BMI Calculator

How BMI is calculated

BMI divides your weight by the square of your height. In metric units: BMI = weight in kg ÷ (height in m)². For 70 kg and 1.75 m: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.

In imperial units, multiply by 703 to account for the units: BMI = 703 × weight in lb ÷ (height in inches)². For 160 lb and 5 ft 9 in (69 in): 703 × 160 ÷ 4,761 = 23.6.

BMI chart for adults

These categories come from the CDC and World Health Organization and apply to adults of all ages from 20 upwards, for both men and women. There is no separate BMI scale for women or men – the same numbers are used, although body fat at a given BMI differs on average between the sexes.

Your healthy weight range by height

Rearranging the formula shows the weights that put you in the 18.5–24.9 range: weight = BMI × height². At 1.75 m that is 18.5 × 3.0625 = 56.7 kg up to 24.9 × 3.0625 = 76.3 kg. At 5 ft 9 in the range is about 125–169 lb. The calculator shows your range automatically; to work backwards from a target BMI, use the reverse BMI calculator.

Lower BMI thresholds for some ethnic groups

Research shows that people from some ethnic backgrounds tend to have a higher risk of type 2 diabetes and heart disease at a lower BMI. In the UK, NICE guidance (NG246) recommends using lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background: a BMI of 23 to 27.4 is classed as overweight and 27.5 or above as obesity. The WHO has described similar 'action points' for Asian populations. Tick the option in the calculator to see your result on this scale.

BMI Prime and waist-to-height ratio

BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A BMI of 22.9 gives a BMI Prime of 0.91; anything above 1.0 is above the healthy range. It simply shows how far you are from that limit as a ratio.

Waist-to-height ratio adds information BMI cannot: where you carry weight. Fat around the abdomen is more closely linked to health risks. NICE recommends keeping your waist measurement to less than half your height – a ratio under 0.5. If your height is 175 cm, aim for a waist below 87.5 cm. See how to measure your waist correctly in our guide.

Limitations of BMI

• It does not measure body fat directly. Muscular people, including many athletes, can have a 'overweight' BMI with low body fat.
• It does not show fat distribution – two people with the same BMI can have very different waist sizes and risks.
• Older adults lose muscle with age, so BMI can underestimate body fat.
• It is not used to assess weight during pregnancy.
• For children and teens (2–19) BMI is interpreted with age- and sex-specific percentiles, not adult categories.
• It is a screening tool: a doctor looks at BMI alongside waist size, blood pressure, blood tests and family history.

If your BMI is outside the healthy range, it is a reason to talk to a health professional rather than a diagnosis. For a fuller picture, combine BMI with waist measurements or a body fat estimate.

This calculator is for general information about adults. It is not medical advice. Speak to a doctor or registered dietitian about your weight, especially if you are pregnant, under 20, have a medical condition or have a history of disordered eating.

BMI for older adults

The same adult categories are used for people over 65, but interpreting them needs care. Muscle and bone mass fall with age, so an older person can have a 'healthy' BMI and still carry a lot of body fat, and some studies find the lowest health risks in older adults at a BMI slightly above 25. Unintentional weight loss in later life is a warning sign worth discussing with a doctor whatever your BMI.

Health risks linked to BMI

On a population level, a higher BMI is linked with greater risk of type 2 diabetes, high blood pressure, heart disease, stroke, some cancers, sleep apnoea and osteoarthritis. A BMI below 18.5 is linked with risks too, including nutritional deficiencies, weakened immunity and osteoporosis. These are associations across large groups – your own risk depends on many other factors, which is why BMI is a starting point for a conversation with a health professional, not a verdict.

What to do with your result

• Healthy range: keep up regular activity and a balanced diet; check your waist measurement too.
• Overweight or obesity range: small, sustained changes help – even a 5–10% weight loss can improve blood pressure and blood sugar. See the calorie deficit calculator for a realistic plan.
• Underweight: speak to a doctor, particularly if you have lost weight without trying.
• Any range: if you are very muscular, pregnant, or under 20, use other measures instead of adult BMI.

Related guides

• How to Calculate Your Target Weight From BMI (Formula + Examples)
• Alternatives to BMI: Better Ways to Check Your Body Measurements
• Reverse BMI vs Ideal Body Weight: Which Should You Use?
• How to Measure Your Waist and Hips Correctly (Step by Step)

Sources

1. CDC – Adult BMI categories — cdc.gov/bmi/adult-calculator/bmi-categories.html
2. CDC – About Body Mass Index — cdc.gov/bmi/about/index.html
3. NICE guideline NG246 – Overweight and obesity management (2025) — nice.org.uk/guidance/ng246
4. CDC – BMI percentile calculator for children and teens — cdc.gov/bmi/child-teen-calculator/index.html
5. NHS – Calculate your body mass index (BMI) for adults — nhs.uk/health-assessment-tools/calculate-your-body-mass-index/calculate-bmi-for-adults