BMI Calculator
A free BMI calculator helps you work out your Body Mass Index (BMI) from height and weight. BMI is widely used as a quick screening measure for underweight, healthy weight, overweight, and obesity in adults. Enter metric (cm, kg) or imperial (feet, inches, pounds) values above to see your score, color-coded category, position on the BMI scale, and the healthy weight range for your height. Optional age and gender fields are for your reference—they do not change the standard adult BMI formula.
What is BMI?
Body Mass Index (BMI) is a simple index of weight-for-height. It is defined as a person’s weight in kilograms divided by the square of their height in metres. The same formula is used worldwide for adult screening:
BMI = weight (kg) ÷ height (m)²
For example, someone who weighs 70 kg and is 1.70 m tall has BMI 70 ÷ (1.70 × 1.70) ≈ 24.2, which falls in the normal range. BMI does not diagnose disease on its own, but it helps flag when further assessment may be useful. Children and teens use age- and sex-specific growth charts rather than adult cut-offs.
BMI Categories Explained
The World Health Organization (WHO) defines these adult BMI categories for most populations aged 18+:
- Underweight — BMI below 18.5. May indicate insufficient nutrition or other health issues; discuss with a clinician if unintentional.
- Normal weight — BMI 18.5 to 24.9. Associated with lower average health risk for many adults, though individual factors still matter.
- Overweight — BMI 25.0 to 29.9. Suggests increased risk of conditions such as type 2 diabetes and cardiovascular disease over time.
- Obese — BMI 30.0 and above. Further classes (I, II, III) exist at 30, 35, and 40 for clinical planning.
Our calculator colors your result and marks your position on the scale bar so you can see how close you are to each band.
Limitations of BMI as a Health Measure
BMI treats all weight equally—it cannot tell muscle from fat. Athletes and strength trainers often score “overweight” despite low body fat. Older adults may have a “normal” BMI but lose muscle and gain visceral fat, which still carries metabolic risk.
BMI also does not account for ethnicity-specific risk (some guidelines suggest lower action thresholds for South Asian populations), pregnancy, or oedema. Waist circumference, body fat percentage, diet quality, activity, sleep, and blood markers give a fuller picture. Use this tool for screening and awareness, not as a substitute for medical advice.
Alternative Body Composition Metrics
Because BMI relies solely on height and total body weight, health professionals often pair it with other metrics to get a more accurate evaluation of body composition and cardiovascular risk. Here are some of the most common alternative measures:
- Waist-to-Height Ratio (WHtR) — Calculated by dividing waist circumference by height. Keeping your waist circumference to less than half your height is associated with lower risks of cardiovascular disease and diabetes. Unlike BMI, this directly accounts for abdominal fat distribution.
- Waist-to-Hip Ratio (WHR) — Divides waist measurement by hip measurement. According to the WHO, a WHR above 0.90 for men or 0.85 for women indicates abdominal obesity and a higher risk of metabolic complications.
- Body Fat Percentage (BFP) — The actual percentage of your total mass that is fat, as opposed to lean muscle mass, bone, and water. This is measured using tools like bioelectrical impedance scales, skinfold calipers, or DEXA scans. Healthy ranges are typically 8–19% for men and 21–33% for women.
- Tri-Ponderal Mass Index (TMI) — Calculated as weight divided by height cubed (weight ÷ height³). Some pediatric studies suggest TMI offers a more stable and accurate estimate of body fat levels in adolescents compared to BMI.
How to Maintain a Healthy BMI
If your BMI is outside 18.5–24.9, small sustainable changes often work better than crash diets. Focus on balanced nutrition (enough protein, fibre, and whole foods), regular physical activity (aim for both cardio and resistance training where safe), adequate sleep, and stress management. The healthy weight range shown on this page is the weight band that would place your height in the normal BMI category.
If you are underweight, a GP or dietitian can help rule out underlying causes and plan safe weight gain. If you are overweight or obese, personalised targets and support improve long-term outcomes. Track progress with our age calculator for milestones, or other AllCalcNow tools for everyday planning—but pair numbers with professional guidance when health is a concern.
Frequently Asked Questions
Healthy ranges, accuracy, sex differences, and athletes.
What is a healthy BMI for adults?
For most adults aged 18 and older, a BMI between 18.5 and 24.9 is considered the healthy weight range by the World Health Organization (WHO). Below 18.5 is classified as underweight; 25.0–29.9 is overweight; 30.0 and above is obese (further divided into Class I, II, and III obesity). These cut-offs apply broadly to both men and women, though individual health depends on many other factors including body composition, fitness level, family history, and metabolic markers like blood pressure and cholesterol.
Is BMI an accurate health measurement?
BMI is a useful screening tool for weight categories at the population level, but it does not measure body fat directly and cannot show where fat is stored. It may misclassify very muscular people as overweight or miss health risks in people with normal BMI but high visceral fat (fat stored around internal organs). For a more complete picture, use BMI alongside waist circumference, body fat percentage, blood pressure, blood glucose, cholesterol levels, and clinical advice from a healthcare provider.
Is BMI different for men and women?
The standard adult BMI formula and WHO category thresholds are the same for men and women. However, body composition differs on average—men typically carry more muscle mass and less body fat at the same BMI compared to women. Some research suggests that health risk thresholds may differ slightly by sex, but official BMI bands do not change. The optional gender field on this calculator is for your personal records only; it does not alter the BMI score or category.
Can BMI be misleading for athletes?
Yes. Athletes and people with high muscle mass may have a BMI in the overweight or obese range despite having very low body fat. Muscle is denser than fat, so height–weight ratios alone overstate "excess weight" in muscular individuals. In those cases, body fat percentage (measured by skinfold calipers, DEXA scans, or bioelectric impedance), waist-to-hip ratio, or sport-specific performance assessments are more informative than BMI alone.
How is BMI calculated for children and teenagers?
For children and teens aged 2–19, BMI is calculated with the same formula (weight ÷ height²) but interpreted differently. Instead of fixed thresholds, the result is plotted on age-and-sex-specific growth charts from the CDC or WHO to determine a percentile. A BMI at or above the 85th percentile is considered overweight; at or above the 95th percentile is obese. This age-adjusted approach accounts for the natural changes in body composition that occur during growth and puberty.
Why is waist circumference important alongside BMI?
Waist circumference measures abdominal fat, which is strongly linked to health risks like type 2 diabetes, heart disease, and metabolic syndrome—even when BMI is in the normal range. A waist measurement above 40 inches (102 cm) for men or 35 inches (88 cm) for women indicates elevated risk. Combining BMI with waist circumference gives a much better picture of health risk than either measurement alone, which is why many health guidelines recommend tracking both.
Disclaimer: This BMI calculator is for general information only—not medical advice. Consult a healthcare provider for personal health decisions.