BMI Calculator — Body Mass Index
& Weight Loss Eligibility
Calculate your Body Mass Index instantly, understand your weight category, health risks, and whether you may qualify for clinically supervised weight loss treatment.
What Is BMI?
Body Mass Index (BMI) is a numerical screening value calculated from your height and weight. It is used by healthcare professionals worldwide as a simple, cost-effective way to identify whether your weight falls within a healthy range for your height.
BMI does not directly measure body fat. However, research published by the World Health Organization (WHO) shows a strong correlation between BMI and body fat percentage across most adult populations, making it a reliable initial screening tool.
In clinical practice, BMI is used alongside other measurements — such as waist circumference, blood pressure, blood glucose, and lipid levels — to build a complete picture of metabolic health. It is one of the primary criteria used when assessing eligibility for medical weight loss treatment.
Fast Screening
Calculated in seconds using just your height and weight — no specialist equipment needed.
Globally Standardised
The same formula is used by the NHS, WHO, and clinical providers in over 150 countries.
Clinical Eligibility
BMI is one of the key criteria used to assess eligibility for GLP-1 weight loss medications.
How Is BMI Calculated?
BMI is calculated by dividing your weight by your height squared. The formula differs slightly depending on whether you’re using metric (kg/cm) or imperial (lbs/inches) measurements.
Metric Formula
Example: 85 kg ÷ (1.75 × 1.75) = 27.8
Imperial Formula
Example: 703 × 187 ÷ (69²) = 27.6
The multiplier of 703 in the imperial formula is a conversion constant that aligns the result with the metric equivalent. Both formulas produce the same BMI number for a given individual — the calculation above is exactly what our calculator uses.
Measurement tip: For the most accurate result, measure your height without shoes on a flat surface and weigh yourself in the morning without clothing, ideally on a calibrated digital scale. Small measurement errors in height can significantly affect your BMI because height is squared in the formula.
BMI Categories & Weight Classifications
The WHO and NHS use the following BMI ranges to classify weight status in adults aged 18 and over.
| BMI Range | Weight Category | Health Risk | Treatment Eligibility |
|---|---|---|---|
| Below 18.5 | Underweight | Increased | Not indicated |
| 18.5 – 24.9 | Healthy Weight | Minimal | Not indicated |
| 25.0 – 26.9 | Overweight | Moderate | Generally not indicated |
| 27.0 – 29.9 | Overweight | Moderate | ✓ May qualify with conditions |
| 30.0 – 34.9 | Obesity Class I | High | ✓ Typically qualifies |
| 35.0 – 39.9 | Obesity Class II | Very High | ✓ Qualifies |
| 40 and above | Obesity Class III | Severe | ✓ Qualifies |
Source: World Health Organization (WHO), 2024. Eligibility criteria are indicative — a full clinical assessment is required.
What Is a Healthy BMI for Adults?
For most adults, a healthy BMI is between 18.5 and 24.9. Within this range, the risk of weight-related chronic disease is generally at its lowest. However, it’s important to note that “healthy” BMI can vary depending on age, sex, and ethnicity — particularly for individuals of South Asian, East Asian, or Black African heritage, for whom lower thresholds may apply.
When Is a BMI Considered Overweight?
A BMI of 25 or above is classified as overweight. This doesn’t mean you are unhealthy — other factors such as muscle mass, fitness level, diet quality, and metabolic markers all matter. However, a BMI of 25–29.9 is associated with a statistically higher risk of developing conditions such as type 2 diabetes, hypertension, and sleep apnoea compared to the healthy weight range.
When Is a BMI Classified as Obese?
Obesity is defined as a BMI of 30 or above by the WHO and NHS. Obesity is further classified into three grades (Class I, II, and III) based on the degree of excess weight, as each class carries progressively greater health risks. Obesity is a recognised medical condition, not a lifestyle choice, and evidence-based medical treatments are now widely available.
Health Risks Associated with Excess BMI
Carrying excess weight is associated with a significantly increased risk of developing several serious long-term health conditions. According to the NHS, obesity reduces life expectancy by an average of 3–10 years depending on severity.
- Type 2 Diabetes — Excess body fat, particularly visceral fat, contributes to insulin resistance and impaired glucose regulation.
- Cardiovascular Disease — Higher BMI is linked to increased risk of heart attack, stroke, and coronary artery disease.
- High Blood Pressure — Excess weight requires the heart to work harder, raising systolic and diastolic blood pressure.
