BMI Calculator
A BMI calculator that goes further than the number: your category on a visual scale, the healthy weight range for your height, your BMR from the Mifflin-St Jeor equation, TDEE across five activity levels and calorie targets with a macro split.
How to use BMI Calculator
- Switch between metric and imperial, then enter your height and weight.
- Read your BMI, the category it falls in, and where you sit on the scale.
- Check the healthy weight range calculated for your height.
- Add your age and sex to get BMR, then pick an activity level for your TDEE.
- Choose a goal — cut, maintain or bulk — to see a daily calorie target and a suggested macro split.
About BMI Calculator
Body mass index is a ratio invented in the 1830s by Adolphe Quetelet to describe populations, not individuals — and that origin explains both its usefulness and its limits. Across a large group, BMI tracks body fat well enough to be a cheap and effective screening signal. Applied to one person it can be badly wrong, because it takes no account of muscle mass, bone density, fat distribution or ancestry. A rugby forward and a sedentary person of the same height and weight get the same BMI and have entirely different health profiles. Treat the number as a prompt to look closer, never as a verdict.
That is why this calculator does not stop at BMI. It shows the healthy weight range for your height, so the result becomes actionable rather than merely a label, and marks your position on a visual scale so you can see how far from a band boundary you actually are. It then estimates your basal metabolic rate with the Mifflin-St Jeor equation — the predictive formula that validation studies have found most accurate for the general population — and multiplies it by a standard activity factor to give total daily energy expenditure.
From TDEE come the practical numbers: a deficit of roughly 500 kcal a day for around 0.45 kg (1 lb) of loss per week, maintenance, or a modest surplus for gaining, each with a suggested protein, fat and carbohydrate split anchored on protein per kilogram of body weight. All of it is an estimate built from population averages, and real metabolic rates vary by 10–15% around the prediction even for people who match the model well. Track what actually happens over two or three weeks and adjust the target, rather than trusting the first number. None of this is medical advice, and none of it substitutes for a qualified clinician.
Frequently asked questions
How is BMI calculated?
BMI is weight in kilograms divided by height in metres squared: BMI = kg ÷ m². Imperial input is converted first — the equivalent form is 703 × lb ÷ in². The tool shows the substituted formula with your figures.
What is a healthy BMI range?
The World Health Organization classifies 18.5–24.9 as healthy weight, under 18.5 as underweight, 25.0–29.9 as overweight and 30.0 or above as obese. Some health bodies apply lower thresholds for people of South Asian, Chinese and other Asian ancestry, where cardiometabolic risk rises at a lower BMI.
Is BMI accurate for athletes or muscular people?
No. BMI measures mass against height and cannot tell muscle from fat, so it systematically misclassifies muscular people as overweight and can miss excess body fat in people with low muscle mass. It is a population screening measure, not a body-composition measurement and not a diagnosis.
What is the difference between BMR and TDEE?
BMR is the energy your body uses at complete rest to keep you alive. TDEE is BMR multiplied by an activity factor and represents everything you burn in a normal day, including movement and digestion. TDEE is the number to compare your food intake against.
Which BMR formula do you use?
Mifflin-St Jeor, which research has repeatedly found to be the most accurate of the widely used predictive equations for people without an obvious body-composition outlier. It is more reliable than the older Harris-Benedict equation for modern populations.
Should I use this instead of seeing a doctor?
No. These are estimates from population averages and cannot account for your body composition, medical history, medication or metabolic health. Use them as a starting point for a conversation with a doctor or registered dietitian, not as a diagnosis or a treatment plan.
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