BMI Calculator

Determining your BMI

Body mass index (BMI) is a simple screening measure that compares your weight with your height. It is calculated as:

BMI = weight (kg) ÷ height (m)²

or, in US units, BMI = 703 × weight (lb) ÷ height (in)².

The BMI calculator above works in kilograms and centimetres or pounds, feet and inches. It shows your BMI, the adult category it falls into, the healthy weight range for your height, and your BMI Prime (your BMI divided by 25, the upper limit of the healthy range). If you are determining BMI for a child or teenager, note that adult categories do not apply – see the section on children below.

Adult BMI categories

The World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC) use the same adult categories:

The CDC applies these categories to adults aged 20 and over, regardless of age or sex. WHO further describes a BMI under 16 as severe thinness.

Lower thresholds for some ethnic groups

At the same BMI, people of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds have, on average, a higher risk of type 2 diabetes and other conditions than people of white European background. A WHO expert consultation in 2004 proposed additional "action points" for Asian populations, and the UK's NICE guideline on overweight and obesity (NG246) recommends using lower thresholds: a BMI of 23 to 27.4 as overweight and 27.5 or above as obesity for people from these backgrounds.

Healthy weight for your height

The calculator gives the exact range for your own height.

Worked examples

• 70 kg, 175 cm: 70 ÷ 1.75² = 70 ÷ 3.0625 = 22.9 – healthy weight.
• 85 kg, 170 cm: 85 ÷ 1.70² = 29.4 – overweight.
• 105 kg, 180 cm: 105 ÷ 1.80² = 32.4 – obesity class 1.
• 160 lb, 5′7″ (67 in): 703 × 160 ÷ 67² = 25.1 – just into the overweight range.

BMI Prime

BMI Prime = BMI ÷ 25. A value under 1.0 means you are below the upper limit of the healthy range; 1.2 means 20% above it. It is simply a different way of expressing the same number, which some people find easier to interpret.

Calculating BMI by hand

You do not need the calculator to work out BMI – just a little arithmetic.

Metric (kg and cm)

1. Convert height to metres: 172 cm = 1.72 m.
2. Square it: 1.72 × 1.72 = 2.958.
3. Divide weight by the result: 68 kg ÷ 2.958 = 23.0.

US units (lb and inches)

1. Convert height to inches: 5′9″ = 69 in.
2. Square it: 69 × 69 = 4,761.
3. Multiply weight by 703 and divide: 180 lb × 703 ÷ 4,761 = 26.6.

The factor 703 converts pounds per square inch into kilograms per square metre.

Stones and pounds (UK)

Convert to pounds first: 1 stone = 14 lb. So 11 st 6 lb = 160 lb. Then use the US formula.

Working backwards: target weight for a BMI

Weight (kg) = target BMI × height (m)². For someone 1.72 m tall, a BMI of 24.9 corresponds to 24.9 × 2.958 = 73.7 kg. The calculator's "healthy weight for your height" line uses exactly this calculation at BMI 18.5 and 24.9.

Determining BMI accurately at home

Small measurement errors shift BMI more than people expect. Being 2 cm out on height changes BMI by about 0.5 for an average adult, and weighing in heavy clothes and shoes can add 1–2 kg. Measuring height against a wall with a book held flat on your head, and weighing in the morning in light clothing, gives the most repeatable result.

What BMI does and does not tell you

BMI became the standard screening tool because it is quick, free, reproducible and – across populations – correlates reasonably well with body fat and with health risks such as type 2 diabetes, high blood pressure, heart disease, sleep apnoea and some cancers. The CDC describes BMI as a screening measure, not a diagnostic test: it should be considered alongside other information when assessing an individual's health.

Limitations

For these reasons, BMI is often combined with:

• Waist circumference – a direct measure of abdominal fat.
• Waist-to-height ratio – NICE advises keeping your waist to less than half your height (ratio under 0.5).
• Blood pressure, A1C and cholesterol – which show actual metabolic health. See the A1C calculator and the cholesterol ratio calculator.

Waist-to-height ratio categories (NICE)

The BMI calculator for men includes a waist field and calculates this ratio automatically.

