⚖️ BMI Calculator

Find out where you stand with our free BMI calculator. Enter your weight in kilograms and your height in centimetres to instantly see your Body Mass Index, your weight category according to NHS and WHO standards, and the healthy weight range for your height. BMI is a quick screening tool used by GPs and other healthcare professionals to flag potential weight-related health risks.

While BMI is not a diagnostic measure on its own, it provides a useful starting point for conversations with your GP about nutrition, exercise, and long-term health goals.

Enter your weight in kilograms (1 stone = 6.35 kg)
Enter your height in centimetres (5'10" = 178 cm)

What Is BMI?

Body Mass Index (BMI) is a numeric value derived from a person’s weight and height. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it was originally called the “Quetelet Index” and later adopted by the World Health Organisation as a simple population-level screening tool for weight categories.

BMI does not measure body fat directly. Instead, it uses the statistical relationship between weight and height to estimate whether a person is likely underweight, at a healthy weight, overweight, or obese. Because it requires only two inputs and no special equipment, it remains the most widely used initial assessment in GP surgeries, public health studies, and insurance underwriting across the United Kingdom.

How to Calculate BMI

The standard metric formula is used throughout the UK:

BMI = Weight (kg) ÷ Height (m)2

Example: A person weighing 80 kg who is 1.75 m tall has a BMI of 80 ÷ (1.75)2 = 80 ÷ 3.0625 = 26.1 (Overweight).

Converting Stones to Kilograms

Many people in the UK weigh themselves in stones and pounds. To convert, multiply the number of stones by 6.35 and add any extra pounds multiplied by 0.4536. Here are some common conversions:

  • 9 stone = 57.2 kg
  • 10 stone = 63.5 kg
  • 11 stone = 69.9 kg
  • 12 stone = 76.2 kg
  • 13 stone = 82.6 kg
  • 14 stone = 88.9 kg
  • 15 stone = 95.3 kg

Converting Feet and Inches to Centimetres

Height in the UK is often given in feet and inches. Multiply feet by 30.48 and inches by 2.54, then add the results:

  • 5’2″ = 157 cm
  • 5’6″ = 168 cm
  • 5’10″ = 178 cm
  • 6’0″ = 183 cm
  • 6’2″ = 188 cm

BMI Categories (NHS / WHO)

The NHS uses the same thresholds as the WHO for most adults:

  • Below 18.5 — Underweight: May indicate malnutrition or an underlying health condition. Associated with weakened immune function and bone loss.
  • 18.5–24.9 — Healthy weight: Generally associated with the lowest risk of weight-related disease.
  • 25.0–29.9 — Overweight: Increased risk of heart disease, type 2 diabetes, and high blood pressure.
  • 30.0–34.9 — Obese (Class I): Moderate health risk. Lifestyle changes are strongly recommended.
  • 35.0–39.9 — Obese (Class II): High risk. Medical intervention may be advised.
  • 40.0 and above — Obese (Class III): Very high risk, sometimes described as severe obesity.

Waist Circumference — NICE Guidance

The National Institute for Health and Care Excellence (NICE) recommends measuring waist circumference alongside BMI. Carrying excess fat around the middle increases the risk of heart disease and type 2 diabetes, even if your BMI is within the healthy range:

  • Men: increased risk above 94 cm (37″), high risk above 102 cm (40″)
  • Women: increased risk above 80 cm (31.5″), high risk above 88 cm (34.5″)

Limitations of BMI

BMI is a screening tool, not a definitive diagnosis. It has several well-documented limitations:

  • Muscle vs. fat: Athletes and people who do strength training may have a high BMI due to muscle mass, not excess fat.
  • Age: Older adults tend to lose muscle and gain fat. A “healthy” BMI in a 75-year-old may mask a concerning body composition.
  • Children and young people: BMI is interpreted differently for under-18s. The NHS uses age- and sex-specific growth charts rather than fixed thresholds.
  • Ethnicity: Research shows that health risks may emerge at different BMI levels for different populations. NICE advises that people of South Asian, Chinese, and Black African or Caribbean heritage may face elevated risks at lower BMI thresholds.
  • Fat distribution: BMI says nothing about where fat is stored. Visceral fat around the organs poses greater health risks than subcutaneous fat under the skin.

For a more complete picture, consider supplementing BMI with waist circumference, waist-to-hip ratio, or a body composition analysis (DEXA scan, bioelectrical impedance).

When to See Your GP

The NHS recommends speaking to your GP if your BMI falls outside the healthy range and you have additional risk factors such as a family history of heart disease, type 2 diabetes, or high blood pressure. Your GP can arrange further assessments—blood tests for cholesterol and blood sugar, blood-pressure checks, and referrals to weight-management programmes available on the NHS.

If your BMI is 30 or above, you may be eligible for specialist support through the NHS Diabetes Prevention Programme or local authority-funded exercise-on-referral schemes. Adults with a BMI of 40 or more, or 35 or more with a serious weight-related condition, may be considered for bariatric surgery after other interventions have been tried.

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Frequently Asked Questions

What is a healthy BMI range according to the NHS?
The NHS considers a BMI between 18.5 and 24.9 to be a healthy weight for most adults. Below 18.5 is classed as underweight, 25–29.9 is overweight, and 30 or above is classified as obese.
How do I convert stones and pounds to kilograms?
Multiply the number of stones by 6.35 and add any remaining pounds multiplied by 0.4536. For example, 11 stone 7 lbs = (11 × 6.35) + (7 × 0.4536) = 69.85 + 3.18 = 73.0 kg approximately.
Is BMI calculated differently for children?
Yes. For children and young people under 18, the NHS uses age- and sex-specific growth charts (BMI centile charts) rather than the fixed adult thresholds. A child’s BMI is compared against typical values for their age group.
What waist circumference is considered high risk?
NICE guidelines state that a waist measurement above 102 cm (40″) for men or above 88 cm (34.5″) for women indicates a high risk of weight-related health problems, regardless of BMI.
Why might BMI be inaccurate for some people?
BMI does not distinguish between muscle and fat. Athletes, pregnant women, and older adults may receive misleading results. Ethnicity also matters—NICE advises that South Asian and Black populations may face elevated health risks at lower BMI thresholds.
When should I see my GP about my weight?
Consider booking an appointment if your BMI is below 18.5 or above 30, if your waist circumference exceeds the NICE thresholds, or if you have symptoms such as persistent fatigue, breathlessness, or joint pain. Your GP can arrange further tests and refer you to appropriate services.