BMI Calculator
Enter your height and weight (metric or imperial) to calculate your Body Mass Index and see your weight category.
Enter your height and weight to see your BMI.
⚕️ This tool is for general information only and is not medical advice. Talk to a healthcare professional about your individual health.
BMI categories and what they mean
The World Health Organization (WHO) groups adult BMI into the categories below. Each band is linked to a different statistical level of health risk — but remember these are population averages, not a diagnosis for any individual.
| BMI range | Category | General health risk |
|---|---|---|
| Below 16.0 | Severe underweight | High |
| 16.0 – 18.4 | Underweight | Increased |
| 18.5 – 24.9 | Normal weight | Lowest |
| 25.0 – 29.9 | Overweight | Increased |
| 30.0 – 34.9 | Obese (Class I) | High |
| 35.0 – 39.9 | Obese (Class II) | Very high |
| 40.0 and above | Obese (Class III) | Extremely high |
Asian populations: because health risks rise at lower values, many guidelines use 23+ for overweight and 27.5+ for obese instead of 25 and 30.
Understanding Body Mass Index
Body Mass Index is a single number that relates your weight to your height. It was never meant to diagnose anyone individually — it's a fast, cheap screening tool that flags whether your weight is likely to be in a healthy range, so doctors, gyms and health apps use it as a starting point for a wider conversation about health.
Its great strength is simplicity: you only need a height and a weight, no special equipment. Its weakness is that a single number can't capture everything about a body — which is why the categories should always be read alongside other measures like waist size, body-fat percentage and overall fitness.
How BMI is calculated
There are two versions of the formula depending on your units:
Metric
BMI = weight (kg) ÷ height (m)²
Example: 70 kg, 1.75 m → 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 (normal).
Imperial
BMI = 703 × weight (lb) ÷ height (in)²
Example: 154 lb, 69 in → 703 × 154 ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7 (normal).
Related quick metrics
- BMI Prime = BMI ÷ 25. Below 1 is healthy; above 1 is over the normal range.
- Ponderal Index = weight (kg) ÷ height (m)³. Often more reliable for very tall or very short people than BMI.
Health implications by category
At a population level, both ends of the BMI scale carry raised risks. The relationship is roughly J-shaped: risk is lowest in the normal range (about 20–25) and increases as you move below or above it.
Underweight
Can signal undernutrition, weakened immunity, bone loss, and reduced muscle and energy.
Overweight
Linked to higher blood pressure, raised cholesterol, and increased risk of type 2 diabetes over time.
Obese
Associated with heart disease, type 2 diabetes, joint problems, sleep apnoea and some cancers.
Limitations of BMI
BMI is a useful screen, but it does not measure several important things:
- It cannot tell muscle from fat — muscular people may read as overweight.
- It ignores where fat is stored (belly fat carries more risk than hip/thigh fat).
- It doesn't account for bone density, age, sex, or ethnicity.
- It is less reliable at very tall or very short heights.
Groups that need different interpretation
- Children & teens — use age- and sex-specific BMI percentiles, not adult bands.
- Athletes & bodybuilders — high muscle mass inflates BMI; use body-fat testing instead.
- Older adults — muscle loss can hide excess fat at a "normal" BMI.
- Pregnant people — standard BMI categories don't apply.
Achieving and maintaining a healthy BMI
🥗 Nutrition
Favour whole foods, vegetables, lean protein and fibre; watch portion sizes and sugary drinks rather than chasing extreme diets.
🏃 Exercise
Aim for regular activity — a mix of cardio and strength training helps manage weight and preserve muscle.
😴 Lifestyle
Sleep, stress and hydration all affect weight. Small, consistent habits beat short crash efforts.
To plan this with numbers, estimate your daily energy needs with the Calorie Calculator and your resting burn with the BMR Calculator.