- Sleep Apnoea — Fat deposits around the upper airway can restrict breathing during sleep, causing intermittent oxygen loss.
- Non-Alcoholic Fatty Liver Disease (NAFLD) — Excess fat can accumulate in liver cells, leading to inflammation and scarring.
- Osteoarthritis — Every extra kilogram of body weight adds approximately 4 kg of force on the knee joints.
- Certain Cancers — Obesity is linked to at least 13 types of cancer including bowel, breast, and kidney.
- Mental Health — Weight-related stigma and physical symptoms can contribute to depression and anxiety disorders.
- Polycystic Ovary Syndrome (PCOS) — Excess weight worsens hormonal imbalance and insulin resistance in women with PCOS.
- Fertility Issues — Obesity can disrupt ovulation in women and reduce testosterone levels and sperm quality in men.
Crucially, research consistently shows that even a modest reduction in body weight of 5–10% leads to clinically meaningful improvements in blood pressure, blood glucose, cholesterol, and other metabolic markers — even before reaching a “healthy BMI” range. This is one of the key reasons why medical weight loss treatment can be life-changing.
BMI & Weight Loss Treatment Eligibility
BMI is one of the primary criteria used by clinicians to determine eligibility for prescription weight loss medication. The following thresholds are used in clinical guidelines in the UK.
Standard Pathway — BMI 30+
- BMI 30 or above qualifies without additional conditions
- Applicable to Wegovy® (semaglutide) and Mounjaro® (tirzepatide)
- Full clinical assessment still required
- No pre-existing weight-related condition needed
With Conditions — BMI 27+
- BMI 27–29.9 may qualify with weight-related health conditions
- Conditions include: type 2 diabetes, hypertension, dyslipidaemia, PCOS, sleep apnoea, joint disease
- Assessment reviews full medical history
- Clinician discretion applies in all cases
Important: BMI is a starting point, not a definitive eligibility decision. Our clinicians take a holistic approach — considering your medical history, current medications, lifestyle, and personal goals — before recommending any treatment. Book a free consultation to discuss your individual circumstances.
Clinically Approved Weight Loss Treatments
At Clinically Proven Weight Loss, we prescribe evidence-based, MHRA-approved medications as part of fully supervised programmes. Our clinicians may recommend one of two leading GLP-1 receptor agonist treatments.
Wegovy is a GLP-1 receptor agonist that mimics the gut hormone GLP-1 to regulate appetite and food intake.
- MHRA-approved for chronic weight management
- Reduces appetite and increases satiety
- Slows gastric emptying to reduce cravings
- Administered via pre-filled auto-inject pen
- Dose titrated gradually over 16–20 weeks
Mounjaro is a dual GIP/GLP-1 receptor agonist — activating two incretin pathways for enhanced metabolic effect.
- MHRA-approved for weight management in the UK
- Dual-action mechanism (GIP + GLP-1)
- Reduces hunger, lowers blood sugar, improves insulin sensitivity
- Administered via pre-filled KwikPen
- May produce greater average weight loss vs semaglutide
Why Medical Supervision Matters
GLP-1 and GIP/GLP-1 medications should only ever be prescribed following a thorough clinical assessment by a regulated healthcare professional. Medical supervision ensures appropriate candidate selection, safe dose titration, management of side effects, and ongoing monitoring of weight, blood pressure, and metabolic markers throughout your programme.
Avoid purchasing weight loss injections from unregulated online sources. Counterfeit and unlicensed semaglutide and tirzepatide products have been identified by the MHRA and can pose serious health risks including dosing errors, contamination, and lack of clinical oversight.
At Clinically Proven Weight Loss, every patient receives a full digital consultation, personalised treatment plan, and clinical support throughout their journey. Learn how our programme works →
Limitations of BMI — What It Doesn’t Tell You
BMI is a valuable population-level screening tool, but it has well-documented limitations when applied to individuals. Understanding these helps put your result in context.
Muscle Mass Not Accounted For
Muscle is denser than fat. Athletes and individuals with high lean muscle mass can have a BMI in the “overweight” range without carrying excess body fat. A professional rugby player or bodybuilder may have the same BMI as someone with obesity.
Fat Distribution Ignored
Where you carry fat matters as much as how much you carry. Visceral fat — stored around the abdominal organs — carries greater metabolic risk than subcutaneous fat. Waist circumference (above 102cm in men, 88cm in women) can flag this risk even at a normal BMI.
