How is BMI calculated?

BMI = weight(kg) / height(m)². In imperial: BMI = (weight(lbs) × 703) / height(in)². Categories: Underweight <18.5, Normal 18.5–24.9, Overweight 25–29.9, Obese ≥30 (WHO classification).

What is a healthy BMI?

According to the WHO, a BMI between 18.5 and 24.9 is considered normal/healthy. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese.

What is Waist-to-Height Ratio?

Waist-to-Height Ratio (WHtR) = waist circumference / height. A ratio below 0.5 is generally considered healthy. It is a strong predictor of cardiovascular risk and metabolic disease.

What are the limitations of BMI?

BMI doesn't distinguish between muscle and fat. Athletes may have high BMI but low body fat. Elderly may have normal BMI but high body fat. Ethnic groups have different risk thresholds at the same BMI. Use BMI as a screening tool, not a definitive health assessment.

Is BMI accurate for everyone?

BMI is less accurate for: athletes (high muscle mass), elderly (low muscle mass), pregnant women, and children (use age-specific percentiles). Ethnic groups also have different health risks at the same BMI — Asian populations face higher diabetes risk at lower BMI.

How can I reduce my BMI?

Evidence-based strategies: create a calorie deficit (500–750 kcal/day for 0.5–1 kg/week weight loss), increase protein intake (25–30% of calories), strength training to preserve muscle, and regular cardio. Aim for 0.5–1 kg/week weight loss for sustainable results.

What is the difference between BMI and body fat percentage?

BMI is a height-weight ratio. Body fat percentage measures actual fat mass vs. total mass. A bodybuilder may have BMI 30 (obese by BMI) but 10% body fat (very lean). Body fat percentage is more accurate for health assessment but requires specialized measurement (DEXA, calipers, BIA scales).

BMI Calculator – Body Mass Index

Your Measurements

Unit System

Metric (cm / kg) Imperial (ft+in / lbs)

Height (cm)

Weight (kg)

Waist Circumference (cm) – optional Used to calculate Waist-to-Height Ratio

Your BMI

0

–

Healthy Range

18.5 – 24.9

Waist-to-Height

–

Ideal Weight

–

BMI Categories (WHO)

Underweight< 18.5

Normal weight18.5 – 24.9

Overweight25 – 29.9

Obese≥ 30

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// The Formula

BMI = weight (kg) ÷ height (m)²

Imperial: BMI = (weight lbs × 703) ÷ height (in)²

< 18.5

Underweight

May indicate malnutrition or underlying health issue

18.5–24.9

Normal Weight

WHO-defined healthy range for most adults

25–29.9

Overweight

Elevated risk for metabolic conditions

≥ 30

Obese

Significantly increased risk for chronic disease

One Metric Among Many

BMI: Useful Starting Point, Not the Full Story

BMI screens for weight-related health risk at population scale, but it cannot distinguish muscle from fat. Athletes regularly score 'overweight' despite low body fat. Use BMI alongside waist-to-height ratio for a more complete picture.

// Understanding your result

How to Read Your BMI

BMI was designed as a population-level screening tool, not a personal health diagnosis. It correlates with body fat at the population level but systematically misclassifies muscular individuals, older adults (who lose muscle), and some ethnic groups.

The Waist-to-Height Ratio (WHtR) is increasingly recommended alongside BMI. A WHtR below 0.5 (waist less than half your height) is associated with lower metabolic and cardiovascular risk across all ethnicities.

Asian populations: WHO guidelines note that health risks begin at lower BMI thresholds for South and East Asian adults — approximately 23+ for overweight and 27.5+ for obese.

// Beyond the number

BMI Limitations

Athletes and muscular individuals

High muscle mass increases BMI without elevated health risk. A professional rugby player may score 27–30 with very low body fat percentage.

Older adults and 'normal-weight obesity'

Age-related muscle loss means normal BMI can coexist with high body fat. This 'skinny fat' state carries similar metabolic risks to obesity. Waist-to-height ratio is a better predictor.

Children and adolescents

WHO adult thresholds do not apply. Children require age- and sex-specific percentile charts. A paediatrician should interpret BMI for anyone under 18.

// Common Questions

Frequently Asked Questions

BMI = weight(kg) / height(m)². In imperial: BMI = (weight lbs × 703) / height(in)². WHO classifies: Underweight (<18.5), Normal (18.5–24.9), Overweight (25–29.9), Obese (≥30).

WHO defines 18.5–24.9 as normal weight for most adults. Research suggests optimal health outcomes at BMI 22–23. For South and East Asian adults, risks emerge at lower thresholds (overweight at 23+, obese at 27.5+).

WHtR = waist circumference ÷ height (same units). Below 0.5 is healthy; 0.5–0.6 elevated risk; above 0.6 high risk. It captures abdominal fat more accurately than BMI and is consistent across ethnicities.

Muscle mass typically decreases with age, so BMI underestimates body fat in older adults. For adults over 65, a BMI of 25–27 may be associated with better outcomes. For children, age- and sex-specific percentile charts replace adult thresholds.

No. BMI cannot distinguish muscle from fat and does not account for fat distribution, age, sex, or ethnicity. Athletes often register 'overweight' despite low body fat. Use it as a screening indicator, not a diagnosis.

Yes — 'normal-weight obesity' or 'skinny fat.' People with normal BMI but high body-fat percentage can have the same metabolic risks as obese individuals. Waist-to-height ratio and biomarkers give a more complete picture.

The calculator shows ideal weight at the midpoint of the healthy BMI range (BMI 21.75). For a 175 cm person, that is approximately 66 kg. The healthy range, not a single number, is more meaningful than a fixed target.

For most adults, once or twice a year is sufficient unless actively tracking a weight change. BMI changes slowly — weekly checks create anxiety without useful data. A 3–6 month trend is more meaningful than any single reading.

// Methodology & References

Formula & Calculation Method

BMI (Metric)

BMI = weight(kg) / height(m)²

weight — Body weight in kilograms
height — Height in meters

Source: World Health Organization (WHO) standard

BMI (Imperial)

BMI = (weight(lbs) × 703) / height(in)²

weight — Body weight in pounds
height — Height in inches

Source: CDC BMI calculator standard

Waist-to-Height Ratio (WHtR)

WHtR = waist / height

waist — Waist circumference (same units)
height — Height (same units)

Source: Ashwell & Hsieh (2005); threshold 0.5 = increased cardiometabolic risk

Authoritative Sources & Standards

WHO: WHO BMI Classification (1995, updated 2004): Underweight <18.5, Normal 18.5–24.9, Overweight 25–29.9, Obese ≥30. Asia-Pacific cutoffs differ (Obese ≥27.5). → WHO
CDC: CDC adopts WHO adult BMI cutoffs but uses growth percentiles for children/teens (2–19 years). → CDC
NIH: NIH National Heart Lung and Blood Institute classifies waist circumference >40in (M) / >35in (F) as elevated cardiovascular risk regardless of BMI. → NIH

Expert Insights & Research

BMI misclassifies body composition in 20–30% of individuals (Romero-Corral et al., 2008). Athletes with high muscle mass register 'overweight'; sedentary 'normal-BMI' individuals may have high visceral fat (Normal Weight Obesity).

— Romero-Corral et al., International Journal of Obesity (2008) (2008)

Waist-to-Height Ratio outperforms BMI as a predictor of cardiovascular mortality (meta-analysis of 31 studies, 300,000+ participants).

— Ashwell, Gunn & Gibson, Obesity Reviews (2012) (2012)

// Explore More

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