BMI Calculator (Body Mass Index)

Body Mass Index is a simple ratio of your weight to the square of your height. It doesn't measure body fat directly — instead, it estimates whether your weight is proportional to your height, using a formula research has linked, on average across large populations, to health risks like heart disease, diabetes, and certain cancers.

A Belgian mathematician named Adolphe Quetelet developed the formula in the 1830s while studying population averages, originally calling it the Quetelet Index. It stayed mostly inside academic circles for over a century until physiologist Ancel Keys renamed it the Body Mass Index in 1972, after finding it worked about as well as more complicated body fat tests while being far easier to calculate.

BMI is now the standard weight-screening tool used by the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC), largely because it's free, fast, and needs no special equipment beyond a scale and a tape measure. Both organizations are explicit on one point that's worth repeating: BMI is a screening tool, not a diagnostic one. A high or low result is a signal to look closer, not a medical diagnosis on its own.

There are two versions of the formula, depending on which unit system you use.

Metric

Imperial

Both formulas return the same BMI for the same person — 703 exists purely so pounds and inches line up with the same scale kilograms and meters produce natively.

Calculating BMI by hand takes three steps: square the height, then divide the weight by it.

Metric example — 70 kg, 1.75 m tall

1. Square the height: 1.75 × 1.75 = 3.0625 m²
2. Divide weight by that number: 70 ÷ 3.0625 = 22.9
3. Result: BMI = 22.9 — Normal category

Imperial example — 160 lb, 5'6" (66 in) tall

1. Square the height: 66 × 66 = 4,356 in²
2. Multiply weight by 703: 160 × 703 = 112,480
3. Divide by the squared height: 112,480 ÷ 4,356 = 25.8 — Overweight category

A 5-pound difference or a couple of inches can shift someone from Normal into Overweight, which is exactly why the calculator above is faster and less error-prone than doing the math by hand — especially when converting between feet/inches and centimeters.

This is the WHO's standard eight-tier adult classification (ages 20 and up), applying equally to men and women.

Find your height, then scan across to the weight closest to yours to see your approximate BMI.

Values are rounded to one decimal place. Use the calculator above for your exact figure and any height or weight not shown here.

For most adults, a BMI between 18.5 and 24.9 is considered the healthy range, linked to the lowest average risk of weight-related conditions like type 2 diabetes, high blood pressure, and heart disease.

"Ideal weight" is a related but different idea — the specific weight that would put your BMI near the middle of that range, given your height. If you'd rather see that number directly instead of working backward from BMI, the Ideal Weight Calculator does that conversion for you. If your goal is to move toward the healthy range, the Weight Loss Calculator can help map out a realistic timeline.

The 18.5–24.9 healthy range and the classification chart above apply directly to adults aged 20 and older, using fixed numeric cutoffs rather than age-adjusted percentiles, since bone and muscle growth has stabilized by adulthood.

The formula and category cutoffs are identical for men and women — there's no separate scale by sex. What differs is typical body composition at the same BMI: men usually carry more muscle and less fat than women at an equivalent score, a normal physiological difference rather than a flaw in the formula. If you want a closer read specific to your body composition, the Body Fat Calculator goes a step further than BMI alone.

Adult BMI categories don't work for children and teens. Kids' bodies change shape constantly as they grow, so a fixed cutoff like "25 = overweight" makes no sense when proportions are still developing. Instead, the CDC and WHO compare a child's BMI against a growth chart of other children the same age and sex, producing a BMI-for-age percentile rather than a fixed category: below the 5th percentile is underweight, 5th–84th is healthy weight, 85th–94th is overweight, and 95th and above is obesity. Parents should use a dedicated pediatric BMI tool or consult a pediatrician rather than applying this adult calculator to a child.

