BMI Calculator

Calculate Body Mass Index from height and weight, classify the result with WHO adult BMI categories, and see the healthy weight range for your height.

BMI Categories — Screening Tool, Not Body Composition

BrainyCalculators editorial insight — unique to this tool

BMI = weight(kg) / height(m)² — WHO classifies 18.5–24.9 as normal, but athletes with high muscle mass may read "overweight" at 27+ despite low body fat. Indian and Asian populations face elevated type-2 diabetes risk at BMI 23+ (lower cutoff than Western charts). BMI ignores fat distribution — waist circumference adds metabolic risk context.

When to use this calculator

Use for population-level weight screening. For body fat percentage, use Body Fat; for calorie targets, use BMR or TDEE.

Reference	Value	Context
Underweight	< 18.5	WHO
Normal	18.5–24.9	WHO
Asian overweight	≥ 23	WHO Western Pacific
Obese Class I	≥ 30	WHO

Need lean mass or ideal-weight formulas?

This page is a BMI screening tool. For body composition and lean mass estimates, start with the Lean Body Mass Calculator →

Height (cm)

Weight (kg)

What is BMI (Weight-to-Height Screening)?

Body Mass Index (BMI) is a weight-to-height screening ratio: weight in kilograms divided by height in metres squared. It is widely used because it is fast, inexpensive, and comparable across adult populations, but it estimates risk category rather than measuring fat directly.

This page is best for quick adult weight-status screening: underweight, normal, overweight, and obesity classes. It also calculates the weight range that corresponds to BMI 18.5-24.9 for your height. BMI is useful as a first-pass flag for health risk, not a diagnosis.

BMI cannot distinguish muscle from fat and can misclassify athletes, older adults, and people with unusual body composition. For non-fat mass and protein targets, use Lean Body Mass. For clinical target-weight formulas, use Ideal Weight.

BMI Formula

Metric: BMI = weight (kg) ÷ height² (m²)

Imperial: BMI = 703 × weight (lbs) ÷ height² (inches²)

How the BMI Calculation Works

The metric formula divides weight in kilograms by the square of height in metres. Height is squared because human body size scales in two dimensions — a taller person has proportionally more body area, not just more length. The imperial formula multiplies by 703 to convert from pounds per square inch into the same kilogram-per-square-metre scale as the metric version.

Once calculated, the BMI value is compared against WHO-defined thresholds to assign a weight category. This calculator also computes the healthy weight range for your height — the weight range that would produce a BMI between 18.5 and 24.9.

Worked Examples

Example 1 — Metric (Normal Weight)

Person: 70 kg, 175 cm tall.

Height in metres = 175 ÷ 100 = 1.75 m

BMI = 70 ÷ (1.75²) = 70 ÷ 3.0625

BMI = 22.9 — Normal Weight

Healthy weight range: 56.7 kg – 76.3 kg

Example 2 — Imperial (Overweight)

Person: 195 lbs, 5 ft 9 in (69 inches) tall.

BMI = 703 × 195 ÷ (69²)

BMI = 137,085 ÷ 4,761

BMI = 28.8 — Overweight

Healthy weight range: 125 lbs – 168 lbs

Real-World Applications

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Clinical Screening

Doctors use BMI as a first-step screening for weight-related health risks including diabetes and cardiovascular disease.

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Public Health Research

Epidemiologists track obesity trends across populations using BMI data from national surveys.

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Fitness Goal Setting

Athletes and gym-goers use BMI to set weight targets, though they often complement it with body fat percentage.

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Drug Dosing

Some medications are dosed based on body weight and BMI, particularly in oncology and bariatric care.

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Insurance Risk Assessment

Life and health insurers may use BMI as one factor among many when evaluating underwriting risk.

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Nutrition Planning

Dietitians use BMI alongside other metrics to estimate calorie needs and set realistic dietary goals.

Advantages of BMI

✓ Fast and free — requires only height and weight
✓ Universally understood with standardised WHO cutoffs
✓ Statistically correlated with obesity-related disease risk
✓ Useful for tracking changes in weight status over time

Limitations of BMI

✗ Cannot differentiate muscle from fat mass
✗ Ignores body fat distribution (visceral vs. subcutaneous)
✗ Less accurate for the elderly, children, and some ethnic groups
✗ Athletes may be misclassified as overweight

Common Mistakes

Using Height in cm Instead of m (Metric)

The metric formula requires height in metres. Forgetting to divide cm by 100 produces a BMI hundreds of times too small.

Mixing Unit Systems

Using kg for weight with inches for height, or lbs with metres, gives a meaningless result. Be consistent with your chosen unit system.

Treating BMI as a Diagnostic Tool

BMI screens for weight status — it does not diagnose disease. A high BMI is a flag to investigate further, not a medical diagnosis on its own.

Ignoring Age and Sex Differences

The same BMI can indicate different health risks in men vs. women or in older vs. younger adults. Context matters.

Applying Adult Cutoffs to Children

Children's BMI is interpreted using age- and sex-specific growth charts, not the fixed adult thresholds of 18.5/25/30.

