How to use the BMI calculator
- Choose US units (feet, inches and pounds) or Metric (centimeters and kilograms). Your values are kept when you switch.
- Enter your height and your current weight, ideally measured in the morning without shoes.
- Read your BMI, its category and the healthy weight range for your height. The marker on the color bar shows where you sit.
- Click Copy to save the result, for example to share with your doctor or track it over time.
The BMI formula
Body mass index compares your weight with your height:
- Metric:
BMI = weight (kg) ÷ height (m)² - US units:
BMI = 703 × weight (lb) ÷ height (in)²
The factor 703 converts pounds and inches to the metric result, so both formulas give the same number. The result is rounded to one decimal place and placed in the World Health Organization categories for adults: under 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, and 30 or more obesity, which is split into classes I (30–34.9), II (35–39.9) and III (40 and above).
Worked example
A person who is 5 ft 9 in (69 inches) tall and weighs 170 lb has a BMI of 703 × 170 ÷ 69² = 119,510 ÷ 4,761 ≈ 25.1, just inside the overweight range. The healthy range for 69 inches is about 125 to 169 lb, so losing only 1–2 lb would move them back to a healthy BMI.
In metric, someone 175 cm tall weighing 70 kg has a BMI of 70 ÷ 1.75² = 70 ÷ 3.0625 ≈ 22.9, a healthy weight. For that height the healthy range is about 56.7 to 76.3 kg.
What BMI can and cannot tell you
BMI is a quick, free screening number that correlates with health risks such as type 2 diabetes, high blood pressure and heart disease across large groups of people. For one individual it has clear limits:
- Muscle weighs more than fat. Athletes and people who lift weights can have a high BMI with little body fat.
- Where fat sits matters. Fat around the waist carries more risk. A waist measurement above 40 inches (102 cm) for men or 35 inches (88 cm) for women is a warning sign even at a normal BMI.
- Age and ethnicity change the picture. Older adults lose muscle, and some guidelines use lower cut-offs for people of Asian descent, for example 23 for overweight.
- Not for children, teens or pregnancy. Under 20, BMI must be compared with age- and sex-specific growth charts.
Tips for a healthier weight
- Aim for small, steady changes. Losing 5–10% of body weight already improves blood pressure and blood sugar for many people.
- Track trends, not single days. Weight moves by 1–2 lb from water alone; compare weekly averages.
- Combine food and movement. Adults are advised to get at least 150 minutes of moderate activity a week plus two days of strength training.
This calculator gives an estimate for adults and is not medical advice. Talk to a doctor or registered dietitian before making major changes to your diet or exercise, especially if you have a health condition.
Frequently asked questions
What is a healthy BMI?
For adults, a BMI from 18.5 to 24.9 is classed as healthy weight by the World Health Organization. Below 18.5 is underweight, 25 to 29.9 overweight and 30 or more obesity.
Is BMI accurate for athletes?
Not always. BMI cannot tell muscle from fat, so muscular people can show an overweight BMI while having little body fat. A waist measurement or body fat test gives a fuller picture.
Can I use this BMI calculator for children?
No. Children and teens need BMI-for-age percentiles that compare them with others of the same age and sex. Ask a pediatrician or use a growth chart designed for children.
Does BMI differ for men and women?
The adult formula and categories are the same for men and women. Women usually carry more body fat at the same BMI, which is one reason BMI is a screening tool rather than a diagnosis.