- How is BMI calculated?
- BMI is your weight in kilograms divided by the square of your height in metres: BMI = kg ÷ m². For example, 70 kg at 1.75 m is 70 ÷ (1.75 × 1.75) ≈ 22.9. If you enter other units, the calculator converts height and weight to metric first and applies the same formula.
- What are the BMI categories?
- The World Health Organization's adult categories are: under 18.5 underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, and 30 or above obese, which it splits into class I (30 to 34.9), class II (35 to 39.9) and class III (40 and over). The scale under your result shows every band, and BMI Prime is your BMI divided by 25, so 1.00 is the top of the healthy band.
- Is there a different scale for Asian adults?
- Yes. Risk starts at a lower BMI in many Asian populations, and the WHO's public-health action points for them are 23 (overweight) and 27.5 (obese). Switch "Cut-offs" to Asia-Pacific to use them. The calculator never picks this for you.
- Does BMI work for children and teenagers?
- No. The categories above are for adults aged 20 and over. For under-20s BMI is read against age and sex on a growth chart, so enter the age and the calculator will show the number but not an adult category. Use your country's growth chart or ask a doctor or nurse.
- What are the limitations of BMI?
- BMI doesn't distinguish muscle from fat, so athletes and very muscular people can show a high BMI while carrying little body fat, and it doesn't account for age, sex, or where fat is distributed. It's a quick screening number, not a diagnosis — talk to a doctor or other qualified health professional about what a healthy weight looks like for you. Adding your waist gives a second check: a waist-to-height ratio under 0.5 is the usual aim.
- Can I mix metric and imperial units?
- Yes. Height and weight each have their own unit, so you can enter your height in centimetres or feet and inches, and your weight in kilograms, pounds, or stone and pounds — in any combination, like 5 ft 9 in and 70 kg. Switching a unit converts what you've typed instead of clearing it, and decimal commas such as 70,5 are understood. If a number looks like the wrong unit (154 kg for a 70 kg person, or 1.75 in the cm box) the calculator offers the likely fix.
- Can I keep track of my weight over time?
- Yes. "Keep a log" saves dated readings in this browser only, draws a small chart of your weight, and downloads them as a CSV. Nothing is uploaded.
- Is my height and weight data stored or uploaded anywhere?
- No. Every calculation runs locally in your browser. Your last inputs and your log are saved only to this browser's local storage — your health data is never sent to a server.
- Is the BMI calculator really free?
- Yes — nothing to pay, nothing to register for, no watermark.