Understanding Your BMI Result

BMI Range	Category	Health Implications
< 18.5	Underweight	Risk of malnutrition, osteoporosis, immune deficiency
18.5 – 24.9	Normal Weight	Lowest risk for weight-related chronic diseases
25.0 – 29.9	Overweight	Increased risk of hypertension, type 2 diabetes
30.0 – 34.9	Obese Class I	High risk — lifestyle changes strongly recommended
35.0 – 39.9	Obese Class II	Very high risk — medical evaluation advised
≥ 40	Obese Class III	Extreme risk — clinical intervention typically required

BMI vs. Other Body Composition Measures

Measure	What It Measures	Equipment Needed
BMI	Weight-to-height ratio proxy	Scale + measuring tape
Body Fat %	Actual fat mass as % of total	Callipers or DEXA scan
Waist Circumference	Abdominal (visceral) fat indicator	Measuring tape
Waist-to-Hip Ratio	Fat distribution pattern	Measuring tape
DEXA Scan	Lean mass, fat, bone density	Clinical DXA machine
Bioelectrical Impedance	Estimated fat + lean mass	BIA scale or handheld device

How the BMI Calculator Works

Formula, assumptions, and calculation steps for this health tool.

Formula Used

BMI = weight(kg) / height(m)^2

Methodology

Health calculators use published screening formulas and common planning rules to estimate body, nutrition, pregnancy, or fitness metrics from user inputs.

Calculation Steps

Enter the personal measurements requested by the tool.
Convert height, weight, age, dates, or activity inputs to standard units.
Apply the health or fitness formula for the selected metric.
Show the estimate with practical ranges or interpretation where available.

Assumptions and Limits

Results are educational estimates, not diagnosis or medical advice.
Individual factors such as medication, pregnancy, and medical history can change interpretation.
Consult a clinician for personal health decisions.

Reference basis: Common public-health and sports-science screening formulas.

Frequently Asked Questions

What is a healthy BMI range for adults?

For adults, the WHO defines a healthy BMI as 18.5 to 24.9. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese. These thresholds apply to most adult populations but may vary slightly by ethnicity — Asian populations have lower risk thresholds.

Is BMI an accurate measure of health?

BMI is a useful screening tool but not a direct health measure. It does not distinguish between fat and muscle, does not measure visceral fat, and does not account for age, sex, or body frame. It should be considered alongside blood pressure, cholesterol, blood sugar, and a physical examination.

Does BMI differ for children and teens?

Yes. Children and teenagers (ages 2–19) are assessed using age- and sex-specific percentile charts, not the fixed adult cutoffs. A BMI at or above the 95th percentile for their age and sex is classified as obese.

Why does the imperial formula use 703?

The factor 703 converts the imperial result (lbs/in²) into the same scale as the metric result (kg/m²). Without it, the numbers would be physically identical but dimensionally different and not comparable to the WHO reference ranges.

Can I have a high BMI but still be healthy?

Yes — particularly for highly muscular individuals such as bodybuilders or rugby players. Their BMI may reach 28–32 despite very low body fat. This is why clinicians use BMI as a starting point and confirm with additional assessments.

What BMI is considered morbidly obese?

A BMI of 40 or above is classified as Obese Class III, sometimes called morbid or extreme obesity. At this level, the associated health risks (sleep apnoea, heart disease, joint problems, type 2 diabetes) are severe enough that surgical intervention is often evaluated.

How accurate is the healthy weight range shown?

The healthy weight range is calculated as the weight that produces a BMI of 18.5 to 24.9 for your height. It is mathematically precise for those formulas but reflects population averages — your ideal weight may differ based on muscle mass, bone density, and individual health factors.

How can I lower my BMI?

BMI is lowered by reducing body weight, which requires a sustained calorie deficit through dietary changes, increased physical activity, or both. Evidence suggests a 500–750 calorie daily deficit produces roughly 0.5–0.75 kg of weight loss per week. Consult a healthcare provider before starting any weight-loss programme.

Does BMI work the same for all ethnicities?

No. Research shows that South Asian, East Asian, and some other populations have higher metabolic risk at lower BMI values. The WHO recommends lower action thresholds for some Asian populations — overweight starting at BMI 23 and obese at BMI 27.5.

What should I do if my BMI is in the obese range?

Consult a healthcare professional for a full assessment. Do not rely solely on BMI. A doctor may evaluate waist circumference, blood lipids, blood pressure, fasting glucose, and family history before recommending a course of action, which may include dietary changes, exercise, medication, or referral to a specialist.

References

World Health Organization. Body mass index — BMI. who.int
Centers for Disease Control and Prevention. About Adult BMI. cdc.gov
National Heart, Lung, and Blood Institute. Assessing Your Weight and Health Risk. nhlbi.nih.gov
Flegal, K.M. et al. Association of All-Cause Mortality With Overweight and Obesity Using Standard Body Mass Index Categories. JAMA, 2013.
Nuttall, F.Q. Body Mass Index: Obesity, BMI, and Health. Nutrition Today, 2015.