BMI Calculator

A healthy BMI for most adults falls between 18.5 and 24.9. This range is linked to the lowest risk of heart disease, type 2 diabetes, and other weight-related conditions. People in this range are more likely to have healthy blood pressure, blood sugar, and cholesterol levels. Note that for adults of South Asian or East Asian descent, health risks may start at a slightly lower BMI — around 23 — so some clinicians use ethnicity-adjusted thresholds.

Obesity is divided into three classes based on how far BMI exceeds 30:

• Obese Class I — BMI 30 to 34.9: Moderate obesity. Increased risk of high blood pressure, high cholesterol, sleep apnea, and joint problems. Lifestyle changes (diet + exercise) are usually the first step.
• Obese Class II — BMI 35 to 39.9: Severe obesity. Risk of serious conditions rises significantly. Doctors often recommend medically supervised weight loss programmes and may discuss prescription medication.
• Obese Class III — BMI 40 and above: Also called morbid or extreme obesity. The highest risk category for life-threatening conditions including heart disease, stroke, type 2 diabetes, and certain cancers. Bariatric surgery may be considered if other methods have failed.

A BMI of 35–39.9 means you are carrying excess weight that puts significant strain on your heart, joints, and metabolism. Here is what most healthcare providers recommend:

• See your doctor first — get a full health check including blood pressure, blood sugar (HbA1c), and cholesterol before starting any weight loss plan.
• Structured diet plan — a deficit of 500–750 calories per day is generally recommended. A registered dietitian can create a sustainable plan that avoids yo-yo dieting.
• Exercise regularly — aim for 150–300 minutes of moderate activity per week. Low-impact options like walking, swimming, or cycling reduce joint stress.
• Prescription weight loss medication — at BMI 35+, medications like orlistat may be prescribed alongside lifestyle changes if you have obesity-related health conditions.
• Consider a structured programme — NHS Weight Management (UK), MOVE! (US Veterans), or similar medically supervised programmes offer guidance, monitoring, and support.

Even a 5–10% reduction in body weight can meaningfully improve blood pressure, blood sugar, and joint pain. Progress takes time — consistency matters more than speed.

A BMI of 40 or above is classified as Obese Class III, sometimes called morbid obesity or severe obesity. At this level, excess body weight significantly increases the risk of:

• Type 2 diabetes and insulin resistance
• Cardiovascular disease, heart attack, and stroke
• Obstructive sleep apnea
• Osteoarthritis and chronic joint pain
• Non-alcoholic fatty liver disease (NAFLD)
• Certain cancers (bowel, breast, kidney, uterine)
• Mental health conditions including depression and anxiety

What are your treatment options?

• Medical supervision is essential — do not attempt rapid weight loss without a doctor's guidance at this BMI level.
• Intensive lifestyle intervention — structured programmes combining diet, behaviour therapy, and supervised exercise have shown results even at BMI 40+.
• Weight loss medications — GLP-1 agonists (e.g. semaglutide/Wegovy) and other approved medications can support significant weight loss when combined with lifestyle changes.
• Bariatric surgery — procedures like gastric sleeve, gastric bypass, or gastric band are clinically recommended options for adults with BMI 40+ (or 35+ with serious health conditions) when other approaches have not worked. Many people achieve 50–70% excess weight loss after surgery.

Reaching out to your GP or a specialist obesity clinic is the most important first step. You do not have to manage this alone — effective treatments exist.

Yes — many people have successfully moved from Class II or Class III obesity to a healthy BMI, though it takes significant time and sustained effort. The key principles are:

• Aim for gradual loss — losing 0.5 to 1 kg (1–2 lbs) per week is considered safe and sustainable. Faster loss often leads to muscle loss and nutrient deficiencies.
• Focus on small milestones — a 5–10% reduction in body weight already delivers measurable health improvements. You do not need to reach a "normal" BMI to benefit.
• Address root causes — hormonal conditions (hypothyroidism, PCOS), medications, and mental health factors can all contribute to weight gain and need to be treated alongside lifestyle changes.
• Long-term support — working with a dietitian, GP, or obesity specialist gives you accountability and clinical safety monitoring.

BMI is a useful population-level screening tool, but it has well-known limitations for individuals:

• Athletes and muscular people — BMI may read as overweight or obese due to high muscle mass, even with low body fat.
• Older adults — BMI can underestimate body fat in people who have lost muscle mass with age.
• Ethnicity — people of South Asian, East Asian, and some other backgrounds may face health risks at lower BMI values than the standard thresholds suggest.
• Pregnant or breastfeeding women — BMI is not an appropriate assessment tool during pregnancy.

For a more complete picture, your doctor may also look at waist circumference, waist-to-hip ratio, or body fat percentage.

Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = (weight (lbs) ÷ height (in)²) × 703

Example: A person who is 170 cm tall and weighs 70 kg has a BMI of: 70 ÷ (1.70)² = 70 ÷ 2.89 = 24.2 (Normal weight).

Example: A person who is 5 ft 6 in (66 inches) tall and weighs 220 lbs has a BMI of: (220 ÷ 66²) × 703 = (220 ÷ 4356) × 703 = 35.5 (Obese Class II).