• Free and instant — no cost, no equipment beyond a scale and a tape measure
• Globally standardized — the same formula and categories are used by WHO, CDC, and health systems worldwide
• No special training required to calculate or understand your own result
• Useful for tracking meaningful weight trends over months or years
• Backed by decades of large-scale population health research
• Gives you a concrete number to bring to a doctor or dietitian

BMI is a screening tool, not a full health assessment, and it has real blind spots worth knowing:

• Muscle mass — BMI can't tell muscle from fat, so muscular individuals can register as overweight despite low body fat
• Pregnancy — expected weight gain means standard adult categories don't apply
• Older adults — muscle naturally declines with age while fat can increase at the same BMI
• Athletes — dense muscle and larger bone structure can push fit individuals into higher categories
• Children — need age- and sex-specific percentile charts, not adult cutoffs
• Body composition — two people with the same BMI can have very different fat, muscle, and bone density
• Ethnicity — some populations face higher health risk at lower BMI thresholds than the standard chart suggests
• Fluid retention — some medical conditions can temporarily inflate BMI without any change in body fat

• Diet — calorie intake and food quality are the biggest lever on weight and BMI
• Exercise — affects both weight and body composition, which can shift BMI even without a scale change
• Sleep — poor sleep disrupts hunger hormones, often increasing appetite
• Age — metabolism slows and muscle mass declines with age, which can raise BMI over time
• Hormones — thyroid function, menopause, and other shifts can influence weight independent of diet
• Medical conditions — conditions like hypothyroidism or PCOS, and some medications, can cause weight change unrelated to lifestyle
• Genetics — family history shapes natural body size, metabolic rate, and fat distribution

1. Track your BMI monthly or quarterly, not obsessively.
2. Prioritize protein at each meal — the Protein Calculator finds your daily target.
3. Know your calorie needs before trying to lose or gain weight — try the Calorie Calculator.
4. Understand your metabolism with the BMR Calculator, the foundation for any weight goal.
5. Add strength training, not just cardio — building muscle improves body composition even when the scale barely moves.
6. Get 7–9 hours of sleep most nights to support healthy hunger regulation.
7. Limit ultra-processed foods in favor of whole foods.
8. Stay consistently hydrated — thirst is often mistaken for hunger.
9. Set realistic weekly goals: 1–2 lb (0.5–1 kg) of change per week is sustainable.
10. Manage stress — chronically high cortisol is linked to increased abdominal fat storage.
11. Move throughout the day, not just during workouts.
12. Check your total daily energy expenditure with the TDEE Calculator for a more precise calorie target than BMR alone.
13. Reassess after major life changes — pregnancy, injury, illness, or a new medication can shift your baseline.
14. Don't chase one "perfect number" — aim for the healthy range as a whole.
15. Combine BMI with other measurements, like waist circumference or body fat percentage, for more context.
16. Talk to a doctor or registered dietitian before making major changes.

• Using outdated height or weight — even small changes can shift your category near a boundary
• Mixing unit systems, like entering height in centimeters but weight in pounds
• Applying adult categories to a child or teen instead of age-adjusted percentiles
• Treating BMI as a diagnosis rather than a screening number
• Ignoring muscle mass when reading a high BMI as automatically unhealthy
• Checking BMI daily — normal water-weight fluctuation isn't meaningful; weekly or monthly is better
• Forgetting that standard categories don't apply during pregnancy

BMI is one tool among several — here's how it compares:

Use the calculator above to check your own exact numbers rather than estimating from a table — even a few pounds can move you into a different category.

Looking for BMI by a specific angle — formula, chart, category, or a particular group? These all use the same calculator and formula above, organized around different questions people search for.

This calculator and the information on this page are for general educational and screening purposes only. BMI is a screening tool, not a diagnostic measurement, and it does not account for individual factors like muscle mass, bone density, or medical history. This content is not a substitute for professional medical advice, diagnosis, or treatment — always consult a qualified healthcare provider before making decisions about your weight, diet, or exercise routine, especially if you have an existing health condition.