What is BMI and how is it calculated?
BMI (Body Mass Index) is a single number that estimates whether your weight is in a healthy range for your height. It is calculated by dividing your weight in kilograms by the square of your height in metres: BMI = kg ÷ m². The imperial version uses BMI = (lbs ÷ in²) × 703, which is mathematically the same thing. Developed in the 19th century and adopted by health organisations worldwide, BMI is a population-level screening tool — a quick first indicator of weight-related health risk, not a diagnosis. Everything in this calculator runs entirely in your browser, so the numbers you type are never sent to a server.
How to use this calculator
Choose Metric (cm / kg) or Imperial (ft / lbs) with the toggle at the top, then enter your height and weight — the results update instantly as you type. Your BMI number and category appear in the result panel, along with the ideal weight range for your height and how much you would need to gain or lose to land in the Normal band. The visual BMI scale shows where you fall among all four categories, with a marker that slides along a 10–40 axis. Add your age, sex and activity level to see the daily-energy figures, and enter your waist and neck measurements to estimate body-fat percentage.
BMI categories at a glance
The World Health Organization defines four adult bands: Underweight below 18.5, Normal weight from 18.5 to 24.9, Overweight from 25 to 29.9, and Obese at 30 and above. These thresholds are the same for men and women. Sitting just inside or outside a boundary is rarely cause for alarm on its own — BMI is most useful as a trend you watch over time, alongside how you feel and other health markers.
BMR, TDEE and daily calories
Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest, just keeping you alive. This calculator uses the Mifflin-St Jeor equation, currently the most accurate widely-used formula: 10 × kg + 6.25 × cm − 5 × age + 5 for men, and the same with − 161 instead of + 5 for women. Multiplying BMR by an activity factor (1.2 for sedentary up to 1.9 for athletes) gives your Total Daily Energy Expenditure (TDEE) — roughly the calories you need to maintain your current weight. A deficit of about 500 calories a day is a common, sustainable target for losing around half a kilogram a week, which is why the tool also shows a weight-loss figure.
Body fat and macronutrients
When you supply waist and neck measurements, the calculator estimates body-fat percentage using the US Navy circumference method, which correlates well with more expensive lab methods for most people. It also suggests a balanced 40 / 30 / 30 split of carbohydrates, protein and fat at your maintenance calories. Carbohydrates and protein supply four calories per gram while fat supplies nine, so the gram amounts are derived directly from your TDEE. These are starting points you can tune to your goals rather than fixed prescriptions.
Limitations of BMI
BMI cannot tell muscle from fat, so muscular athletes can read as "Overweight" despite very low body fat, while someone with little muscle can read as "Normal" while carrying excess fat. Older adults naturally hold more fat at the same BMI, and women typically have a higher healthy body-fat percentage than men. Research also shows that some Asian populations face elevated risk at lower BMI thresholds, leading several guidelines to use an overweight cut-off of 23 rather than 25. Very tall or very short people can get misleading scores too. Pair BMI with waist measurement, body-fat percentage and a conversation with a healthcare professional for the fullest picture.