Body Fat Calculator

What body fat percentage tells you

The scale gives you one number, your weight, but it cannot tell you what that weight is made of. Two people can weigh exactly the same and look completely different, because one is carrying more muscle and the other more fat. Body fat percentage is the number that separates the two: how much of your total weight is fat.

It is a more useful figure than weight alone, and often more useful than BMI, which also cannot tell muscle from fat and will happily label a lean, muscular person "overweight". This calculator estimates your body fat using the tape-measure method developed by the US Navy, which needs nothing more than a soft measuring tape and a couple of measurements. Below is how to take them, the formula behind the result, and how far to trust it.

How to measure yourself properly

This is the part that decides whether your result is any good. The method is only as accurate as your tape work, and small slips in placement can shift the answer by a percentage point or two. Use a soft tape, keep it snug but not digging in, and stand relaxed without sucking in.

• Neck. Measure just below the Adam's apple, with the tape sloping very slightly downward at the front. Do not flex or flare your neck.
• Waist. For men, measure horizontally at navel level. For women, measure at the narrowest point of the waist. Breathe out normally first, and do not pull your stomach in.
• Hip (women only). Measure around the widest part of the hips and buttocks, keeping the tape level all the way round.

A good habit is to take each measurement twice and use the average, and to measure at the same time of day when you track it over time. Morning, before eating, is the usual choice.

The formula this calculator uses

The result comes from the US Navy circumference equations, developed by Hodgdon and Beckett at the Naval Health Research Center in 1984. They estimate body density from the ratios between your measurements, then convert that to a fat percentage. In their sex-separated form, with every measurement in centimetres:

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) + 0.221 × log₁₀(height)) − 450

The women's version adds the hip measurement, because body fat tends to sit differently, which is why the tool asks women for that extra number. The log terms look intimidating, but they are just the shape the Navy's data took when researchers fit measurements against laboratory body fat. You do not have to touch any of it; the calculator handles the maths once you enter your tape readings.

A worked example

Take a man who is 180 cm tall, with a 38 cm neck and a 90 cm waist. His waist minus neck is 52 cm. Dropping that into the men's equation:

Body fat % = 495 ÷ (1.0324 − 0.19077 × log₁₀(52) + 0.15456 × log₁₀(180)) − 450
Body fat % = 495 ÷ 1.0536 − 450
Body fat % ≈ 19.8%

So he is sitting around 19.8 percent body fat, which lands in the average range for a man. Notice what drives the result: for the same height, a bigger waist relative to the neck pushes the number up. That is the whole idea. Your waist is standing in as a proxy for the fat you carry.

What is a healthy body fat percentage?

There is no single "correct" number, and healthy ranges differ quite a bit between men and women, largely because women carry more essential fat. These are the widely used categories from the American Council on Exercise:

Treat these as general fitness bands, not a medical diagnosis. Essential fat is the minimum your body needs to function; dropping to or below it is not a goal to chase, and is genuinely risky, especially for women. Where you want to sit within the healthy range is a personal call that depends on your goals and your health.

How accurate is it, and where it slips

For a method that needs only a tape measure, it holds up well. The Navy equations land within roughly 3 to 4 percentage points of hydrostatic (underwater) weighing, a laboratory reference, which is close enough to track real change over time. But it is still an estimate, and a few honest caveats:

• Technique is everything. Because the whole thing rests on a few circumferences, sloppy or inconsistent tape placement is the biggest source of error. Consistency matters more than precision when you are tracking progress.
• It is least accurate at the extremes. For very lean or very heavy people, and for very muscular builds, the circumference method drifts furthest from the truth.
• It reads shape, not fat directly. It infers fat from where your body is wider or narrower, so an unusual build can throw it off. Methods like DEXA measure fat more directly, if you want a lab-grade number.

As a free, repeatable way to estimate body fat and watch it move, it is one of the better ones. It is not a medical assessment, and it will not diagnose anything. If a body fat number matters for a health reason, use this as a starting point and get a professional measurement to go with it.

Questions people ask

Is this better than BMI?

For understanding body composition, generally yes. BMI only knows your height and weight, so it cannot tell whether your weight is muscle or fat, and it misjudges muscular and "skinny-fat" people in opposite directions. Body fat percentage aims straight at the thing BMI can only guess at.

Why is this different from my smart scale?

Smart scales use bioelectrical impedance, sending a tiny current through your body, and the reading swings with how hydrated you are, so it can jump around day to day. The tape method is usually more stable and repeatable, as long as you measure consistently.

How do I lower my body fat percentage?

Two levers, and they work together: lose fat through a modest, sustained calorie deficit, and build or keep muscle through resistance training. Because the percentage is fat relative to total weight, adding muscle can nudge it down even if the scale barely moves. There are no shortcuts worth trusting here.

References

Where the figures come from. The equations this tool runs are the US Navy circumference formulas from Hodgdon and Beckett's 1984 Naval Health Research Center reports, and they are reproduced above as the tool applies them. The healthy-range categories are the body composition norms published by the American Council on Exercise.

1. Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men (and women) from body circumferences and height. Naval Health Research Center, San Diego, 1984. Technical Reports 84-11 and 84-29. https://apps.dtic.mil/sti/citations/ADA143890
2. American Council on Exercise (ACE). Percent body fat categories (Essential, Athletes, Fitness, Average, Obese).