (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450

The "495 ÷ density − 450" part is the standard Siri equation for turning body density into a fat percentage.

A worked example

Say you're a man, 170 cm tall, with a 38 cm neck and an 88 cm waist.

1. Waist − neck = 50. log₁₀(50) = 1.699. log₁₀(170) = 2.230.
2. Density = 1.0324 − (0.19077 × 1.699) + (0.15456 × 2.230) = 1.0324 − 0.3241 + 0.3447 = 1.0530
3. Body fat = 495 ÷ 1.0530 − 450 = 20.1%

If you weigh 72 kg, that's about 14.5 kg of fat and 57.5 kg of lean mass (muscle, bone, organs, water, everything that isn't fat). That's what the calculator shows by default.

What do the body fat categories mean?

The scale in the result uses the American Council on Exercise (ACE) categories, which are the ones most trainers quote:

Women carry more essential fat than men for hormonal and reproductive reasons, so a woman at 22% and a man at 15% are in the same "fitness" band. Getting into the essential range on purpose isn't a goal. Bodybuilders only touch it for a few days around a show, and staying there affects hormones, mood and, in women, periods.

For more detail on what each range looks like in practice, see the body fat percentage chart for Indians.

Why does the result also show a BMI-based estimate?

As a cross-check. Deurenberg and colleagues (1991) worked out a formula that predicts body fat from BMI, age and sex:

Body fat % = 1.2 × BMI + 0.23 × age − 10.8 × sex − 5.4 (sex = 1 for men, 0 for women)

For the same 30-year-old man (BMI 24.9), that gives 1.2 × 24.9 + 6.9 − 10.8 − 5.4 = 20.6%, close to the tape result.

When the two numbers disagree a lot, the tape number is usually the better one for people who train. BMI can't see muscle, so a lifter with a slim waist will get a much higher BMI estimate than tape estimate. The opposite also happens: someone with a "normal" BMI but little muscle and a soft middle can have more fat than their BMI suggests.

Body fat in Asian Indians

At the same BMI, Asian Indians tend to have more body fat than people of European origin. A Singapore study by Deurenberg-Yap and colleagues found that, among Chinese, Malays and Indians, Indians had the highest body fat at any given BMI. Indians also tend to store more of it around the abdomen.

Two practical consequences:

• Don't trust a "normal" BMI on its own. If your BMI is 23 but your waist is above 90 cm (men) or 80 cm (women), the cut-offs used in the Indian consensus guidelines, your belly fat is worth working on. The calculator flags this under the result.
• The Navy formula wasn't built on Indian bodies. It was developed on US Navy personnel, so treat your number as an estimate. The trend over a few months tells you more than any single reading.

How to get an accurate reading

Most bad results come from bad tape work, not the formula. A 2 cm error on the waist moves the answer by 1 to 2 percentage points.

1. Use a soft, non-stretch tailor's tape.
2. Measure in the morning, before eating, with bare skin.
3. Keep the tape level all the way round. Use a mirror or ask someone at the gym.
4. Snug, not tight. The tape shouldn't dent the skin.
5. Take each measurement twice. If they differ by more than 1 cm, take a third and average them.
6. Re-measure every 2 to 4 weeks, under the same conditions. Daily changes are mostly water.

Limitations

• Error of a few points. Hodgdon's own review puts the standard error at about 3.5 percentage points, so a reading of 20% means "somewhere around 17 to 23%". DEXA scans and underwater weighing are more accurate but cost money and need a clinic.
• Very lean or very heavy people get less accurate results, as with most prediction equations.
• It ignores where else fat sits. Two people with the same waist and neck can have different fat on their arms and legs.
• Not for children, pregnancy, or people with a lot of fluid retention.

Using the number

Pick a realistic target band, not a single figure. For most men who train, the high end of "fitness" to low "athletes" (about 12 to 17%) is a sustainable look. For most women, about 20 to 25%. To lose fat, you need a modest calorie deficit; the calorie calculator gives you a daily target. To keep muscle while you do it, eat enough protein (the protein calculator gives you a number) and keep lifting, ideally following a plan like the beginner gym workout plan. Then watch the lean mass figure: if body fat drops while lean mass holds, you're doing it right.

This calculator is for general information and isn't medical advice.