What BMI Is Morbidly Obese? Class III Obesity Explained
Morbid obesity — also called Class III obesity or severe obesity — is a BMI of 40 or higher, per the World Health Organization and most national health agencies. This guide shows you exactly where the cutoff sits, what it means for your health, and what treatment options are available.
Try it: BMI Calculator
Plug in your height and weight (US or metric) to see your BMI, your WHO/CDC weight class, and how far you are from the morbid obesity cutoff.
BMI Calculator
What is BMI and Why Does It Matter?
Body Mass Index (BMI) is a simple, widely-used screening tool that estimates body fat based on your height and weight. Developed in the 1830s by Belgian mathematician Adolphe Quetelet, BMI provides a quick way to categorize weight status and assess potential health risks.
BMI Categories and Health Implications
Healthy Range: 18.5 - 24.9
This range is associated with the lowest risk of weight-related health problems. Maintaining a healthy BMI supports cardiovascular health, reduces diabetes risk, and promotes overall well-being.
Overweight: 25.0 - 29.9
This range indicates increased risk for conditions like high blood pressure, type 2 diabetes, and certain cancers. Modest weight loss (5-10%) can significantly improve health outcomes.
Obese: 30.0+
Obesity significantly increases risk for serious health conditions including heart disease, stroke, type 2 diabetes, and sleep apnea. Medical consultation is recommended for comprehensive management.
Underweight: < 18.5
Being underweight can lead to nutritional deficiencies, weakened immune function, and osteoporosis. Focus on nutrient-dense foods and consult healthcare providers if unintentional weight loss occurs.
Important Limitations of BMI
While BMI is useful for population-level assessments, it has significant individual limitations:
- Muscle vs. Fat: BMI doesn't distinguish between muscle mass and fat mass, so athletes and very muscular individuals may be classified as overweight despite having low body fat.
- Body Composition: Two people with the same BMI can have very different body fat percentages and health risks based on where fat is distributed (visceral vs. subcutaneous).
- Age and Gender: BMI standards don't account for natural changes in body composition with age or differences between biological sexes.
- Ethnicity: Different ethnic groups may have different health risks at the same BMI level, with some Asian populations showing higher risks at lower BMIs.
Professional Guidance: BMI is a screening tool, not a diagnostic instrument. Always consult with healthcare professionals for personalized health assessments and recommendations. Your overall health depends on many factors beyond BMI, including diet quality, physical activity, sleep, stress management, and genetic predispositions.
How our BMI calculator works
Enter weight and height
Type your weight (in kg or lbs) and height (in cm or inches). The calculator auto-converts to metric for the BMI formula.
Get your BMI number
BMI = weight in kg divided by height in meters squared. A 70 kg person who is 1.75 m tall has a BMI of 22.9.
See your category
Below 18.5 is underweight, 18.5-24.9 is normal, 25-29.9 is overweight, 30+ is obese. The calculator also shows the healthy BMI range for your height.
Key things to know about our bmi calculator for guys:
- BMI limitations: BMI doesnt distinguish muscle from fat. Athletes often score overweight despite low body fat. Use the body fat calculator for more accuracy.
- Healthy range: For most adults, 18.5-24.9 is considered healthy. Asians face elevated risk starting at 23; the WHO uses different cutoffs for some populations.
- BMI in children: For kids and teens, BMI is interpreted using age and sex percentiles rather than fixed cutoffs. Our calculator is for adults.
For related calculations, try our body fat calculator.
The full BMI calculator also supports pediatric percentiles and shows the weight ranges for each BMI class at your height.
Open the full health calculatorQuick Answer: Weight That Equals BMI 40 (Morbid Obesity)
For adults, BMI ≥ 40 means morbid (Class III) obesity per WHO and CDC. Use the calculator above to verify, or read the table:
| Height (US) | Height (metric) | BMI 35 (Class II) | BMI 40 (morbid, Class III) | BMI 45 (super obese) |
|---|---|---|---|---|
| 5'2" | 157 cm | 191 lbs | 218 lbs | 245 lbs |
| 5'4" | 163 cm | 205 lbs | 234 lbs | 263 lbs |
| 5'6" | 168 cm | 218 lbs | 249 lbs | 280 lbs |
| 5'9" | 175 cm | 236 lbs | 270 lbs | 304 lbs |
| 6'0" | 183 cm | 255 lbs | 291 lbs | 328 lbs |
| 6'3" | 191 cm | 274 lbs | 313 lbs | 352 lbs |
Worked example: a person 5'9" (175 cm) weighing 270 lbs (122 kg) has BMI = 122 ÷ 1.75² = 39.8, just under the morbid obesity cutoff. At 271 lbs (123 kg), BMI crosses 40.0 and the classification is Class III (morbidly obese).
The full WHO/CDC BMI classification
| BMI | WHO class | CDC class |
|---|---|---|
| < 18.5 | Underweight | Underweight |
| 18.5 – 24.9 | Normal weight | Healthy weight |
| 25.0 – 29.9 | Overweight | Overweight |
| 30.0 – 34.9 | Obese Class I | Obesity |
| 35.0 – 39.9 | Obese Class II | Severe obesity |
| ≥ 40.0 | Obese Class III | Severe (morbid) obesity |
How BMI is calculated
BMI = weight (kg) ÷ height (m)²
Or in US units: BMI = (weight_lb ÷ height_in²) × 703
Example in metric: 100 kg, 1.75 m → BMI = 100 ÷ 1.75² = 100 ÷ 3.0625 = 32.7 (Obese Class I). Example in US units: 220 lb, 5'9" (69 in) → BMI = (220 ÷ 69²) × 703 = (220 ÷ 4761) × 703 = 32.5 (Obese Class I).
Health risks at BMI ≥ 40
- Type 2 diabetes (10× population risk)
- Hypertension (3× population risk)
- Coronary heart disease and stroke
- Obstructive sleep apnea (~50% prevalence)
- Non-alcoholic fatty liver disease (NAFLD)
- Joint disease (osteoarthritis of knees and hips)
- Reduced life expectancy of 5–20 years depending on age and comorbidities
Treatment options for Class III obesity
- Structured lifestyle change — diet, exercise, behavioral counseling. Average 5–10% weight loss in motivated patients.
- Pharmacotherapy — newer GLP-1 agonists (semaglutide, tirzepatide) deliver 15–25% weight loss on average.
- Bariatric surgery — Roux-en-Y gastric bypass or sleeve gastrectomy delivers 25–35% sustained weight loss and is the most effective long-term treatment for BMI ≥ 40.
BMI limitations to know
BMI is a screening tool, not a diagnosis. It overestimates body fat in muscular athletes (think NFL linemen at BMI 35 with 8% body fat) and underestimates it in sedentary older adults. For a fuller picture, pair BMI with waist circumference (men > 40 in, women > 35 in indicates elevated cardiometabolic risk) or a body-fat measurement.
BMI and Obesity Classification Sources
- WHO — Obesity and overweight fact sheet. who.int
- CDC — Adult BMI calculator and classification. cdc.gov
- NIH / NHLBI — Clinical Guidelines on Obesity. nhlbi.nih.gov
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Frequently asked questions
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Morbid obesity (also called Class III obesity or severe obesity) is a BMI of 40.0 or higher in adults, per the World Health Organization and CDC. The terms 'morbid obesity' and 'Class III obesity' are now used interchangeably in clinical guidelines.