What Is BMI?
Body Mass Index (BMI) is a numerical value derived from a person's weight and height. It is one of the most widely used screening tools in medicine and public health to broadly categorise an adult's weight status — whether they are underweight, at a healthy weight, overweight, or obese.
BMI does not measure body fat directly. Instead, it provides a quick, low-cost proxy that clinicians and individuals can use as a starting point for health conversations. First developed by Belgian mathematician Adolphe Quetelet in the 1830s and later adopted by the World Health Organization (WHO), BMI remains the global standard for population-level weight assessments.
It is important to understand that BMI is a screening tool, not a diagnostic one. Two people with the same BMI can have very different body compositions, metabolic health, and disease risk. That is why BMI results should always be interpreted alongside other health markers such as waist circumference, blood pressure, blood glucose, and cholesterol levels.
BMI Formula – Metric & Imperial
The BMI calculation is straightforward regardless of which unit system you use. Here are both formulas:
Metric Formula (kg & cm)
BMI = weight(kg) ÷ height(m)²
Example: 70 kg ÷ (1.75 m)² = 70 ÷ 3.0625 = 22.86
Imperial Formula (lbs & inches)
BMI = (weight(lbs) × 703) ÷ height(in)²
Example: (154 lbs × 703) ÷ (69 in)² = 108262 ÷ 4761 = 22.74
In the metric system, remember to convert your height from centimetres to metres before squaring it. For example, 175 cm becomes 1.75 m. In the imperial system, the factor 703 converts the result into the same scale as the metric formula.
How to Calculate BMI Step by Step
While our calculator does all the work automatically, here is the manual process so you understand what is happening under the hood:
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Measure your weight — Use a reliable scale. Weigh yourself in the morning, before eating, wearing minimal clothing for the most consistent reading.
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Measure your height — Stand against a wall without shoes. Use a measuring tape or stadiometer. Record in centimetres (or feet and inches).
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Apply the formula — Divide your weight (kg) by your height in metres squared, or use the imperial formula above.
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Find your category — Match your BMI number to the WHO category table above.
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Consult a professional — If your BMI falls outside the healthy range, speak with your doctor or a registered dietitian for personalised advice.
Tip: Use our BMI calculator above to skip manual maths entirely. It supports both metric (kg / cm) and imperial (lbs / ft / in) inputs, and gives you an instant result with your healthy weight range.
BMI Examples – Metric & Imperial
Below are worked examples showing BMI at a range of heights and weights, for both unit systems. These help you understand where your number lands before using the calculator.
| Height |
Weight (kg) |
Weight (lbs) |
BMI |
Category |
| 160 cm / 5'3" | 45 kg | 99 lbs | 17.6 | Underweight |
| 160 cm / 5'3" | 62 kg | 137 lbs | 24.2 | Normal |
| 175 cm / 5'9" | 70 kg | 154 lbs | 22.9 | Normal |
| 175 cm / 5'9" | 88 kg | 194 lbs | 28.7 | Overweight |
| 180 cm / 5'11" | 95 kg | 209 lbs | 29.3 | Overweight |
| 180 cm / 5'11" | 110 kg | 243 lbs | 33.9 | Obese Class I |
Values are rounded to one decimal place. Use our calculator above for your precise BMI.
What Is a Healthy BMI?
For most adults aged 18 to 65, a BMI between 18.5 and 24.9 is considered healthy. People within this range statistically have the lowest risk of developing weight-related health conditions such as type 2 diabetes, cardiovascular disease, and certain cancers.
However, the "ideal" BMI is not one single number. The healthiest point within the normal range varies by individual. Research consistently shows that a BMI of around 21 to 23 is associated with the lowest all-cause mortality for adults in most populations.
Certain populations have adjusted thresholds. For people of South Asian, East Asian, or Southeast Asian descent, health risks increase at a lower BMI. The WHO recommends a lower cut-off of 23 for overweight and 27.5 for obesity in these populations. Conversely, for children and teenagers, BMI is interpreted using age- and sex-specific percentile charts rather than the fixed adult thresholds.
Limitations of BMI — What It Cannot Tell You
BMI is useful precisely because it is simple and requires only two measurements. But simplicity comes at a cost. Here are the key situations where BMI alone may be misleading:
💪 Muscular Athletes
Muscle is denser than fat. A highly muscular person — a rugby player or bodybuilder — may have a BMI in the "overweight" or even "obese" range despite having very low body fat.
👴 Older Adults
As people age, they tend to lose muscle mass and gain fat, even without weight changes. Someone over 65 may have a "normal" BMI but carry a high proportion of body fat (sarcopenic obesity).
👶 Pregnancy
BMI is not an appropriate tool for pregnant women, as weight naturally increases during pregnancy. Gestational weight gain guidelines use pre-pregnancy BMI as a baseline, not current BMI.
🌍 Ethnicity
People of Asian descent tend to develop metabolic risk factors at lower BMIs than people of European descent. Standard WHO thresholds may underestimate health risks in these populations.
