BMI Calculator

BMI Calculator — What Your Body Mass Index Actually Means

Body Mass Index (BMI) is one of the most widely used health screening tools in the world. It is a simple numerical value derived from your height and weight that helps determine whether your weight is in a healthy range. Use our free online BMI calculator above to instantly get your BMI score, BMI category, and healthy weight range — in both metric (kg/cm) and imperial (lbs/ft) units.

The BMI formula is straightforward: BMI = weight (kg) ÷ height² (m²). For imperial measurements the equivalent is: BMI = (weight in lbs ÷ height² in inches²) × 703. For example, a person who weighs 70 kg and stands 175 cm tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which sits comfortably in the normal-weight range.

According to the World Health Organization (WHO) , BMI is commonly used to classify underweight, normal weight, overweight, and obesity in adults.

BMI Categories — Understanding the Numbers

The World Health Organisation (WHO) has established four standard BMI categories for adults aged 18 and over:

• Below 18.5 — Underweight: May signal insufficient calorie intake, nutritional deficiencies, or an underlying medical condition. Being underweight is associated with weakened immunity, bone loss, and fatigue.
• 18.5 to 24.9 — Normal (Healthy) Weight: This is the target range associated with the lowest risk of weight-related health complications for most adults. Maintaining a BMI within this band through a balanced diet and regular exercise is considered ideal.
• 25.0 to 29.9 — Overweight: Slightly elevated risk of type 2 diabetes, high blood pressure, heart disease, and joint problems. Even modest weight loss of 5–10% of body weight can meaningfully reduce these risks.
• 30.0 and above — Obese: Significantly increased risk of cardiovascular disease, type 2 diabetes, sleep apnoea, fatty liver disease, and certain cancers. Class I obesity is 30–34.9, Class II is 35–39.9, and Class III (severe obesity) is 40 and above.

Healthy BMI Range by Age and Gender

While the standard BMI ranges apply broadly to all adults, research suggests that the relationship between BMI and body fat percentage varies by age and sex. Women naturally carry a higher proportion of body fat than men at the same BMI. As people age, muscle mass tends to decrease and fat mass increases even when body weight stays the same — so a BMI at the upper end of the normal range may carry slightly more risk in older adults.

For adults over 65, some research suggests that a slightly higher BMI (up to 27) is associated with lower mortality risk — sometimes called the "obesity paradox" — though this remains a topic of active research. Children and teenagers should be assessed using age- and sex-specific BMI-for-age percentile charts rather than adult cut-offs.

BMI for Men vs. BMI for Women

Despite differences in body composition, the same WHO BMI cut-off values apply to both men and women in most clinical settings. However, at the same BMI, women tend to have a higher body fat percentage than men, and men at the same BMI tend to have more lean muscle mass. For a 30-year-old male, a BMI of 24 might correspond to approximately 17–22% body fat; for a woman of the same age and BMI, body fat percentage is typically 5–8 points higher. This is why BMI should always be interpreted alongside other indicators such as waist circumference.

Limitations of BMI — What It Does Not Tell You

BMI is a population-level screening tool, not a definitive individual diagnostic measure. Its most significant limitation is that it cannot distinguish between muscle mass and fat mass. A professional bodybuilder or elite rugby player may register a BMI in the "overweight" or even "obese" range while having very low body fat and excellent metabolic health. Conversely, someone with a "normal" BMI may carry excess visceral (abdominal) fat — a condition sometimes called "normal weight obesity" or "skinny fat" — which carries meaningful metabolic risk.

BMI also does not capture fat distribution. Abdominal fat — measured by waist circumference — is a stronger predictor of cardiovascular risk and insulin resistance than total body weight. A waist circumference above 94 cm (37 inches) for men or 80 cm (31.5 inches) for women is considered an independent risk factor by the WHO, regardless of BMI category.

For people of South Asian, East Asian, and some other ethnic backgrounds, health risks associated with excess body fat tend to occur at lower BMI values. Many clinical guidelines for these populations use adjusted thresholds — for example, a BMI of 23 as the overweight cut-off and 27.5 as the obese cut-off.

How to Calculate BMI Without a Calculator

If you want to calculate BMI by hand, the metric formula is simple. Convert your height to metres (e.g., 162 cm = 1.62 m), square it (1.62 × 1.62 = 2.6244), then divide your weight in kilograms by that number (e.g., 65 ÷ 2.6244 ≈ 24.8). For imperial units, multiply your weight in pounds by 703, then divide by your height in inches squared. Of course, our BMI calculator above does all of this instantly — just enter your numbers and hit calculate.

How to Achieve and Maintain a Healthy BMI

If your BMI is outside the healthy range, the evidence strongly supports modest, sustainable lifestyle changes over aggressive short-term interventions. Here are the key strategies backed by clinical research:

• Calorie balance: A consistent deficit of 400–600 calories per day — through a combination of eating less and moving more — typically produces safe, sustained weight loss of around 0.5 kg per week. Crash diets tend to cause muscle loss and are difficult to maintain.
• Physical activity: The WHO and NHS recommend at least 150 minutes of moderate-intensity aerobic activity (such as brisk walking, cycling, or swimming) per week, plus muscle-strengthening activities on two or more days. Regular exercise helps build lean muscle, which improves metabolic rate and body composition even when BMI changes slowly.
• Dietary quality: Focus on whole foods — vegetables, fruits, legumes, whole grains, lean protein, and healthy fats. Reducing ultra-processed food, added sugars, and refined carbohydrates has a disproportionate impact on weight and cardiovascular health. The Mediterranean diet and DASH diet both have strong evidence for improving BMI and associated health markers.
• Sleep and stress: Poor sleep and chronic stress elevate cortisol levels, which promotes fat storage — particularly around the abdomen. Aiming for 7–9 hours of quality sleep per night and managing stress through mindfulness, exercise, or social connection supports healthy weight regulation.
• Professional guidance: Before making major dietary or exercise changes, consult a qualified GP, registered dietitian, or clinical nutritionist. For people with a BMI above 35 or with obesity-related health conditions, evidence-based medical interventions — including pharmacotherapy or bariatric surgery — may be appropriate options to discuss with a doctor.

Frequently Asked Questions About BMI

What is a good BMI for adults? For most adults, a BMI between 18.5 and 24.9 is considered healthy. However, optimal BMI may vary slightly based on age, ethnicity, and individual health history.

Is BMI accurate for athletes? No — athletes with high muscle mass often have an elevated BMI that overestimates their body fat. For athletes, body fat percentage measured by DEXA or skinfold calipers is a more useful metric.

How often should I check my BMI? For most healthy adults, checking BMI once or twice a year is sufficient. If you are actively managing your weight, monthly tracking alongside waist measurements gives a more complete picture of progress than BMI alone.

Can BMI predict disease risk? At a population level, yes — higher BMI correlates with increased risk of type 2 diabetes, hypertension, heart disease, and certain cancers. But individual disease risk depends on many factors beyond body weight, including genetics, lifestyle, diet quality, and physical fitness.