BMR and TDEE — the calories behind weight
Your Basal Metabolic Rate (BMR) is the energy your body uses at rest. The widely used Mifflin-St Jeor equation is:
- Men: 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
- Women: 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
Your Total Daily Energy Expenditure (TDEE) multiplies BMR by an activity factor:
| Activity level | Multiplier |
|---|---|
| Sedentary (little/no exercise) | × 1.2 |
| Light (1–3 days/week) | × 1.375 |
| Moderate (3–5 days/week) | × 1.55 |
| Active (6–7 days/week) | × 1.725 |
| Very active (hard training/physical job) | × 1.9 |
Body composition beyond BMI
Because BMI can't see inside your body, these measures give a fuller picture of health:
| Metric | What it shows | Healthy guide |
|---|---|---|
| Waist circumference | Abdominal fat | <94 cm men / <80 cm women |
| Waist-to-hip ratio | Fat distribution | <0.90 men / <0.85 women |
| Body fat % | Fat vs lean mass | ~10–20% men / ~18–28% women |
| Muscle mass | Strength & metabolism | Higher is generally protective |
Estimate yours with the Body Fat Calculator and the Ideal Weight Calculator.
A short history of BMI
The formula was created in the 1830s by the Belgian mathematician Adolphe Quetelet, who was studying the "average man" — it was a tool of statistics, not medicine, and was originally called the Quetelet Index. It only entered mainstream health use in 1972, when the researcher Ancel Keys popularised it and gave it the name "Body Mass Index" as a cheap way to compare weight across large populations.
Today, alternatives such as bioelectrical-impedance scales, DEXA scans and indices like ABSI offer more detail — but BMI endures because it needs nothing more than a height and a weight, which keeps it useful as a first, fast screen.
Key takeaways
- BMI = weight ÷ height² — a quick screen, not a diagnosis.
- 18.5–24.9 is the normal range for most adults; risk rises above and below it.
- It can't tell muscle from fat, so pair it with waist size and body-fat measures.
- Athletes, children, older adults and pregnant people need different assessment.
- Use it as a starting point for a conversation with a healthcare professional.
Frequently asked questions
What is BMI?
Body Mass Index is a simple measure that uses your height and weight to estimate whether your weight is in a healthy range. The formula is weight in kilograms divided by height in metres squared (kg/m²).
How is BMI calculated?
In metric units, BMI = weight (kg) ÷ height (m)². In imperial units, BMI = 703 × weight (lb) ÷ height (in)². For example, 70 kg at 1.75 m gives 70 ÷ 3.0625 = 22.9.
What are the BMI categories?
For adults: below 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30 or above is obese. Obesity is sometimes split into Class I (30–34.9), II (35–39.9) and III (40+).
What is a healthy BMI range?
A BMI between 18.5 and 24.9 is generally considered healthy for most adults, though the ideal point within that range varies by individual.
Is BMI accurate for everyone?
No. BMI does not distinguish muscle from fat or account for bone density, fat distribution, age, sex, or ethnicity. Athletes, older adults, pregnant people and children need different assessment.
Why can a muscular athlete have a high BMI?
Muscle is denser and heavier than fat, so a lean, muscular person can weigh more for their height and score as “overweight” on BMI despite having low body fat.
Do Asian populations use different BMI cut-offs?
Yes. Because health risks appear at lower BMI values, many Asian guidelines treat 23+ as overweight and 27.5+ as obese, rather than 25 and 30.
What is BMI Prime?
BMI Prime is your BMI divided by 25 (the upper limit of normal weight). A value below 1 is within the healthy range; above 1 means you are over it. For example, a BMI of 27.5 gives a BMI Prime of 1.1.
Should BMI be used for children?
Not directly. Children and teens are assessed using age- and sex-specific BMI percentiles rather than the fixed adult categories.
How often should I check my BMI?
For most people, occasionally — monthly or when your weight changes noticeably — is enough. BMI changes slowly, and daily checking adds little value.
Related tools
Try the Calorie Calculator or Ideal Weight Calculator.