BMI Calculator — Metric and Imperial
Calculate Body Mass Index in kg/m² or imperial units and see the category. Free BMI calculator from Calculator Direct — not a diagnosis. Free to use.
Body Mass Index (BMI) is a quick screening tool that estimates whether your weight is in a healthy range for your height. Developed in the 1830s by Belgian mathematician Adolphe Quetelet, BMI is now used by doctors, fitness professionals, and public health agencies worldwide because it is simple, free, and reasonably accurate for most adults.
Our BMI calculator works in both metric (kg/cm) and imperial (lbs/inches) units. Just enter your height and weight, and it instantly tells you your BMI value along with the official World Health Organization category — underweight, normal, overweight, or obese. While BMI is not perfect for everyone (especially athletes with high muscle mass), it remains one of the fastest ways to spot potential weight-related health risks.
How BMI is calculated
The BMI formula is simple: BMI = weight (kg) ÷ height (m)². For imperial units, the formula becomes BMI = (weight in pounds × 703) ÷ height in inches². The result is a single number that can be compared against standard categories.
The standard categories defined by the World Health Organization are: under 18.5 is underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or higher is obese. The obese range is further divided into Class I (30-34.9), Class II (35-39.9), and Class III (40+).
These categories were established based on large population studies showing how mortality risk changes with BMI. People with BMI in the normal range generally have the lowest rates of weight-related disease, while both underweight and overweight ranges show elevated risk for different health conditions.
What your BMI category means
An underweight BMI (below 18.5) may indicate insufficient calorie intake, an underlying medical condition, or naturally lean body type. It can be associated with weakened immunity, fertility issues, and bone density loss. If you are underweight, talk to a doctor about whether weight gain would benefit your health.
A normal BMI (18.5-24.9) is associated with the lowest overall health risks for most adults. People in this range generally have lower rates of heart disease, type 2 diabetes, and many cancers compared to people with higher BMIs.
An overweight BMI (25-29.9) increases the risk of conditions like type 2 diabetes, high blood pressure, and joint problems, but does not always mean immediate health danger. Many people in this range are healthy, and small lifestyle changes — better diet, regular activity — can move you back into the normal range.
An obese BMI (30+) significantly increases the risk of heart disease, stroke, type 2 diabetes, sleep apnea, and several types of cancer. The good news: even modest weight loss of 5-10% can substantially improve health markers.
Limitations of BMI
BMI does not distinguish between fat and muscle. A muscular athlete might be classified as overweight or obese while having very low body fat. Similarly, an older person who has lost muscle mass might have a normal BMI while having an unhealthy amount of body fat.
BMI also does not account for fat distribution. Visceral fat (around the organs) is far more dangerous than subcutaneous fat (under the skin), and BMI cannot tell the difference. Waist circumference is often used alongside BMI to provide a better picture — a waist over 40 inches for men or 35 inches for women indicates higher risk regardless of BMI.
BMI standards also vary by ethnicity. Some populations, particularly people of South Asian descent, develop weight-related health risks at lower BMI thresholds and are sometimes assessed using adjusted ranges (overweight at 23+ instead of 25+). For a more complete picture, combine BMI with body fat percentage, waist measurement, and other health markers.
How to use this calculator
- Choose your unit system: metric (kg and cm) or imperial (pounds and inches).
- Enter your current weight.
- Enter your height.
- Optionally select your gender for a more accurate body fat estimate.
- Click Calculate to see your BMI value, category, and estimated body fat percentage.
Worked example: 70 kg adult, 175 cm tall
BMI = 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.86. This falls into the normal weight category (18.5-24.9), indicating a healthy weight for this height.
For comparison, the same person at 85 kg would have a BMI of 27.76 — overweight. At 95 kg, BMI would be 31.02 — obese Class I. This shows how a 25 kg change can move someone across three categories.
In imperial units, a 5'9" (69 inches) person weighing 154 pounds would have a BMI of (154 × 703) ÷ 69² = 22.74 — normal weight.
Frequently asked questions
Is BMI accurate for athletes?
Not always. BMI cannot distinguish muscle from fat, so very muscular athletes often score as overweight or obese despite having low body fat. Athletes should use body fat percentage measurements instead.
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy. This range is associated with the lowest risk of weight-related diseases.
Does BMI work for children?
Not with the adult categories. For children aged 2 to 17, BMI is compared with percentiles for the child's age and sex. Child mode on this calculator uses the US CDC growth charts and shows the percentile and the CDC category (underweight, healthy weight, overweight, obesity or severe obesity). This uses the US CDC 2000 growth charts and US categories (overweight from the 85th percentile, obesity from the 95th). The NHS uses UK growth charts with different cut-offs (overweight from the 91st, very overweight from the 98th), so the category here can differ from the NHS calculator. For a UK assessment, use the NHS calculator or speak to your GP. Anyone aged 18 or over uses the adult categories. It is a screening guide, not a diagnosis, so speak to your GP, practice nurse, health visitor or school nurse about your child's weight.
How often should I check my BMI?
Once every few months is plenty for most people. BMI changes slowly, and daily fluctuations in weight from water and food intake can give misleading short-term readings.
Can my BMI be too low?
Yes. A BMI under 18.5 is classified as underweight and can be associated with nutritional deficiencies, low bone density, weakened immunity, and hormonal issues. Very low BMI is just as concerning as very high BMI.