The most common way to calculate body mass is using the Body Mass Index formula: divide your weight in kilograms by your height in meters squared. If you weigh 70 kg and stand 1.75 m tall, your BMI is 70 ÷ (1.75 × 1.75) = 22.9. For pounds and inches, the formula is slightly different, and there are also methods that go beyond BMI to estimate how much of your weight is fat versus muscle.
The BMI Formula in Metric and Imperial
BMI gives you a single number that represents your weight relative to your height. It doesn’t measure fat directly, but it’s the standard screening tool used worldwide. Here are the two versions of the formula:
- Metric: BMI = weight (kg) ÷ [height (m) × height (m)]
- Imperial: BMI = [weight (lb) ÷ (height (in) × height (in))] × 703
The 703 multiplier in the imperial version is a conversion factor that accounts for the difference between pounds/inches and kilograms/meters. So if you weigh 160 pounds and are 5’8″ (68 inches), the calculation is: 160 ÷ (68 × 68) × 703 = 24.3.
If you’re working in centimeters instead of meters, you can use this variation: weight (kg) ÷ height (cm) ÷ height (cm) × 10,000. The result is identical.
What BMI Categories Mean
The World Health Organization defines four main ranges for adults:
- Underweight: below 18.5
- Normal weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obese: 30 or higher
These thresholds apply to adults of all ages but not to children. For kids and teens aged 2 through 19, BMI is plotted on growth charts and expressed as a percentile compared to other children of the same sex and age. A child at the 75th percentile, for example, has a higher BMI than 75% of peers. Healthy weight falls between the 5th and 85th percentiles, overweight is the 85th to 95th, and obesity is at or above the 95th percentile.
Why BMI Has Limitations
BMI treats all weight the same. It can’t tell whether your 200 pounds comes from muscle, fat, bone, or water. Research from Cedars-Sinai found that BMI misclassifies roughly 60% of women and 13% of men when used to determine obesity status. A muscular person can register as overweight despite having low body fat, while someone with little muscle and a normal BMI might carry more fat than is healthy.
This is why many health professionals now recommend pairing BMI with at least one other measurement.
Waist-to-Height Ratio
One of the simplest alternatives is dividing your waist circumference by your height, both in the same unit. A ratio under 0.5 is generally considered healthy for younger adults. For people over 60, a threshold closer to 0.6 appears more appropriate, based on a large study of over 29,000 participants published in The Lancet Regional Health. That study found waist-to-height ratio was the strongest single predictor of cardiometabolic risk, outperforming BMI.
All you need is a tape measure. Wrap it around the narrowest part of your waist, typically near your belly button for men, and divide that number by your height.
Relative Fat Mass (RFM)
Relative Fat Mass is a newer formula that estimates body fat percentage using only your height and waist circumference. No scale required.
- Men: 64 − (20 × height ÷ waist circumference) = RFM
- Women: 76 − (20 × height ÷ waist circumference) = RFM
Height and waist need to be in the same unit (both inches or both centimeters). A study by the team that developed RFM found it predicts body fat percentage more accurately than BMI, particularly for women. The result represents an estimated percentage of your total weight that is fat.
The U.S. Navy Body Fat Method
If you want a more detailed body fat estimate using a tape measure, the U.S. Navy method uses circumference measurements and a logarithmic formula. Men need to measure their neck and waist. Women need neck, waist, and hip circumference.
For men, the formula is: body fat (%) = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76, with all measurements in inches. For women: body fat (%) = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387.
To take the measurements correctly: measure your neck at its narrowest point just below the Adam’s apple, your waist at the narrowest point above your hip bones, and (women only) your hips at the widest point of your glutes with feet together. Pull the tape snug but not tight, and keep it level.
Lean Body Mass
Lean body mass is everything in your body that isn’t fat: muscle, bone, organs, water. One widely used formula is the Boer equation, which uses your weight in kilograms and height in centimeters:
- Men: (0.407 × weight) + (0.267 × height) − 19.2
- Women: (0.252 × weight) + (0.473 × height) − 48.3
For a man weighing 80 kg at 180 cm, lean body mass would be roughly (0.407 × 80) + (0.267 × 180) − 19.2 = 61.4 kg. That means about 18.6 kg of his weight is fat. Knowing your lean mass is useful for setting protein intake targets, tracking muscle changes over time, or understanding whether weight loss is coming from fat or muscle.
Getting Accurate Measurements
Any formula is only as good as the numbers you feed it. Small errors in height or weight can shift your BMI by a full point or more.
For height, stand barefoot against a wall with your heels, buttocks, and shoulder blades touching the surface. Look straight ahead so your chin is level, not tilted up or down. Have someone place a flat object (a book works fine) on top of your head, pressing gently, and mark where it meets the wall. Measure in the morning if possible, since you can lose up to a centimeter of height over the course of a day as your spinal discs compress.
For weight, use the same scale at the same time of day, ideally in the morning before eating, wearing minimal clothing. Digital scales on a hard, flat surface tend to be most consistent. Weigh yourself twice and take the average if the readings differ.
For waist and hip measurements, use a flexible tape measure directly against your skin rather than over clothing. Stand naturally and breathe out gently before reading the tape. Take two measurements and average them.
Body Mass vs. Body Weight
You might notice that “body mass” and “body weight” are used interchangeably in everyday life, and for practical purposes that’s fine. Technically, mass is the amount of matter in your body, measured in kilograms, while weight is a force that depends on gravity. Your mass stays the same whether you’re on Earth or the moon, but your weight changes. On a bathroom scale at sea level, the distinction is meaningless: the number you read effectively represents your mass. In biology and medicine, kilograms refers to mass even though scales technically measure gravitational force.