Off the Clock Labs

How to estimate body fat with a tape measure (the US Navy method)

· 4 min read

The scale cannot tell fat from muscle. A tape measure can give you a rough answer. A method many body fat calculators use comes from two 1984 reports by Hodgdon and Beckett at the US Naval Health Research Center. They measured hundreds of Navy men and women, then found the tape measurements that best predicted body fat.

What to measure

You need a soft tape measure and your height. Measure in centimetres if you can, because the equations below use them. If your tape is in inches, multiply each number by 2.54.

For men:

For women:

Keep the tape level and snug, but do not press it into the skin. Measure twice and use the average if the two differ.

The equations

The reports do not give body fat directly. They give body density, and a second step by Siri, from 1961, turns density into a percentage.

For men:

density = 1.0324 − 0.19077 × log10(abdomen − neck) + 0.15456 × log10(height)

For women:

density = 1.29579 − 0.35004 × log10(waist + hips − neck) + 0.22100 × log10(height)

Then, for both:

body fat % = 495 ÷ density − 450

“log10” is the base-10 logarithm. Any phone calculator has it as the “log” key.

Worked examples

A man 177 cm tall, with a 90 cm abdomen and a 38 cm neck:

density = 1.0324 − 0.19077 × log10(52) + 0.15456 × log10(177) = 1.0525
body fat = 495 ÷ 1.0525 − 450 = about 20.3%

A woman 165 cm tall, with a 75 cm waist, 100 cm hips and a 33 cm neck:

density = 1.29579 − 0.35004 × log10(142) + 0.22100 × log10(165) = 1.0325
body fat = 495 ÷ 1.0325 − 450 = about 29.4%

A trap with the inch version

Many websites print a different formula that takes inches, with numbers such as 86.010 and 70.041. That formula does not appear in either Navy report. In the normal range it gives answers close to the originals. Outside that range it drifts away from them. The safe way is to convert your inches to centimetres and use the equations above.

How accurate is it?

The men’s equation was fitted on 602 men. Its typical error is about 3.5 percentage points of body fat. The women’s was fitted on 214 women, with a typical error of about 3.7 points. So for a result of 25%, your real figure is likely to be within about 4 points either way, somewhere around 21% to 29%.

That is too loose to compare yourself with a friend. It is good enough to follow your own change over time. Downtrend works out this estimate from your latest tape measurements and splits your weight into fat and lean. Whatever does the sum, measure the same way each time:

  1. Measure at the same time of day, for example in the morning before you eat.
  2. Use the same tape, and the same spots on your body.
  3. Measure every two to four weeks, not every day. A tape is less precise than a scale, and fat changes slowly.

If your waist shrinks while the scale stays still, you may be losing fat while your body holds more water, or while you add muscle. That is one more reason to look at more than one number. The waist-to-height ratio uses the same tape and takes a few seconds.

Sources

  1. Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men from body circumferences and height. Naval Health Research Center report 84-11, 1984.
  2. Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy women from body circumferences and height. Naval Health Research Center report 84-29, 1984.
  3. Siri WE, 1961, the conversion from body density to percent fat, as cited in both reports.

This article is general information, not medical advice. Talk to a doctor before a big change to what you eat or how you train, and especially if you are pregnant, under 18 or have a health condition.