Body Mass Index is the screening measure used by the NHS, the WHO and doctors worldwide to flag whether your weight is in a range associated with higher or lower health risk. It is not a diagnosis and it does not measure body fat directly — but as a quick, free first check it is hard to beat. This calculator gives your BMI from your height and weight, and shows which category you fall into.

How it works

Enter your height and weight — in metric (kilograms and centimetres) or imperial (pounds, feet and inches) — and the calculator produces your BMI as a single number, along with the healthy weight range for your height. That second figure is often the more useful one: instead of an abstract score, you get a concrete weight band to compare yourself against.

BMI is designed for adults. Children and teenagers need age- and sex-specific percentile charts, because their bodies are still growing, so this calculator should not be used for under-18s.

How the calculation works

The formula is your weight divided by the square of your height:

BMI = weight (kg) ÷ height (m)²

For imperial measurements, the equivalent is BMI = (weight in lb ÷ height in inches²) × 703.

The result is placed into four standard adult categories:

• Underweight: below 18.5
• Healthy weight: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obese: 30 or above

Because the formula only uses height and weight, it cannot distinguish muscle from fat or see where fat is carried. A rugby player can score as “obese” with low body fat, and an older adult can score “healthy” while carrying risky abdominal fat and little muscle. Waist circumference — under 94 cm for men and 80 cm for women as a lower-risk guide — is a useful second check alongside BMI.

Worked example

An adult who weighs 82 kg and is 1.76 m tall:

1. Square the height: 1.76 × 1.76 = 3.0976
2. Divide the weight by that figure: 82 ÷ 3.0976 = 26.5
3. BMI of 26.5 falls in the overweight category (25–29.9).

The healthy weight range for 1.76 m runs from 18.5 × 3.0976 ≈ 57 kg up to 24.9 × 3.0976 ≈ 77 kg. So this person would need to lose around 5 kg to move just inside the healthy band — a far more actionable target than “lower your BMI”.

Tips

• Measure your height properly. Stand barefoot against a wall, heels and head touching it. Many adults overstate their height by a centimetre or two, which flatters the result — a 2 cm error at 70 kg changes BMI by about 0.6.
• Weigh yourself consistently. First thing in the morning, after the bathroom and before breakfast, gives the most repeatable number. Day-to-day fluctuations of 1–2 kg from water and food are normal; look at weekly averages.
• Use trends, not single readings. Weigh weekly and track the direction over a month. A single BMI reading is a snapshot; the trend tells you whether your habits are working.
• Pair BMI with a tape measure. If your BMI is borderline but your waist is well within the healthy range and you exercise regularly, the overall picture is likely better than the number suggests — and vice versa.
• Know the exceptions. BMI categories are adjusted lower for people of South Asian, Chinese and some other ethnic backgrounds, because health risks rise at lower BMI values; the NHS suggests increased risk from 23 and high risk from 27.5 for these groups.

FAQ

What is a healthy BMI for an adult?
Between 18.5 and 24.9. Below 18.5 is classed as underweight, 25 to 29.9 as overweight, and 30 or above as obese.

Is BMI accurate for athletes and muscular people?
Often not. Muscle is denser than fat, so very muscular people can have a high BMI without excess body fat. In that case, waist measurement and body composition matter more than the BMI number.

Can I use this calculator for my child?
No. Children need BMI percentiles that account for age and sex, because healthy body composition changes rapidly as they grow. Your GP or school nurse can plot a child’s BMI correctly.

Does a high BMI mean I am unhealthy?
Not necessarily — it is a risk flag, not a diagnosis. Many people with a BMI over 25 are metabolically healthy, and some with a “healthy” BMI are not. Blood pressure, blood sugar, cholesterol and lifestyle all matter, and a GP can assess them together.

How quickly can I change my BMI?
Slowly, and that is a good thing. Losing 0.5–1 kg a week is considered a safe, sustainable rate. For someone 1.76 m tall, each kilogram lost lowers BMI by about 0.3, so moving down one category typically takes a few months.

This calculator is for general information only and is not medical advice. If you are concerned about your weight, speak to your GP or another qualified health professional.