How to calculate your BMI
Body Mass Index (BMI) estimates whether your weight is in a healthy range for your height, and this BMI calculator works it out instantly. Enter your height and weight in metric or imperial, and it shows your BMI on a visual scale, the category you fall in, and how far you are from the neighbouring ranges. Switch to ideal-weight mode to see the weight range that corresponds to a healthy BMI for your height.
How to use it
- Choose your sex (it affects interpretation, not the number itself).
- Enter your weight in kilograms or pounds.
- Enter your height in centimetres or feet and inches.
The result updates as you type. Ideal-weight mode swaps the weight field for a target BMI and returns the matching weight.
The BMI formula
BMI categories and what they mean
The World Health Organization classifies adult BMI into these ranges:
| Category | BMI range |
|---|---|
| Underweight | below 18.5 |
| Normal weight | 18.5 to 24.9 |
| Overweight | 25.0 to 29.9 |
| Obese class I | 30.0 to 34.9 |
| Obese class II | 35.0 to 39.9 |
| Obese class III | 40.0 and above |
These apply to adults aged 20 and over, regardless of sex. Children and teens use age- and sex-specific percentile charts instead, which this calculator does not cover.
What these categories actually indicate: each step up the scale is associated with progressively higher risk of conditions like type 2 diabetes, hypertension, heart disease, and certain cancers. However, BMI is a population-level screening tool — it does not diagnose anything for an individual.
BMI limitations: what it can and cannot tell you
BMI uses only height and weight, so it cannot distinguish between muscle mass and fat mass. A muscular athlete can score as "overweight" while carrying low body fat, and an older adult who has lost muscle can score as "normal" while having excess fat.
Other limitations to keep in mind:
- Ethnic variation: some populations (particularly South and East Asian) carry higher metabolic risk at lower BMI thresholds. Several countries use adjusted categories (e.g. overweight starting at 23 rather than 25).
- Age: BMI tends to underestimate fat in older adults who have lost muscle, and may overestimate risk in young, fit people.
- Body composition: two people at BMI 27 can have very different health profiles depending on where fat is stored. Visceral (abdominal) fat carries more risk than subcutaneous fat.
- It is not a health score. A "normal" BMI does not guarantee good health, and an "overweight" BMI does not guarantee poor health. It is one data point among many.
For a fuller picture, pair BMI with waist circumference (a proxy for visceral fat), body-fat percentage (from a DEXA scan or impedance scale), blood pressure, and blood markers.
Ideal-weight mode: finding your healthy range
Switch to ideal-weight mode to reverse the formula: enter your height and a target BMI (default 22, the midpoint of the healthy range) and it returns the weight that achieves it, along with the full healthy span (BMI 18.5–24.9) for your height.
This gives you a concrete weight range to aim for rather than a single number. If you are currently above or below, aim for the edge of the range first rather than the midpoint — sustainable progress beats drastic targets.
How to use your BMI result
- In the healthy range (18.5–24.9): maintain current habits. Recheck periodically if your weight changes significantly.
- Overweight (25–29.9): consider whether it is muscle or fat. If fat, modest changes to diet and activity often bring the most benefit. A waist circumference over 94 cm (men) or 80 cm (women) adds independent risk.
- Underweight (below 18.5): can indicate insufficient nutrition or an underlying condition. Worth discussing with a healthcare provider.
- Obese (30+): higher priority to address, as risk rises steeply. A healthcare professional can help distinguish between BMI-driven risk and other factors.
BMI is a starting point, not the final word. If your result concerns you, talk to a healthcare professional who can weigh it against your full health picture, including blood markers, fitness level, and family history.
Frequently asked questions
Is BMI accurate for muscular people?
Not always. BMI cannot distinguish between muscle and fat, so athletes and people who strength-train often score as 'overweight' while having low body fat. If you train regularly, body-fat percentage (via DEXA scan or a calibrated impedance scale) is a more accurate measure.
Does BMI apply differently across ethnicities?
Yes. South and East Asian populations tend to carry higher metabolic risk at lower BMIs. Several countries use adjusted thresholds (overweight starting at 23 rather than 25). If you are of Asian descent, discuss with your doctor whether lower thresholds apply to you.
What is a healthy BMI range?
18.5 to 24.9 per the WHO classification. This range is associated with the lowest risk of weight-related conditions like type 2 diabetes, heart disease, and hypertension. However, health depends on many factors beyond BMI alone.
How does BMI differ for men and women?
The WHO categories are the same for both sexes, but women naturally carry higher body fat at equivalent BMIs. A woman at BMI 24 may carry more fat than a man at the same BMI. Some practitioners use slightly different interpretation thresholds for women.
Should I aim for the lowest BMI in the healthy range?
Not necessarily. Very low-normal BMI (18.5–20) can indicate insufficient muscle mass, especially in older adults. The midpoint (21–23) is generally where population health outcomes are best. Aim for sustainable health, not the lowest number.
How often should I check my BMI?
BMI changes slowly. Monthly is sufficient, or whenever your weight changes significantly. Day-to-day weight fluctuations of 1–2 kg from water and food don't meaningfully affect your BMI category. Track the trend over months, not individual readings.