BMI Calculator
Your weight relative to height, measured
The BMI calculator (Body Mass Index calculator) estimates your body fat based on your height and weight. It works in both metric (kg, cm) and imperial (lbs, feet, inches) units and instantly tells you whether you fall in the underweight, normal, overweight, or obese range.
BMI categories — reference chart
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency risk |
| 18.5 – 24.9 | Normal weight | Lowest risk |
| 25.0 – 29.9 | Overweight | Moderate risk |
| 30.0 – 34.9 | Obese (Class I) | High risk |
| 35.0 – 39.9 | Obese (Class II) | Very high risk |
| 40 or above | Obese (Class III) | Extremely high risk |
BMI formula
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = (weight (lbs) × 703) ÷ height (inches)²
Example (metric): a person weighing 70 kg at 175 cm has a BMI of 70 ÷ (1.75)² = 22.9 — Normal.
Example (imperial): a person weighing 154 lbs at 5 ft 9 in (69 inches) has a BMI of (154 × 703) ÷ 69² = 22.7 — Normal.
Limitations of BMI
BMI does not distinguish between muscle and fat, so it can be misleading for athletes (who may show "overweight" BMI due to muscle mass) and older adults (who may show "normal" BMI despite excess body fat). For a fuller picture, use BMI alongside body fat percentage and daily calorie needs (TDEE).
BMI also applies the same thresholds across sexes, though research suggests women naturally carry more body fat than men at the same BMI. The standard WHO thresholds above apply to adults aged 18–65.
Example
Let's walk through an example using imperial units. You weigh 180 lbs and are 5 ft 11 in tall (71 inches total).
BMI = (180 × 703) ÷ 71² = 126,540 ÷ 5,041 = 25.1 — Overweight.
Now the same person in metric: 81.6 kg at 180 cm. BMI = 81.6 ÷ (1.80)² = 81.6 ÷ 3.24 = 25.2 — Overweight. Minor rounding difference, same category.
To move to the top of the "Normal" range (BMI 24.9), this person would need to weigh about 178 lbs (80.7 kg). Small targets like this are more motivating than abstract BMI numbers.
Frequently Asked Questions
What is a normal BMI?
A normal (healthy) BMI is between 18.5 and 24.9. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese according to WHO guidelines.
How is BMI calculated in metric units?
BMI = weight (kg) ÷ height (m)². For example, a person weighing 70 kg at 1.75 m has a BMI of 70 ÷ (1.75²) ≈ 22.9, which is in the normal range.
How do I calculate BMI in pounds and inches?
BMI = (weight in lbs × 703) ÷ height in inches². For example, 154 lbs at 5 ft 9 in (69 inches): (154 × 703) ÷ 69² = 108,262 ÷ 4,761 ≈ 22.7.
Is BMI accurate for athletes?
BMI may overestimate body fat in athletes because muscle weighs more than fat. A 200 lb athlete could show an 'overweight' BMI despite having very low body fat. Use our body fat calculator for a more accurate measure.
What BMI is considered obese?
A BMI of 30 or higher is classified as obese. 30–34.9 is Class I obesity, 35–39.9 is Class II, and 40+ is Class III (severe obesity).
Is BMI the same for men and women?
The same BMI thresholds are used for both sexes, but women naturally carry more body fat than men at any given BMI. A BMI of 22 in a woman represents a higher body fat percentage than in a man of the same BMI.
What is a healthy BMI for seniors over 65?
Some research suggests a slightly higher BMI of 23–27 may be healthier for adults over 65, as modest body reserves can help during illness. Always consult a doctor for age-specific guidance.
Why does my BMI say overweight if I work out?
Because BMI only uses height and weight — it can't tell the difference between muscle and fat. Muscle is denser than fat, so a muscular person can have a high BMI despite low body fat. If you lift regularly, use body fat percentage as a better measure.
Is a BMI of 27 bad?
A BMI of 27 falls in the 'overweight' range (25–29.9), which carries a moderately increased risk of conditions like high blood pressure and type 2 diabetes. However, it's close to the normal range and not as high-risk as obesity. Small changes in diet and activity can bring it under 25.
What weight do I need to be to have a BMI of 25?
It depends on your height. Multiply 25 by your height in metres squared. For example, at 175 cm (1.75 m): 25 × (1.75²) = 25 × 3.0625 = 76.6 kg (about 169 lbs). Our ideal weight calculator can give you a personalised healthy weight range.
Is a BMI of 22 good?
Yes — 22 sits comfortably in the middle of the healthy range (18.5–24.9) and is associated with the lowest risk of weight-related health problems. Most health guidelines consider 20–22 an optimal target for long-term health.
What BMI is considered morbidly obese?
Morbid obesity (now also called Class III obesity) is a BMI of 40 or higher. At this level the health risks — including heart disease, sleep apnoea, and type 2 diabetes — are severe. A BMI of 35–39.9 with obesity-related conditions is also sometimes classified as morbid.
Can I have a healthy BMI but still be unhealthy?
Yes. BMI doesn't measure where fat is stored or how much muscle you have. Someone can be 'normal weight' but carry excess fat around the abdomen — sometimes called 'skinny fat' — which raises metabolic risk even with a BMI under 25. Pair BMI with your daily calorie needs (TDEE) for a fuller picture.
How much does height affect BMI?
Height has a squared effect on BMI, so small differences matter a lot. Two people both weighing 80 kg: at 170 cm the BMI is 27.7 (overweight), but at 180 cm it drops to 24.7 (normal). That's why tall people often have lower BMIs despite weighing more.