What is BMI?
BMI stands for Body Mass Index. It is a simple numerical measure calculated from your height and weight that is widely used as a screening tool to categorise people into weight ranges — underweight, healthy weight, overweight, and obese.
BMI was developed in the 1830s by a Belgian mathematician called Adolphe Quetelet, which is why it is sometimes still referred to as the Quetelet Index. It was originally designed as a statistical tool for studying populations — not as a diagnostic tool for individual health.
📏 In simple terms: BMI is a quick, easy calculation that gives a rough indication of whether your weight is in a healthy range for your height. It is a screening tool, not a definitive health measure.
How is BMI Calculated?
BMI is calculated using a straightforward formula that divides your weight in kilograms by your height in metres squared:
Example: A person weighing 75kg who is 1.75m tall:
BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5
Our TDEE calculator automatically calculates your BMI alongside your calorie targets — so you don't need to do the maths yourself.
Calculate Your BMI Instantly
Our free calculator gives you your BMI, BMI category, TDEE, and personalised calorie targets all in one place.
Calculate My BMI Free →BMI Categories — What Does Your Score Mean?
The World Health Organisation (WHO) uses the following BMI categories for adults:
| BMI Range | Category | What it means |
|---|---|---|
| Below 18.5 | Underweight | May indicate insufficient nutrition or an underlying health condition |
| 18.5 – 24.9 | Healthy weight | Associated with the lowest risk of weight-related health issues for most people |
| 25.0 – 29.9 | Overweight | Slightly elevated risk of certain health conditions — lifestyle changes may be beneficial |
| 30.0 and above | Obese | Associated with increased risk of type 2 diabetes, heart disease, and other conditions |
The Limitations of BMI
While BMI is a useful population-level screening tool, it has significant limitations when applied to individuals. It is important to understand what BMI does — and does not — tell you about your health.
⚠️ Important: BMI does not directly measure body fat. Two people with the same BMI can have very different body compositions and health profiles. Always consider BMI alongside other health measures.
BMI does not account for muscle mass
This is the most well-known limitation of BMI. Muscle is denser and heavier than fat. A very muscular athlete may have a high BMI that classifies them as "overweight" or even "obese" despite having very low body fat and excellent health markers. Many professional athletes fall into this category.
BMI does not account for fat distribution
Where you carry body fat matters as much as how much you carry. Fat stored around the abdomen (visceral fat) is associated with significantly higher health risks than fat stored around the hips and thighs. Two people with identical BMIs but different fat distributions can have very different health risks.
BMI does not account for age
Older adults naturally lose muscle mass over time, meaning a person in their 60s or 70s with a "healthy" BMI may still have a relatively high body fat percentage. In older populations, slightly higher BMIs are sometimes associated with better outcomes — a phenomenon sometimes called the "obesity paradox".
BMI does not account for sex differences
Women naturally carry a higher percentage of body fat than men at the same BMI. The standard BMI formula uses the same thresholds for both sexes, which means it may underestimate health risks in women at any given BMI.
BMI does not account for ethnicity
Research has shown that people of Asian descent tend to have higher health risks at lower BMI values than people of European descent. Some health organisations recommend lower BMI thresholds for Asian populations as a result.
✅ Advantages of BMI
- Quick and easy to calculate
- Requires no equipment
- Free and universally available
- Useful for population-level research
- Good starting point for health conversations
❌ Limitations of BMI
- Doesn't measure body fat directly
- Ignores muscle mass
- Doesn't account for fat distribution
- Less accurate for older adults
- Varies in accuracy by ethnicity
Better Measures to Use Alongside BMI
For a more complete picture of your health, consider these measures alongside your BMI:
Waist circumference
Measuring around your waist at the level of your navel gives an indication of visceral (abdominal) fat. General guidelines suggest health risks increase above 80cm for women and 94cm for men.
Waist-to-height ratio
Dividing your waist circumference by your height gives a simple ratio. A waist-to-height ratio below 0.5 is generally considered healthy for most adults — in other words, your waist should be less than half your height.
Body fat percentage
A direct measure of how much of your body weight is fat versus lean tissue. This can be estimated visually, measured with skinfold callipers, or assessed more accurately with DEXA scanning. Our TDEE calculator includes a visual body fat estimation tool to help you get a rough idea of your body fat range.
Blood markers
Blood pressure, cholesterol levels, blood glucose, and other clinical markers give a far more accurate picture of cardiovascular and metabolic health than BMI alone.
Calculate Your BMI and More
Our free calculator gives you BMI, TDEE, body fat estimate, maintenance calories and personalised macro targets — all in seconds.
Calculate My BMI Free →Frequently Asked Questions
What is a healthy BMI for adults?
For most adults, a BMI between 18.5 and 24.9 is considered a healthy weight range. However, this is a general guideline and does not apply equally to everyone — particularly very muscular individuals, older adults, and people of Asian descent.
Is BMI accurate for women?
BMI uses the same formula and thresholds for both men and women, despite the fact that women naturally carry more body fat. This means BMI may underestimate health risks in women at the same BMI compared to men. Waist circumference is a useful additional measure for women.
Can you be healthy with a high BMI?
Yes — particularly if a high BMI is driven by high muscle mass rather than excess body fat. Many athletes have BMIs in the overweight or obese range while having excellent cardiovascular fitness, low body fat, and healthy blood markers. BMI alone does not determine health status.
Can you be unhealthy with a normal BMI?
Yes — this is sometimes called "normal weight obesity" or being "skinny fat". A person can have a BMI in the healthy range while carrying a relatively high proportion of body fat and low muscle mass, which can still be associated with metabolic health risks.
How often should I check my BMI?
BMI is most useful when tracked over time rather than as a single snapshot. Checking every few months — or whenever your weight changes significantly — gives you a useful trend to monitor. Day-to-day fluctuations in weight are normal and not meaningful.