Where BMI came from

The formula dates back to the 1830s, when the Belgian mathematician and statistician Adolphe Quetelet observed that, in adults, weight tends to increase in proportion to the square of height. For more than a century it was known as the Quetelet index. In 1972 the American physiologist Ancel Keys and colleagues compared several weight-for-height indices and concluded that this one correlated best with body fat in population studies, and named it the body mass index. Through the 1980s and 1990s, WHO and national health agencies adopted BMI cut-offs to standardise how overweight and obesity were defined and monitored across countries.

It is worth remembering that BMI was designed to describe populations, which is where it works best. For an individual, it is a starting point that needs context.

BMI Prime and other height–weight indices

BMI is one of several indices that relate weight to height. You may come across these alternatives:

None of these replaces a clinical assessment. BMI remains the measure used in most guidelines and research, so it is the one your clinician is most likely to use; the others are best treated as extra context.

Why height is squared

Weight does not increase in direct proportion to height – taller people are heavier partly because they are bigger in every dimension. As Quetelet noted, adult weight scales roughly with the square of height, which is why BMI divides by height squared. The fit is not perfect, which is one reason why very tall people tend to have slightly higher BMIs and very short people slightly lower BMIs for the same body shape.

What the evidence says about BMI and health

Large studies consistently link BMI with health outcomes. The Global BMI Mortality Collaboration, which pooled individual data from 239 prospective studies on four continents, found that among never-smokers without pre-existing chronic disease, the lowest risk of death was in the BMI range of about 20 to 25, with risk increasing progressively above 25 and also rising at low BMIs. The association with higher risk above 25 was seen across continents, and was stronger in younger than in older adults and stronger in men than in women.

Higher BMI is associated with higher rates of:

• Type 2 diabetes and prediabetes.
• High blood pressure, high triglycerides and low HDL cholesterol.
• Coronary heart disease and stroke.
• Fatty liver disease – see the ALT blood test.
• Obstructive sleep apnoea.
• Osteoarthritis of the knees and hips.
• Several cancers, including oesophageal, colorectal, endometrial, kidney and post-menopausal breast cancer.

A very low BMI is associated with nutritional deficiencies, osteoporosis, reduced immunity, fertility problems and, in older adults, frailty.

Other ways to assess body fat

None of these replaces BMI in routine screening, but they help when BMI might mislead – for example in muscular people, older adults, or people of ethnic groups where risk starts at lower BMIs.

BMI for children and teenagers

For people aged 2 to 19 years (CDC) or 5 to 19 years (WHO), BMI is calculated with the same formula but interpreted against age- and sex-specific growth charts, because body fat changes as children grow and differs between boys and girls. The result is a BMI-for-age percentile:

The calculator on this page uses adult categories. For children, use a paediatric BMI tool or ask your child's healthcare provider.

BMI in older adults

Some research suggests that, in older adults, the lowest risk of death occurs at slightly higher BMIs than in younger adults, and that unintended weight loss is a bigger concern than a BMI in the overweight range. Muscle strength and function matter a great deal at older ages. A low or falling BMI in an older person should always be discussed with a clinician.

BMI in pregnancy

BMI is calculated from pre-pregnancy weight and used to guide recommended weight gain during pregnancy. It is not used to assess weight during pregnancy itself. For pregnancy dates and milestones, see the due date calculator.

Why BMI changes over time

BMI changes with weight, and – more slowly – with height loss in older age. Short-term weight changes of 1–2 kg from water, food and salt intake shift BMI by about 0.5. Look at trends over weeks and months rather than day-to-day changes.

How to measure accurately

• Weigh yourself in the morning, after using the toilet, in light clothing, on a flat hard floor.
• Measure height without shoes, standing straight against a wall, heels together, looking straight ahead.
• Measure waist midway between the lowest rib and the top of the hip bone, after breathing out normally.
• Use the same scale for repeat measurements.

If your BMI is outside the healthy range

A BMI outside the healthy range is a reason to look more closely at your health, not a judgement. Useful next steps include checking blood pressure, A1C and cholesterol, considering waist size, and talking with a healthcare professional about what matters for you. Gradual changes to eating, activity and sleep are the foundation of most approaches; the TDEE calculator and calorie calculator give evidence-based starting estimates for energy needs. If your BMI is low, especially with unintended weight loss, speak with a clinician.