Ethnicity Variations
For people of South Asian, East Asian, and some Black African heritage, health risks may arise at lower BMI thresholds. Many clinicians apply a lower cut-off (e.g. 23 for overweight, 27.5 for obesity) for these populations. Adjust your interpretation accordingly.
Age-Related Changes
Body composition naturally shifts with age — muscle mass tends to decrease while fat mass can increase even at a stable body weight. Older adults may have a healthy BMI but carry an unfavourable fat-to-muscle ratio. BMI does not capture these changes.
Sex Differences
Women naturally carry more body fat than men at the same BMI. A woman and a man with the same BMI will typically have different body fat percentages, yet the same BMI thresholds are applied to both.
Pregnancy
BMI calculated during pregnancy is not clinically meaningful as body weight changes substantially throughout gestation. BMI-based weight loss treatment is not suitable during pregnancy.
Better alternatives alongside BMI: Consider measuring your waist circumference (associated with visceral fat risk), requesting a body composition scan (DEXA or bioimpedance), or asking your GP for a metabolic health panel (HbA1c, fasting glucose, lipid profile). These tools together provide a much more complete picture of your health than BMI alone.
BMI Across Different Populations
Standard WHO BMI thresholds were largely derived from studies of European white populations. Research has since shown that risk profiles differ across ethnicities, ages, and sexes.
BMI & Ethnicity
The NICE guidelines (PH46) recommend lower BMI intervention thresholds for people of South Asian, East Asian, Chinese, and Middle Eastern heritage:
| Ethnicity | Overweight Threshold | Obesity Threshold |
|---|---|---|
| White European | BMI ≥ 25 | BMI ≥ 30 |
| South Asian (Indian, Pakistani, Bangladeshi) | BMI ≥ 23 | BMI ≥ 27.5 |
| East Asian / Chinese | BMI ≥ 23 | BMI ≥ 27.5 |
| Black African / Caribbean | BMI ≥ 25 | BMI ≥ 30* |
*Some studies suggest risk may increase at BMI 27.5 for some Black populations. Consult your clinician for personalised guidance.
BMI & Age
The relationship between BMI and health risk changes across the lifespan. Research published in The Lancet and other journals suggests:
- In older adults (65+), a BMI slightly above 25 may be protective, particularly for bone density and recovery from illness.
- In younger adults and those under 50, the benefits of maintaining a BMI in the 18.5–24.9 range are well-established.
- BMI percentile charts (not absolute values) are used for children and adolescents (under 18).
This calculator is designed for adults aged 18 and over. For children and young people, please consult the NHS Healthy Weight Calculator for Children.
How to Use This BMI Calculator
Our BMI calculator is free, instant, and clinically accurate. Here’s how to get the most reliable result.
1. Choose Your Unit System
Select Metric (cm/kg) or Imperial (ft-in/lbs). Both systems produce the same result.
2. Enter Your Height
Measure without shoes on a flat surface. In metric, enter in centimetres. In imperial, enter feet and inches separately.
3. Enter Your Weight
Weigh yourself in the morning, before eating, on a calibrated digital scale. Remove heavy clothing and shoes.
4. Read Your Result
Your BMI, weight category, health risk level, and treatment eligibility are shown instantly alongside a visual scale.
5. Consult a Clinician
If your BMI is 27 or above, speak to one of our clinicians to explore your treatment options in a no-obligation consultation.
Privacy & Accuracy
No data is stored or transmitted. All calculations are performed locally in your browser in real time.
Frequently Asked Questions
Everything you need to know about BMI, weight categories, and weight loss treatment eligibility.
For most adults in the UK, a healthy BMI is between 18.5 and 24.9. This is the range used by the NHS and aligned with WHO international guidelines. Within this range, the risk of developing weight-related conditions such as type 2 diabetes, heart disease, and high blood pressure is at its lowest.
It’s worth noting that for individuals of South Asian or East Asian heritage, the NHS and NICE recommend a lower healthy weight range — a BMI above 23 may warrant additional monitoring in these groups.
In the UK, NICE guidelines and MHRA-approved prescribing criteria generally indicate:
- BMI 30 or above — typically qualifies for prescription weight loss medication (Wegovy® or Mounjaro®) without additional conditions being required
- BMI 27–29.9 — may qualify if you also have one or more weight-related health conditions such as type 2 diabetes, high blood pressure, sleep apnoea, or dyslipidaemia
Final eligibility always depends on a full clinical assessment. Book a free consultation to discuss your individual circumstances with one of our clinicians.
