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BMI Calculator

Calculate your Body Mass Index with the WHO formula. See your BMI value, your WHO category, and the healthy weight range for your height in metric or imperial units.

BMI inputs

This calculator is for informational purposes only and is not a substitute for professional medical advice. Consult a qualified healthcare provider before making changes to your diet, exercise, or health regimen.

What is BMI and why does it matter?

Body Mass Index (BMI) is a single number that describes how heavy a person is relative to their height. It was proposed by the Belgian statistician Adolphe Quetelet in the 1830s and formally standardised by the World Health Organization in the 1990s as the global screening tool for underweight, overweight, and obesity in adults. Despite its limitations it remains the most widely used measure in public health research because it requires only a scale and a tape measure, correlates reasonably well with body fatness at the population level, and is the same everywhere in the world.

BMI matters because weight sits at the centre of many chronic disease risks. Sustained elevated BMI is linked to type 2 diabetes, cardiovascular disease, several cancers, osteoarthritis, and obstructive sleep apnoea. Very low BMI carries its own risks — osteoporosis, anaemia, fertility problems, and in extreme cases organ failure. The point of calculating your BMI is not to shame yourself into a number, but to take a quick look at where you sit on the distribution and decide whether a more detailed conversation with a clinician is worth having.

How this calculator works

The formula, as defined by the World Health Organization, is:

  • Metric: BMI = weight (kg) ÷ height (m)²
  • Imperial: BMI = (weight (lb) ÷ height (in)²) × 703

This calculator converts imperial inputs to metric internally and applies the metric formula, so you get exactly the same answer either way. We then look up your BMI in the WHO adult classification table and return the category plus the range of weights that would land you in the healthy 18.5–24.9 band for your specific height.

The WHO thresholds for adults (aged 20 and older) are:

  • Underweight: BMI < 18.5
  • Normal weight: BMI 18.5 – 24.9
  • Overweight: BMI 25.0 – 29.9
  • Obese class I: BMI 30.0 – 34.9
  • Obese class II: BMI 35.0 – 39.9
  • Obese class III (severe): BMI ≥ 40.0

Worked example

Consider an adult who weighs 72 kg and stands 175 cm tall. Converting height to metres gives 1.75 m, and squaring that is 3.0625. Dividing weight by height squared: 72 ÷ 3.0625 ≈ 23.5. That places them comfortably in the normal weight category. The healthy weight range for their height is 18.5 × 3.0625 ≈ 56.7 kg at the low end and 24.9 × 3.0625 ≈ 76.3 kg at the top, so there is about four kilograms of headroom before they would cross into the overweight band.

The same person in imperial units weighs about 159 lb and stands 5′9″ (69 in). (159 ÷ 69²) × 703 = (159 ÷ 4,761) × 703 ≈ 23.5 — the same number. The units don't change the physics, only the presentation.

How to interpret your BMI

Think of BMI as a weather forecast, not a diagnosis. A value in the normal range is reassuring but does not guarantee good health — a sedentary, smoking, junk-food eater with a BMI of 22 is a worse bet than an active, non-smoking vegetable eater with a BMI of 27. Conversely, a high BMI does not automatically mean poor health: Olympic weightlifters often register as obese on BMI alone. The right response is the same in either direction: if the number surprises you, look deeper.

For people in the overweight or obese categories the practical interpretation is that modest, sustained weight loss (5–10% of current body weight) produces measurable improvements in blood pressure, blood sugar, cholesterol, and joint load. You do not need to "hit" normal BMI to see benefits. Conversely, for people in the underweight category, the action item is usually to investigate the cause — insufficient intake, malabsorption, chronic illness, or an eating disorder — before focusing on weight gain.

Limits of BMI

BMI treats muscle and fat identically and ignores where fat is stored. That is fine for population statistics but can mislead in individuals. A few notable situations where BMI alone falls short:

  • Athletes and strength-trained people — high muscle mass inflates BMI without health risk.
  • Elderly adults — muscle loss (sarcopenia) can keep BMI "normal" while body fat percentage creeps up.
  • Asian populations — tend to carry more visceral fat at lower BMIs; many Asian countries use lower cut-offs.
  • Pregnant or breastfeeding women — BMI does not apply; use pre-pregnancy BMI for planning.
  • Children and adolescents — use age- and sex-specific percentile charts, not adult thresholds.

