How to Calculate Your BMI, and What It Can't Tell You About Body Composition

Body mass index is your weight in kilograms divided by the square of your height in meters. That is the whole formula. It needs a scale and a wall to stand against, which is why clinics, insurers and public health surveys all use it. What it cannot do is say whether the weight is muscle, fat, bone or water, or where on the body the fat sits. The first half of this page works out BMI by hand; the second half adds the measurements that fill in what it leaves out.

Working out BMI step by step

Metric: Lena, 165 cm and 68 kg

Convert height to meters and square it: 1.65 × 1.65 = 2.7225. Divide weight by that figure: 68 ÷ 2.7225 = 24.98. The BMI calculator rounds to one decimal place before it picks a category, so Lena sees 25.0 and the label Overweight, even though the unrounded value sits a hair under the line. Rounding first keeps the label consistent with the number on screen, but it is also a reminder that a reading at a cut-off carries no more meaning than one a fraction either side of it.

The same tool reports the weight range that would put someone of her height between 18.5 and 24.9: 50.4 to 67.8 kg. Lena is 0.2 kg above the top of that band, which is less than the swing most people see on a scale between morning and evening.

US units: Marcus, 5 ft 10 in and 200 lb

The pound-and-inch version multiplies by 703 to make up for the unit change. Height is 70 inches, so 70 × 70 = 4,900. Then 200 × 703 = 140,600, and 140,600 ÷ 4,900 = 28.69. The calculator itself converts to kilograms and meters with the exact definitions (1 lb = 0.45359237 kg, 1 in = 0.0254 m) rather than using 703, and lands on the same 28.7. At 70 inches its 18.5 to 24.9 range runs from 128.9 to 173.5 lb, so Marcus would need to lose about 26 lb to reach the upper edge.

The adult categories and where they come from

The ratio was described by the Belgian statistician Adolphe Quetelet in the nineteenth century. Ancel Keys and colleagues compared several weight-for-height indices in 1972, found this one tracked body fat best among them, and gave it the name body mass index. The World Health Organization classification for adults is:

BMI	WHO class
Below 18.5	Underweight
18.5 to 24.9	Normal range
25.0 to 29.9	Pre-obese (overweight)
30.0 to 34.9	Obese class I
35.0 to 39.9	Obese class II
40.0 and above	Obese class III

The site's calculator uses the four broad labels (underweight, normal weight, overweight, obese) and does not split obesity into classes. None of these cut-offs apply to children and teenagers. For ages 5 to 19, WHO reads BMI against growth references for the child's age and sex instead.

The categories were drawn from population data, where higher average BMI goes with higher rates of type 2 diabetes and heart disease. Applied to one person, the number is a screening flag. It tells a clinician whether to look further, not what they will find.

Same BMI, different bodies

Marcus has a twin on paper. Dave is also 5 ft 10 in and 200 lb, so his BMI is also 28.7. Marcus lifts weights five days a week; Dave drives for a living and has not trained in years. A tape measure separates them where the scale cannot.

The body fat calculator uses the US Navy circumference equations, which for men need height, waist at the navel, and neck:

• Marcus: waist 33 in, neck 17 in. Estimated body fat 11.1%, in the athletes band (6% to 13%) of the American Council on Exercise chart the tool uses. Of his 200 lb, about 22.2 lb is fat and 177.8 lb is lean mass.
• Dave: waist 41 in, neck 16 in. Estimated body fat 27.8%, in the chart's obese band (25% and above). That is about 55.6 lb of fat and 144.4 lb of lean mass.

Fat mass is simply body weight times the body fat fraction (200 × 0.111 for Marcus), and lean mass is what remains. Two men with an identical BMI differ by more than 33 lb of fat. BMI would rate them the same; their doctors would not.

The same check works for women, who add a hip measurement. Lena, at 165 cm with a 70 cm waist, 33 cm neck and 98 cm hips, comes out at 26.2%, which is the ACE average band for women (25% to 31%). Her borderline BMI and an unremarkable body fat estimate point the same way: nothing here needs action.

