BMI Calculator
Body Mass Index from weight and height, with category and healthy weight range.
- bmi
- body mass index
- weight
- height
- overweight
About BMI Calculator
Body Mass Index (BMI) is the standard population-level screening figure used by doctors, public health agencies, and most life-insurance forms. It's a simple ratio — weight in kilograms divided by height in metres squared — and it sorts adults into bands that correlate, on average, with health risks: under 18.5 is underweight, 18.5–24.9 is normal, 25–29.9 is overweight, and 30+ is obese (split into Class I, II, and III).
BMI's value is that it's quick, free, and consistent enough to compare populations and track trends. Its limits are that it doesn't know the difference between fat and muscle, and it doesn't account for frame size or body composition — which is why a heavily-muscled athlete can score "overweight" without being unhealthy, and a sedentary person can score "normal" while carrying risky levels of body fat. Treat the number as one input among many, not a diagnosis.
How to use
Pick metric or imperial with the toggle at the top, then enter your weight and height. The BMI figure and category appear immediately, along with a colour-coded scale showing where you fall relative to the WHO band boundaries.
The healthy-weight panel calculates the weight range that would put you in the 18.5–24.9 "normal" band for your specific height, in the units you chose. The copy button beside the BMI value lifts just the figure (to one decimal place) onto your clipboard.
Frequently asked questions
How is BMI calculated?
BMI = weight in kilograms ÷ (height in metres)². The calculator handles imperial input by converting to metric internally — 1 lb = 0.45359237 kg; 1 inch = 0.0254 m — so the final figure is the same regardless of which unit system you used.
What are the BMI categories?
The WHO uses these bands for adults: under 18.5 underweight, 18.5–24.9 normal weight, 25–29.9 overweight, 30–34.9 obese Class I, 35–39.9 obese Class II, 40+ obese Class III. The colour-coded scale beneath the result shows where you sit relative to each boundary.
Is BMI accurate for athletes or older adults?
Not always. Muscle weighs more than fat, so a heavily-muscled athlete can score "overweight" without being unhealthy. Older adults lose muscle mass with age, which can hide unhealthy levels of body fat behind a "normal" BMI. BMI is a quick population-level screen, not a diagnosis — treat the number as one input among many.
Does BMI apply to children?
Not the adult categories shown here. For children and teenagers, paediatric BMI is interpreted using age- and sex-specific percentile charts published by the CDC or WHO, because healthy ranges shift considerably during growth. This calculator is designed for adults.
What's a better measure than BMI?
Body composition (the percentage of body weight that's fat versus lean tissue), waist-to-hip ratio, and waist circumference all correlate better with health outcomes than BMI alone. A DEXA scan or bioelectrical impedance scale gives a body-fat percentage; a tape measure around the waist captures most of the cardiovascular-risk information BMI was originally a proxy for.