Common mistakes

  • Weighing yourself at random times. Daily weight fluctuates by 1–2 kg from food, water, and glycogen. Use a morning, post-toilet, pre-breakfast weight and average over a week.
  • Measuring height in shoes. A centimetre of shoe sole can change your BMI by 0.1–0.2. Measure barefoot, standing straight.
  • Comparing BMIs across ages. BMI norms differ for adolescents, adults, and the elderly. Do not plug a child's numbers into this calculator.
  • Treating the number as permanent. BMI is a snapshot. It changes when your weight or height change.
  • Using BMI to set appearance goals. BMI is a health screening tool, not a beauty metric. Normal BMI covers a very wide range of body shapes.

When to consult a professional

A BMI result outside the normal band is a signal to have a conversation, not a prescription. Book an appointment with a doctor or registered dietitian if your BMI is below 18.5 or above 30, if it has changed substantially without intentional effort, or if you have symptoms — unusual fatigue, breathlessness, joint pain, changes in eating patterns — that might be related. Clinicians can order more informative measurements (waist circumference, body fat percentage, lab work, sometimes a DEXA scan) and put your number in context. This calculator is a starting point, not a substitute for medical advice.

Frequently Asked Questions

What is a healthy BMI?
According to the World Health Organization, a BMI between 18.5 and 24.9 is considered healthy for most adults. Below 18.5 is classified as underweight, 25.0–29.9 as overweight, and 30.0 or above as obese. These cut-offs were derived from large epidemiological studies linking weight-for-height to mortality risk and were formally adopted by the WHO in the 1990s.
How is BMI calculated?
BMI equals your weight in kilograms divided by the square of your height in metres: BMI = kg / m². For imperial units the formula becomes (lb ÷ in²) × 703. The calculator handles the conversion automatically — you just pick your preferred units, enter weight and height, and we return a single number plus the WHO category.
Is BMI accurate for athletes or muscular people?
Not very. BMI treats all body mass the same, so a muscular person with low body fat can register as "overweight" or even "obese". Bodybuilders, rugby players, sprinters, and weightlifters routinely fall outside the normal range despite being lean. For those populations, body fat percentage, waist-to-hip ratio, or DEXA scans give a far better picture than BMI alone.
Does BMI apply to children?
The adult thresholds do not apply to anyone under 20. Children and adolescents are evaluated against age- and sex-specific percentile charts (BMI-for-age), published by the CDC and WHO. A paediatrician can plot a child's BMI on those charts and interpret the percentile. This calculator is intended for adults only.
Why does BMI use height squared and not cubed?
Statistically, in human populations, body mass scales with height roughly to the power of 2, not 3. This was observed by Adolphe Quetelet in the 1830s (hence "Quetelet index") and confirmed by modern data. Cubing height would overestimate expected weight for tall people and underestimate it for short people. BMI squared is a pragmatic compromise that correlates well with body fat across typical height ranges.
What is a "healthy weight range" for my height?
It is simply the range of weights that put your BMI between 18.5 and 24.9. For example, a person who is 170 cm tall has a healthy weight window of about 53.5 to 72.0 kg (118 to 159 lb). The calculator shows this range automatically so you can see how far you are from the boundaries rather than fixating on a single "target" number.
Is a BMI of 24 "better" than a BMI of 22?
Within the normal range the differences are small and largely noise. Epidemiology suggests the lowest mortality risk across large populations sits somewhere in the 22–24 band, but individual optimum depends on muscle mass, fitness, ethnicity, and age. Obsessing over tenths of a point is not useful — staying inside the normal band and being physically active matters far more.
Should different populations use different BMI cut-offs?
Yes — the WHO has acknowledged that Asian populations tend to carry proportionally more visceral fat at lower BMIs than European populations. Some countries (Japan, China, Singapore) use lower cut-offs (overweight ≥ 23, obese ≥ 25). The WHO still publishes the global thresholds as the reference for international comparison, which is what this calculator uses.
Can I use BMI during pregnancy?
No. BMI is not meaningful during pregnancy or in the first months postpartum because of fluid shifts, uterine growth, and the developing baby. Pre-pregnancy BMI is used to set weight-gain targets during pregnancy, but calculation and interpretation should be handled by your obstetrician or midwife.
Is this calculator a substitute for medical advice?
No. BMI is a screening tool, not a diagnosis. A high or low value is a prompt to look more closely — with blood work, body composition measurement, and a conversation with a qualified healthcare professional — not an instruction to diet or gain weight. Always consult a doctor or registered dietitian before making significant changes to how you eat or train.