📍 Fat Distribution
Where fat is stored matters as much as how much fat there is. Abdominal (visceral) fat is more dangerous than fat stored in the hips or thighs. Waist circumference captures this; BMI does not.
🧒 Children & Teens
Children's BMI must be assessed using age- and sex-specific growth charts. The adult categories do not apply — a BMI of 22 could be normal for an adult but overweight for a 10-year-old.
Better together: For a more complete picture of your metabolic health, combine BMI with your waist circumference, waist-to-hip ratio, body fat percentage, blood pressure, and fasting blood glucose. These together give clinicians far more information than BMI alone.
Tips to Reach or Maintain a Healthy BMI
Whether you need to lose weight, gain weight, or simply stay in the healthy range, the following evidence-based strategies can help:
🥗 Balanced Nutrition
Focus on whole foods — vegetables, legumes, wholegrains, lean proteins, and healthy fats. Reduce ultra-processed foods, added sugars, and refined carbohydrates, which promote fat accumulation.
🏃 Regular Physical Activity
Aim for at least 150 minutes of moderate aerobic activity (brisk walking, cycling) per week, plus 2 sessions of strength training. Exercise helps both lose fat and build metabolically active muscle.
😴 Quality Sleep
Sleeping 7–9 hours per night regulates appetite hormones (ghrelin and leptin). Poor sleep is strongly associated with weight gain and higher BMI in large population studies.
💧 Hydration
Drinking adequate water (roughly 2–3 litres daily for most adults) supports metabolism, reduces appetite, and can replace calorie-dense beverages like sodas and juices.
🧘 Stress Management
Chronic stress raises cortisol, which promotes fat storage — particularly visceral fat. Mindfulness, yoga, or even regular outdoor walks can meaningfully lower stress levels and support weight management.
📊 Regular Monitoring
Track your BMI every 1–3 months rather than daily. Weight fluctuates naturally day to day (by up to 2 kg) due to water retention, food volume, and hormonal changes. Monthly trends matter more than daily numbers.
Frequently Asked Questions About BMI
What is a healthy BMI for adults? +
For most adults (18–65), a BMI between 18.5 and 24.9 is considered healthy. A BMI below 18.5 indicates underweight, 25–29.9 is overweight, and 30 or above is classified as obese. These are general population guidelines and should be considered alongside other health markers.
How do I calculate my BMI in kg? +
Divide your weight in kilograms by the square of your height in metres. For example, if you weigh 68 kg and are 170 cm (1.70 m) tall: BMI = 68 ÷ (1.70 × 1.70) = 68 ÷ 2.89 = 23.5. You can also use our calculator above — enter your weight in kg and height in cm, and the result is instant.
How do I calculate BMI in pounds (lbs)? +
Use the imperial formula: BMI = (weight in lbs × 703) ÷ (height in inches)². For example, a person who weighs 150 lbs and is 5 feet 6 inches (66 inches) tall: BMI = (150 × 703) ÷ (66²) = 105,450 ÷ 4,356 = 24.2. Switch our calculator to "Imperial" mode to do this in seconds.
Is BMI accurate for women and men equally? +
The same BMI formula and thresholds apply to both men and women, but their body compositions differ. Women typically carry more body fat than men at the same BMI. A woman and a man both at a BMI of 25 may have different health risk profiles. Some clinicians argue that sex-specific BMI thresholds would be more accurate, though current WHO and NHS guidelines use unified thresholds for adults.
Can I use this BMI calculator for children? +
No. The adult BMI calculator above uses fixed thresholds that do not apply to children (under 18). For children and teenagers, BMI must be interpreted using age- and sex-specific growth percentile charts, because body composition changes significantly as children grow. Your GP or paediatrician can advise on healthy weight for children.
How often should I check my BMI? +
For most people, checking BMI once every 1–3 months is sufficient. Daily weight measurements can be influenced by normal fluctuations (hydration, food mass, hormones) of up to 2 kg, which can be misleading. Monthly checks give a more accurate picture of your trend over time. If you're actively managing your weight under medical supervision, your doctor will advise an appropriate frequency.
What is a healthy weight range for my height? +
A healthy weight is the range of body weights that correspond to a BMI between 18.5 and 24.9 for your specific height. Use our BMI calculator above — after calculating, it displays your personalised healthy weight range in both kg and lbs. For example, someone 170 cm tall has a healthy weight range of approximately 53–72 kg (117–158 lbs).
Does BMI measure body fat percentage? +
No. BMI does not directly measure body fat. It is a ratio of weight to height squared. To accurately measure body fat percentage, methods like DEXA scan, hydrostatic weighing, BodPod, or skinfold callipers are used. However, research shows that BMI correlates reasonably well with body fat at a population level, making it a useful (if imperfect) screening tool.
⚕️ Medical Disclaimer
The BMI calculator and information on this page are for general informational and educational purposes only. BMI is a screening tool, not a diagnostic instrument. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of your physician, dietitian, or other qualified health provider with any questions you may have regarding your weight, health, or a medical condition. Never disregard professional medical advice or delay seeking it because of something you have read here.