BMI Calculator — Body Mass Index
Calculate your Body Mass Index (BMI) to check if your weight is healthy. Supports metric and imperial units. See also Ideal Weight Calculator and Calorie Calculator.
BMI Categories Chart
What is BMI (Body Mass Index)?
Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. It provides a simple screening tool to categorize individuals into weight categories: underweight, normal weight, overweight, and obese. BMI was devised by Adolphe Quetelet in the 1830s and is used worldwide by healthcare professionals as a quick assessment of body fat levels. While BMI does not directly measure body fat, research shows it correlates moderately with more direct measures of body fat like skinfold thickness and underwater weighing.
BMI Formula — How to Calculate BMI
Metric Formula (kg and meters)
Imperial Formula (pounds and inches)
Example
Weight: 70 kg, Height: 1.75 m
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625
BMI = 22.9 (Normal Weight)
BMI Chart — Weight Categories
| BMI Range | Category | Health Risk |
|---|---|---|
| < 18.5 | Underweight | Malnutrition risk, osteoporosis |
| 18.5 – 24.9 | Normal Weight | Low risk — healthy range |
| 25.0 – 29.9 | Overweight | Increased risk of heart disease, diabetes |
| 30.0 – 34.9 | Obese Class I | High risk of heart disease, diabetes |
| 35.0 – 39.9 | Obese Class II | Very high risk |
| ≥ 40.0 | Obese Class III | Extremely high risk — morbid obesity |
BMI Prime and Ponderal Index
BMI Prime is the ratio of your BMI to the upper limit of normal (25). A BMI Prime of 1.0 means you are at the upper boundary of normal weight. Values below 1.0 indicate normal or underweight; above 1.0 indicates overweight or obese.
Ponderal Index (also called Corpulence Index) uses height cubed instead of squared: PI = weight / height³. It is considered more accurate for very tall or very short individuals because it scales more proportionally with body size.
Limitations of BMI
BMI is a useful screening tool but has important limitations. It does not distinguish between muscle mass and fat mass — a muscular athlete may have a high BMI but low body fat. It does not account for age, sex, ethnicity, or body composition. BMI may underestimate body fat in older adults who have lost muscle mass, and overestimate it in athletes. For a more complete health assessment, combine BMI with waist circumference, body fat percentage, and other health markers. BMI is not recommended as the sole diagnostic tool for individuals — it is best used for population-level health screening.
Frequently Asked Questions
What is a healthy BMI?
A BMI between 18.5 and 24.9 is considered normal/healthy weight. This range is associated with the lowest risk of weight-related health problems.
Is BMI accurate for athletes?
No. Athletes with high muscle mass often have elevated BMI values that classify them as overweight or obese, even though they have low body fat. For athletes, body fat percentage or DEXA scans are more accurate measures.
Does BMI differ for men and women?
The BMI formula and categories are the same for both sexes. However, women naturally have more body fat than men at the same BMI. Some health organizations suggest slightly different risk thresholds by sex.
How is BMI calculated for children?
For children and teens (ages 2-20), BMI is calculated the same way but interpreted using age- and sex-specific percentile charts from the CDC. A child's BMI percentile shows how their BMI compares to other children of the same age and sex.
What should I do if my BMI is too high or too low?
Consult a healthcare professional. They can assess your overall health, body composition, and risk factors to provide personalized advice. BMI is a starting point, not a diagnosis.
