BMI Calculator — Check Your Body Mass Index
Calculate your Body Mass Index (BMI) to determine if your weight falls within a healthy range relative to your height.
Enter your details and press Calculate BMI to see your score and category.
Enter your height above
What is Body Mass Index?
Body Mass Index (BMI) is a numerical measure of health that uses your height and weight to estimate your body fat. It's a screening tool used by healthcare providers to identify potential weight problems for adults.
Limitations
- closeDoes not distinguish between muscle mass and fat.
- closeLess accurate for athletes or very muscular individuals.
- closeDoes not account for age-related bone density loss.
- closePregnancy significantly alters the accuracy.
BMI Category Chart
Adults 20+Related Performance Tools
View All arrow_forwardHow to Use This BMI Calculator
Using the Calorie Fit BMI calculator takes under 30 seconds. Follow these steps to get an accurate reading:
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Select your sex. Use the Male/Female toggle at the top of the calculator. BMI uses the same formula for both sexes, but knowing your sex allows the insight panel to give you more relevant context about healthy body fat levels.
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Enter your age. Age doesn't affect the core BMI formula for adults, but it does inform the contextual advice — the healthy weight range interpretation shifts slightly for adults over 65.
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Enter your height in centimetres. Stand straight against a wall, feet flat, and measure from the floor to the top of your head. Convert feet/inches if needed: multiply feet by 30.48 and add inches × 2.54.
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Enter your weight in kilograms. Weigh yourself first thing in the morning after using the bathroom, without clothes, for the most consistent reading. To convert pounds: divide by 2.205.
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Press Calculate BMI. Your BMI score, weight category, healthy weight range for your height, and a gauge visualization all appear instantly. No sign-up required.
For best accuracy, always measure under the same conditions — same time of day, same scale, same clothing (or none). Weight naturally fluctuates 1–2 kg across a day due to food, fluids, and bowel contents, so a single measurement is just a snapshot, not your true weight.
Understanding Your BMI Score
Underweight (Below 18.5)
A BMI below 18.5 suggests you are carrying less mass than is typical for your height. This can be perfectly normal for naturally lean individuals with a small frame, but persistent underweight status raises concerns around nutritional deficiency, low bone density (increasing fracture risk), impaired immune function, and — in more extreme cases — hormone disruption including loss of menstrual cycle in women.
If you are underweight unintentionally, consider tracking your calorie intake to ensure you're eating enough. A structured weight gain protocol with a 300–500 kcal daily surplus, prioritising protein and resistance training, is the safest approach to reaching a healthy weight.
Normal Weight (18.5–24.9)
The normal weight range is associated with the lowest risk of weight-related disease. Research consistently shows that adults in this range have lower rates of cardiovascular disease, type 2 diabetes, certain cancers, sleep apnoea, and joint degeneration.
Being in the normal weight range does not guarantee good health — diet quality, physical activity, sleep, and stress all matter enormously. But from a pure weight-risk perspective, maintaining a BMI of 18.5–24.9 is a meaningful, evidence-backed goal for most adults.
Overweight (25.0–29.9)
The overweight category indicates above-average mass for your height. The associated health risks are real but moderate — elevated risk of high blood pressure, elevated LDL cholesterol, insulin resistance, and joint stress. However, context matters enormously here. A muscular 85 kg athlete at 175 cm will score 27.8 and show zero of these risk markers, while a sedentary 80 kg person at the same height may genuinely benefit from weight loss.
If you're in this range and concerned, pair your BMI with a waist circumference measurement (health risk rises sharply above 94 cm for men and 80 cm for women) and our Body Fat % calculator for a more complete picture.
Obese (30.0 and Above)
Obesity is classified in three grades: Grade I (30–34.9), Grade II (35–39.9), and Grade III / morbid obesity (40+). Risk of serious chronic conditions — type 2 diabetes, heart disease, stroke, sleep apnoea, certain cancers, and osteoarthritis — increases progressively with each grade.
Even modest weight loss of 5–10% of body weight has been shown to meaningfully reduce metabolic risk markers. A structured approach combining a calorie deficit (calculated with our Calorie Calculator), adequate protein, and progressive exercise is the foundation of evidence-based weight management.
The Science Behind BMI
Body Mass Index was developed by Belgian mathematician Adolphe Quetelet between 1830 and 1850, originally as a statistical tool for population studies — not as a clinical diagnostic for individual health. It was later adopted by the medical community in the 1970s when obesity researcher Ancel Keys confirmed that Quetelet's formula correlated reasonably well with body fat estimates in large population samples.
The formula itself is deceptively simple: BMI = weight (kg) ÷ height² (m²). This mathematical simplicity is both its greatest strength and most significant weakness. It requires no equipment, no blood test, and no specialist training to calculate — making it useful for population-level screening. However, the formula treats all mass the same, regardless of whether it is muscle, bone, organ, or fat tissue.