Keep the limits in view. The Navy equations were fitted to military personnel, and errors of several percentage points for an individual are normal, especially for older adults and unusual builds. The ACE bands are not adjusted for age. Gallagher and colleagues, working from the BMI cut-offs, proposed healthy body fat ranges that rise with age and run higher for women than men at every age. If you want a personal target, a clinician can tell you which reference fits your age, sex and measuring method.

Other ways to estimate body fat

Skinfold calipers depend heavily on who holds them and which equation they use. Bioelectrical impedance scales pass a small current through the body and infer fat from how easily it flows, so hydration, a recent meal or a workout can shift the reading. DEXA scanning, done in clinics and research labs, also maps bone density and where fat and lean tissue sit by body region. For tracking change at home, the tape method is adequate if you measure the same way at the same time of day every few weeks and watch the trend rather than any single result.

Waist size: the measurement BMI ignores

Fat stored around the abdominal organs carries more metabolic risk than fat under the skin of the hips and thighs, and BMI has no way to see it. The WHO expert consultation on waist circumference reviewed the evidence and set out the cut-offs most often quoted:

• Waist above 102 cm (about 40 in) for men or 88 cm (about 35 in) for women: substantially increased risk of metabolic complications.
• Waist-to-hip ratio of 0.90 or more for men, or 0.85 or more for women: also substantially increased risk.

Dave's 41-inch waist is past the men's threshold. Marcus, at 33 inches, is well under it. Lena's 70 cm waist is far below the women's figure. Of the three, only Dave has a measurement that independently says his BMI deserves a closer look.

Energy: BMR uses the same inputs and has the same blind spot

The BMR calculator estimates resting energy use with the Mifflin-St Jeor equation, which takes weight, height, age and sex. For men it is 10 × weight in kg + 6.25 × height in cm − 5 × age + 5.

• Marcus, 34: BMR 1,853 kcal a day. At the moderately active multiplier of 1.55 his maintenance estimate is 2,873 kcal.
• Dave, 46: BMR 1,793 kcal, and 2,152 kcal at the sedentary multiplier of 1.2.

The 60 kcal gap between their BMRs comes entirely from the 12-year age difference. The equation has no input for muscle, so it treats 177.8 lb of lean mass and 144.4 lb exactly alike. Read the figure as a starting point, then adjust it against what your weight actually does over a few weeks of steady eating.

Protein: sizing it for the body you have

The US adult Recommended Dietary Allowance for protein is 0.8 g per kg of body weight. A 2016 joint position from the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine puts athletes at 1.2 to 2.0 g/kg. The protein needs calculator applies a preset rate within those bounds for each activity level and goal.

Both men weigh 90.7 kg. At the RDA that is 73 g a day. Marcus picks Active and Maintain Weight, which uses 1.2 g/kg: 109 g a day, or about 27 g at each of four meals. Dave picks Sedentary and Lose Weight, which uses 1.0 g/kg: 91 g a day. A higher share of protein while eating less is meant to protect lean mass during weight loss, which is the point of tracking body fat rather than just weight. To fit those grams into a calorie budget, the macro calculator splits a daily total into protein, carbohydrate and fat.

Anyone with kidney disease, or anyone told to limit protein, should take a target from their own clinician rather than from a per-kilogram rule.

Using the numbers together

Each measure answers a narrower question than the one before it. BMI asks whether weight is unusual for height. Body fat asks what that weight consists of. Waist size asks where the fat is. BMR and protein targets turn the answers into a daily plan. For Marcus, a high BMI is explained by muscle and a small waist. For Dave, a high BMI, a high body fat estimate and a large waist agree, and that agreement is the useful signal.

If you change body composition, the scale can mislead: losing fat while gaining muscle may barely move it. Re-measure waist and body fat every few weeks under the same conditions. WHO advises adults to do muscle-strengthening activity on at least two days a week, and the CDC advises adults to sleep at least 7 hours a night; both matter more to the outcome than the precision of any formula.

These tools give general information, not a diagnosis. See a doctor if your BMI is under 18.5, if your waist is above the WHO thresholds, if you are losing weight without trying, or before starting a weight-loss plan when you have diabetes, heart disease or another long-term condition. The ideal weight calculator shows four clinical height-based formulas side by side; their disagreement is a fair summary of how much any single target weight should be trusted.