Solved Examples
Example 1: Calculate BMI for a 70 kg Person at 170 cm
Solution:
Step 1: Convert height to meters: 170 cm = 1.70 m
Step 2: Square the height: 1.70 × 1.70 = 2.89 m²
Step 3: Divide weight by height²: 70 ÷ 2.89 = 24.2
Step 4: Check category: 18.5–24.9 = Normal Weight
Answer: BMI = 24.2 (Normal Weight)
Example 2: Calculate BMI Using Imperial Units (180 lbs, 5'10")
Solution:
Step 1: Convert height to inches: 5'10" = 70 inches
Step 2: Square the height: 70 × 70 = 4,900 in²
Step 3: Apply imperial formula: (180 × 703) ÷ 4,900 = 126,540 ÷ 4,900 = 25.8
Step 4: Check category: 25.0–29.9 = Overweight
Answer: BMI = 25.8 (Overweight)
Example 3: Find Ideal Weight Range for a Person 165 cm Tall
Solution:
Step 1: Convert height: 165 cm = 1.65 m
Step 2: Height² = 1.65 × 1.65 = 2.7225 m²
Step 3: Minimum healthy weight: 18.5 × 2.7225 = 50.4 kg
Step 4: Maximum healthy weight: 24.9 × 2.7225 = 67.8 kg
Answer: Healthy weight range is 50.4 – 67.8 kg
Example 4: How Much Weight to Lose to Reach Normal BMI (95 kg, 175 cm, BMI 31.0)
Solution:
Step 1: Current BMI = 95 ÷ (1.75)² = 95 ÷ 3.0625 = 31.0 (Obese Class I)
Step 2: Target BMI = 24.9 (upper limit of normal)
Step 3: Target weight = 24.9 × 3.0625 = 76.3 kg
Step 4: Weight to lose = 95 − 76.3 = 18.7 kg
Answer: Need to lose 18.7 kg to reach BMI 24.9 (Normal Weight)
Practice Questions
Q1: A woman weighs 55 kg and is 160 cm tall. What is her BMI and category?
Answer: BMI = 55 ÷ (1.60)² = 55 ÷ 2.56 = 21.5 (Normal Weight)
Q2: A bodybuilder weighs 100 kg at 180 cm. His BMI is 30.9. Is he obese?
Answer: By BMI alone, yes (Obese Class I). However, BMI does not distinguish muscle from fat. With low body fat percentage (e.g., 12%), he is not clinically obese — this is a known BMI limitation for muscular individuals.
Q3: What is the BMI of a person who weighs 200 lbs and is 6'0" tall?
Answer: Height = 72 inches. BMI = (200 × 703) ÷ (72²) = 140,600 ÷ 5,184 = 27.1 (Overweight)
Q4: A person has a BMI of 17.5. What category are they in and what weight would bring them to normal range at 170 cm?
Answer: Category: Underweight. Target weight for BMI 18.5 = 18.5 × (1.70)² = 18.5 × 2.89 = 53.5 kg (need to gain about 2.9 kg from current 50.6 kg).
Q5: Calculate the BMI Prime for someone with a BMI of 22.0.
Answer: BMI Prime = BMI ÷ 25 = 22.0 ÷ 25 = 0.88. A value below 1.0 indicates the person is within or below normal weight range.
Common Mistakes to Avoid
The most common BMI calculation error is mixing up units — using pounds with the metric formula or kilograms with the imperial formula will give wildly incorrect results. Always ensure you convert properly: multiply pounds by 0.4536 for kg, or use the 703 multiplier for imperial. Another frequent mistake is forgetting to square the height, or squaring it in the wrong unit (squaring cm instead of meters gives a number 10,000× too large). People also often misinterpret their BMI result by treating it as a definitive health diagnosis rather than a screening tool — BMI does not account for muscle mass, bone density, age, sex, or ethnicity. Athletes, elderly individuals, and people of certain ethnic backgrounds may have misleading BMI values. Finally, comparing BMI across children and adults is incorrect — children and teens use age-specific percentile charts, not the same fixed categories as adults.
Key Takeaways
- BMI = weight (kg) ÷ height² (m²). For imperial: BMI = (weight lbs × 703) ÷ height² (in²).
- Normal BMI range is 18.5–24.9. Below 18.5 is underweight, 25–29.9 is overweight, 30+ is obese.
- BMI Prime (BMI ÷ 25) provides a quick ratio — values below 1.0 are normal or underweight.
- BMI is a screening tool, not a diagnostic tool — it cannot distinguish muscle from fat or assess individual health risks.
- The healthy weight range for your height is calculated as: BMI 18.5 × height² to BMI 24.9 × height².
- For athletes or muscular individuals, body fat percentage or waist-to-height ratio are more reliable health indicators than BMI alone.