Body Mass Index (BMI) is a single number derived from your weight and height — calculated as weight in kilograms divided by height in metres squared. The World Health Organization classifies values below 18.5 as underweight, 18.5–24.9 as healthy, 25–29.9 as overweight, and 30 and above as obese. BMI is a quick screening tool, not a full health assessment.
What is BMI Calculator?
BMI was developed in the 1830s by a Belgian statistician named Adolphe Quetelet, originally as a population-level measure rather than an individual diagnostic. It became widely used by the medical community in the late 1900s as a simple way to screen large groups for weight-related health risks. The math is unchanged from Quetelet's original — weight in kilograms divided by height in metres squared.
BMI is intentionally simple. It does not measure body fat directly, does not distinguish between muscle and fat, and does not account for where the fat sits on your body. A muscular gym-goer at 85 kg and 175 cm has the same BMI as a sedentary person of the same dimensions. This is BMI's biggest limitation — and why doctors use it as a starting point, not a final verdict.
Despite its bluntness, BMI correlates well with body fat percentage at the population level. For most adults who are not athletes, a BMI in the healthy range (18.5–24.9) genuinely tracks with lower disease risk. For more nuanced individual assessment, BMI should be paired with waist-to-hip ratio, body fat percentage, and resting heart rate.
The BMI formula
BMI is one of the simplest formulas in clinical medicine. It produces a single number which is then mapped to a category — underweight, healthy, overweight, or obese. The categories were set by the WHO in 2000 and are used globally, with minor regional adjustments (Asian populations sometimes use a slightly lower overweight threshold of 23).
- Weight
- Body weight—in kilograms — convert from pounds with × 0.4536
- Height
- Height—in metres squared — convert cm to m by dividing by 100, then square it
How to use this calculator
Pick metric or imperial units, then enter your height and weight. The calculator returns your BMI, the category it falls into, and the healthy-weight range for your height.
Choose your unit system
Metric (kilograms and centimetres) or Imperial (pounds and inches). Most of the world uses metric for medical reporting; the US uses imperial. The result is the same number either way.
Enter your height
If using metric, in centimetres (e.g., 170). If imperial, in inches (e.g., 67 — that is 5'7''). Be precise — a 2 cm error changes the BMI by about 0.6 points.
Enter your weight
Best taken in the morning, after using the bathroom, with minimal clothing — that is when you weigh least and most consistently. Weigh on the same scale at the same time each measurement.
Read your category
The calculator shows your BMI value and the WHO category — underweight, healthy, overweight, or obese. It also shows the healthy-weight range for your height, which is more actionable than the BMI number alone.
Pair with other measurements
BMI alone is not enough for athletes, the elderly, pregnant women, or growing children. Combine it with waist circumference (over 94 cm for men or 80 cm for women indicates abdominal obesity) and body fat % for a clearer picture.
When BMI is and is not useful
Routine health screening
Most useful for adults aged 18-65 with average activity levels. A first quick check before deeper assessment.
Population-level public health studies
BMI shines at this scale — comparing obesity rates between countries, tracking trends over decades, identifying high-risk groups.
Insurance underwriting
Health insurance companies use BMI as a baseline metric for premium calculation. Out-of-range BMI may add a loading.
Fitness goal-setting
Use the healthy-weight range as a target. If you are 175 cm, the healthy range is roughly 56.7 kg to 76.3 kg — pick a target inside that range based on your build.
Pre-op risk assessment
Surgeons use BMI as one factor for surgical risk. Very high or very low BMI increases complication risk.
Pediatric growth tracking
Child BMI is plotted against age-and-sex-specific WHO growth charts (different from adult thresholds). The calculator above is for adults; pediatric BMI requires its own percentile chart.
Common mistakes to avoid
Using clothed-weight or evening-weight readings
Weigh first thing in the morning, post-bathroom, with minimal clothing. Same time, same scale, same conditions for any tracking.
Trusting BMI for muscular athletes
If you train heavily and have visible muscle, use a body fat percentage measurement instead — DEXA scan, bioimpedance, or skinfold calipers.
Calculating BMI for children using adult thresholds
Pediatric BMI uses age-and-sex-specific percentiles, not the 18.5-24.9 adult range. Use a WHO child growth chart instead.
Setting weight loss goals based on BMI alone
Target a healthy waist circumference and body fat percentage, not just a BMI number. You can hit a 'healthy' BMI while still being unhealthily fat or sarcopenic.
Ignoring the trajectory and only looking at the static value
A BMI of 27 trending downward is healthier than 26 trending upward. Track changes over months, not single snapshots.
Glossary
- BMI (Body Mass Index)
- Weight in kilograms divided by height in metres squared. A simple ratio used to screen for weight categories.
- Underweight
- BMI below 18.5. May indicate undernutrition, hyperthyroidism, or other underlying conditions. Worth discussing with a doctor.
- Healthy weight
- BMI between 18.5 and 24.9. The range associated with the lowest all-cause mortality risk in epidemiological studies.
- Overweight
- BMI 25 to 29.9. Increased risk of cardiovascular disease, diabetes, and joint problems. Often reversible with diet and exercise.
- Obese
- BMI 30 or above. Substantially elevated risk of metabolic syndrome, type 2 diabetes, sleep apnoea, and several cancers.
- Severely obese
- BMI 35 or above. Often classified as Class II (35-39.9) and Class III (40+) obesity. May warrant medical or surgical intervention.
- Body Fat Percentage
- Proportion of total body weight that is fat. More accurate than BMI for individual assessment. Healthy ranges are roughly 8-19% for men and 21-32% for women.
- Waist-to-Hip Ratio
- Waist circumference divided by hip circumference. Above 0.9 for men or 0.85 for women indicates abdominal obesity, regardless of BMI.
- Visceral fat
- Fat that surrounds internal organs, particularly dangerous metabolically. Distinct from subcutaneous fat. Best assessed by DEXA or MRI.
- Sarcopenic obesity
- Low muscle mass combined with high body fat — common in elderly. BMI may appear normal while muscle wasting is present.
Frequently asked questions
Is BMI accurate?
Why is my BMI in 'overweight' if I'm fit?
References
- WHO — Body mass index (BMI) classification— World Health Organization
- ICMR — Nutrient requirements and dietary guidelines for Indians— Indian Council of Medical Research
- WHO Global Health Observatory — Adult overweight and obesity— World Health Organization