Modern research has refined how BMI is interpreted. Studies such as the Framingham Heart Study and EPIC cohort have confirmed that while BMI correlates reasonably with metabolic disease risk at the population level, individual predictions are unreliable. Two people with an identical BMI of 27 can have body fat percentages ranging from 15% to 35% depending on their muscle mass, frame size, age, and sex.
Despite these limitations, BMI remains a first-line screening tool used by the World Health Organization (WHO) and the National Institutes of Health (NIH) because of its zero cost and ease of application. It should always be interpreted alongside other metrics — particularly waist circumference, waist-to-hip ratio, and body fat percentage — for a complete picture.
Formula
weight ÷ height²
kg and metres
Developed by
Adolphe Quetelet
c. 1835 — originally called the Quetelet Index
Accuracy vs DEXA
~±5% body fat
Body Fat % calculator is more accurate
Endorsed by
WHO, NIH, NHS
For population screening, not individual diagnosis
BMI vs. Other Body Composition Metrics
BMI is the starting point, not the full picture. Use it alongside these complementary metrics to build a more complete understanding of your health and fitness.
How to Improve Your BMI
Whether you want to lower or raise your BMI, the fundamentals are the same: consistent nutrition, appropriate training, and patience. Here is the evidence-based approach for each direction.
trending_down Lowering BMI (Weight Loss)
- doneCreate a calorie deficit of 300–500 kcal/day. This produces ~0.3–0.5 kg of fat loss per week — fast enough to make progress, slow enough to preserve muscle. Use our Calorie Calculator to find your maintenance calories first.
- doneHit 1.6–2.2g protein per kg bodyweight. High protein intake during a deficit spares lean mass, keeps you fuller longer, and has a higher thermic effect (burns more calories digesting it) than carbs or fat.
- doneResistance train 2–4 times per week. Lifting weights stimulates muscle protein synthesis, preserving the muscle that diets tend to strip away. More muscle also means a higher resting metabolic rate.
- doneAdd low-intensity movement (NEAT). Walking 8,000–10,000 steps per day burns 200–400 extra kcal without the recovery cost of cardio sessions.
- donePrioritise sleep. Poor sleep elevates ghrelin (hunger hormone) and suppresses leptin (satiety hormone), making calorie control significantly harder. Aim for 7–9 hours per night.
trending_up Raising BMI (Weight Gain)
- doneEat in a 300–500 kcal surplus. A controlled surplus prioritises lean mass gain over fat accumulation. Larger surpluses lead to faster but fattier weight gain. Our Macro Calculator can help structure your meals.
- donePrioritise calorie-dense, nutrient-rich foods. Nuts, nut butters, whole milk, oily fish, avocado, eggs, rice, and oats let you hit your calorie target without needing to eat uncomfortably large volumes.
- doneProgressive overload in the gym. Without a training stimulus, surplus calories go primarily to fat. Compound lifts (squat, deadlift, bench, row) across the major muscle groups provide the stimulus for muscle growth.
- doneEat frequently if you struggle with appetite. 4–6 smaller meals per day is easier than forcing 3 large ones if you have a naturally low appetite. Liquid calories (smoothies, milk, shakes) also reduce the bulk load.
- doneTrack your intake for consistency. Most people underestimate how much (or little) they're eating by 20–30%. Tracking with an app for even a few weeks calibrates your portions accurately.
BMI Accuracy Across Different Groups
Athletes
BMI consistently misclassifies highly muscular individuals. A 90 kg powerlifter at 175 cm has a BMI of 29.4 — "overweight" — yet may have under 12% body fat. Use the Body Fat % calculator for a more meaningful assessment of athletic body composition.
Older Adults (65+)
As we age, muscle mass declines and fat mass tends to increase even at the same BMI. Research suggests a slightly higher BMI (23–27) may be protective against fracture risk and frailty in adults over 65. Discuss with your GP before making weight changes in this age group.
Ethnicity
WHO and the International Obesity Task Force note that people of South Asian, East Asian, and Southeast Asian descent tend to carry more visceral fat at lower BMIs. Lower action thresholds — approximately 23 for overweight and 27.5 for obesity — are used clinically for these populations in several countries.
BMI Questions
A BMI between 18.5 and 24.9 is considered healthy weight for most adults. Below 18.5 is underweight, 25.0-29.9 is overweight, and 30.0 or higher is obese.
BMI is calculated by dividing weight in kilograms by height in metres squared (kg/m squared). Our calculator does this instantly when you enter your height and weight.
No. BMI does not distinguish between muscle and fat, so muscular athletes often score as overweight or obese despite having very low body fat. Use our Body Fat calculator for a more accurate picture.
BMI measures total mass relative to height. Body fat percentage measures the proportion of fat tissue in your body. Two people with the same BMI can have very different body fat percentages.
Combine a moderate calorie deficit (300-500 kcal per day) with resistance training to preserve muscle. Use our Calorie Calculator to find your maintenance calories, then subtract your target deficit.
Yes. The same formula and the same category cut-offs apply to both sexes for adults aged 20+. The interpretation can differ slightly because women naturally carry more essential fat.