BMI is mathematically accurate — it correctly calculates the ratio of weight to height squared. However, it is a screening tool, not a direct measure of body fat or overall health. Its main limitations are:
- It does not distinguish between fat mass and muscle mass
- It does not account for fat distribution (e.g. visceral vs subcutaneous fat)
- The standard thresholds may not be appropriate for all ethnic groups
- It does not account for age-related changes in body composition
For a comprehensive health assessment, BMI should be used alongside waist circumference, metabolic blood tests, and clinical evaluation.
Yes — this is one of the most well-known limitations of BMI. Muscle tissue is significantly denser than fat tissue. As a result, athletes, bodybuilders, and highly active individuals can have a BMI in the “overweight” or even “obese” range without carrying excess body fat.
If you have high muscle mass and your BMI is elevated, consider requesting a body composition analysis (such as a DEXA scan or bioimpedance assessment) alongside other metabolic health markers. In most cases, a high-muscle BMI is not a contraindication to general health — but clinical assessment is always recommended before starting any weight management programme.
Wegovy® (semaglutide) is a GLP-1 receptor agonist — it mimics the gut hormone GLP-1 to reduce appetite, increase feelings of fullness, and slow gastric emptying. It is administered as a weekly injection and is approved by the MHRA for chronic weight management in adults with a BMI of 30+ (or 27+ with conditions).
Mounjaro® (tirzepatide) is a dual GIP/GLP-1 receptor agonist — it activates both the GIP and GLP-1 incretin pathways. This dual mechanism may produce greater weight loss. In the SURMOUNT-1 clinical trial, participants on the highest dose (15mg) lost an average of 22.5% of body weight over 72 weeks.
The best option for you depends on your medical history, existing conditions, and clinical assessment. Learn more about our treatments →
Clinical trial data shows significant weight loss with GLP-1 and dual GIP/GLP-1 treatments when combined with lifestyle modification:
- Wegovy® (semaglutide 2.4mg): ~15% average body weight reduction over 68 weeks (STEP 1 trial)
- Mounjaro® (tirzepatide 15mg): ~22.5% average body weight reduction over 72 weeks (SURMOUNT-1 trial)
Individual results vary depending on starting weight, adherence, diet, activity levels, and other health factors. Results are most sustainable when treatment is part of a comprehensive programme including nutrition and lifestyle support — which is exactly how we approach treatment at Clinically Proven Weight Loss.
Waist circumference is used alongside BMI to assess abdominal (visceral) fat, which carries a higher metabolic risk than fat stored elsewhere. NHS thresholds are:
| Sex | Increased Risk | High Risk |
|---|---|---|
| Men | Above 94 cm (37 in) | Above 102 cm (40 in) |
| Women | Above 80 cm (32 in) | Above 88 cm (35 in) |
If your BMI is in the healthy range but your waist circumference is above these thresholds, you may still benefit from medical assessment and lifestyle intervention.
To calculate BMI using stones and pounds, first convert your weight to pounds (1 stone = 14 pounds). For example, 13 stone 4 pounds = (13 × 14) + 4 = 186 pounds. Then use the imperial formula: BMI = 703 × weight (lbs) ÷ height² (inches).
Our calculator handles this automatically — select Imperial mode and enter your height in feet/inches and your weight in total pounds.
Ready to Take the Next Step?
If your BMI suggests you may be eligible for weight loss treatment, our clinical team is here to help. Book a free, no-obligation consultation today.
Book Free Consultation →Clinically supervised · MHRA-approved treatments · Fully regulated
References & Sources
- World Health Organization (2024). Obesity and overweight — fact sheet. who.int
- NHS (2023). BMI healthy weight calculator. nhs.uk
- NICE (2014). Obesity: identification, assessment and management (CG189). nice.org.uk
- NICE (2013). BMI: preventing ill health and premature death in black, Asian and other minority ethnic groups (PH46). nice.org.uk
- Wilding, J.P.H. et al. (2021). Once-weekly semaglutide in adults with overweight or obesity (STEP 1). New England Journal of Medicine. nejm.org
- Jastreboff, A.M. et al. (2022). Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). New England Journal of Medicine. nejm.org
- GBD 2015 Obesity Collaborators (2016). Health effects of overweight and obesity in 195 countries. The Lancet. thelancet.com
- Cancer Research UK (2023). Excess weight and cancer risk. cancerresearchuk.org
- MHRA (2023). Warning on unlicensed weight loss injections. mhra.gov.uk
