What is obesity?

The definition of obesity

Body mass index (BMI) is a widely used screening tool based on height and weight. It does not measure body fat directly, but it helps clinicians classify whether weight may be affecting health.

BMI ranges commonly used in UK practice include:

• 18.5 to 24.9 — healthy weight range for most adults
• 25 to 29.9 — overweight
• 30 to 39.9 — obese range
• 40 and above — severely obese

Clinicians may also consider waist measurement, ethnicity, muscle mass, age, and other conditions when assessing whether treatment is appropriate. You can use our BMI calculator as a starting point, then discuss results with a prescriber.

Lifestyle changes and weight management

Even a modest reduction in body weight — around 10% for many people — can improve breathing, mobility, and control of conditions such as hypertension or type 2 diabetes.

Sustainable change usually combines:

• A balanced, calorie-aware diet rich in fruit, vegetables, and wholegrains
• Gradual increases in activity — for example building toward 30 minutes of moderate exercise on most days
• Realistic goals — many people aim for around 0.5–1 kg (1–2 lb) loss per week when actively losing weight
• Support when mobility or pain makes exercise difficult — walking and gentle progression are often a good start

Prescription treatment works best alongside these habits, not instead of them. Read more in our resource library, including tips to help you lose weight and the obesity epidemic in the UK.

Surgery and other care

Bariatric surgery (such as gastric bypass or sleeve gastrectomy) may be considered on the NHS or privately when other approaches have not succeeded and clinical criteria are met. These procedures are invasive and require lifelong follow-up — your GP or specialist team can advise whether referral is appropriate.

PrivateDoc provides regulated online prescribing and ongoing support for eligible patients using licensed medicines; we do not provide surgical weight loss procedures.