Why use this calculator
BMI is the figure clinicians, insurers and public health bodies reach for first, so knowing yours tells you how you will be categorised before anyone else does it for you. It is genuinely useful as a starting point: it correlates well with body fat across large groups, it costs nothing, and it is reproducible anywhere with scales and a tape measure.
What it cannot do is assess an individual, because two inputs cannot distinguish muscle from fat or say where that fat sits. This calculator gives you the number, the band it falls in and your healthy weight range, and is explicit about the situations in which the result should be ignored rather than acted on.
How to use it
- Choose metric or imperial first, because the fields change with it.
- Weight should be measured first thing in the morning, after using the bathroom and before eating, since food and fluid move scale weight by one to two kilograms across a day.
- Height is the input people most often get wrong from memory.
- Adults lose height slowly from their thirties, so measure against a wall without shoes rather than using the figure on your licence.
- Read the healthy weight range alongside the number: it is usually more useful than the BMI itself, because it gives you a target in units you can act on.
What each result means
- Your BMI
- Your weight relative to your height on the standard scale. It is a screening figure, not a diagnosis, and it cannot see body composition.
- Category
- The World Health Organization band your figure falls in. Some guidelines apply lower thresholds for people of South and East Asian descent.
- To reach the healthy band
- The smallest weight change that puts your BMI between 18.5 and 24.9, measured to the nearest edge of the band rather than its middle. If you are already in the band it shows how much room you have either way.
- Healthy Weight Range
- The weight range that would put you between 18.5 and 24.9 for your height. Usually more actionable than the BMI number itself.
- Above healthy midpoint
- How far you sit above the centre of the healthy range. Being above it is not automatically a problem, particularly if you carry muscle.
- Below healthy midpoint
- How far below the centre of the healthy range you sit. Persistent underweight carries its own risks to bone density and immunity.
- Ponderal Index
- Weight divided by height cubed rather than squared. It distorts less at the extremes of height, which is why it suits very tall and very short people.
- WHO healthy BMI band
- The reference range your figure is compared against, shown so the category is not a black box.
The formula
BMI = weight in kilograms divided by height in metres squared
In imperial units the same relationship
BMI = 703 x weight in pounds divided by height in inches squared
The World Health Organization bands are underweight below 18.5, healthy 18.5 to 24.9, overweight 25 to 29.9, and obese 30 and above. The healthy weight range shown is the weight that would place you at a BMI of 18.5 and at 24.9 for your measured height.
How this number is used in practice
The single most useful companion to BMI is a tape measure. Where fat sits matters more than how much there is: visceral fat around the organs drives most of the metabolic risk, while fat on hips and thighs carries far less. Commonly cited waist thresholds are 94 centimetres for men and 80 for women for increased risk, and waist-to-height ratio, keeping your waist under half your height, travels better across body types than any single BMI cut-off does.
BMI misclassifies predictably. A rugby player or a regular lifter reads as overweight while carrying low body fat. An older adult who has lost muscle can read as healthy while carrying excess fat, a pattern sometimes called normal weight obesity. Pregnancy, growth in children and adolescents, and conditions causing fluid retention all make the figure meaningless. In each case the number is not wrong so much as answering a question you did not ask.
Worked examples
5 ft 10 in, 176 lb adult
BMI of 24.4. At the top of the healthy band, with a healthy weight range of 129 to 174 lb.
175 cm, 95 kg adult
BMI of 31.0, which falls in the obese band, against a healthy range of about 57 to 76 kg.
160 cm, 45 kg adult
BMI of 17.6, below the 18.5 threshold, suggesting underweight worth discussing with a clinician.
When people use it
- Checking where you sit before a health assessment or insurance application
- Setting a realistic weight target from the healthy range rather than a round number
- Tracking a trend over months rather than reacting to a single reading
- Understanding a BMI figure quoted by a clinician or on a form
- Explaining why an athlete's BMI reads high without any health concern
Tips
- Weigh yourself at the same time under the same conditions. Daily swings of 1 to 2 kg come from water and food, not fat.
- Pair BMI with a waist measurement. Under 94 cm for men and 80 cm for women is the commonly cited lower-risk threshold.
- Waist-to-height ratio under 0.5 is a simple rule that works across different builds and ancestries.
- Some guidelines use lower BMI thresholds for people of South and East Asian descent, where risk rises at lower values.
- Ignore BMI entirely if you are pregnant, under 18, or carrying substantial muscle. It is not measuring what you think.
- Track the trend across months. A single reading says far less than six readings in a row.
What it assumes, and where it stops being accurate
- Standard World Health Organization adult thresholds are used, without ancestry-specific adjustments.
- The formula cannot distinguish muscle from fat, or say where body fat is distributed.
- It is not valid for children, adolescents, pregnant people, or anyone with significant fluid retention.
- Height and weight are assumed accurate; remembered figures are frequently out by several centimetres.
- The healthy weight range comes purely from the BMI band and ignores frame size and body composition.
Terms used on this page
- Body mass index
- Weight in kilograms divided by height in metres squared, used to screen weight relative to height.
- Ponderal index
- Weight divided by height cubed, which distorts less than BMI at very tall and very short heights.
- Visceral fat
- Fat stored around the abdominal organs, carrying more metabolic risk than fat stored under the skin.
- Waist-to-height ratio
- Waist circumference divided by height; under 0.5 is a commonly used lower-risk guide.
Questions people ask
- What is a healthy BMI?
- The World Health Organization defines 18.5 to 24.9 as the healthy band for adults, with overweight from 25 and obese from 30. Some guidelines apply lower thresholds for people of South and East Asian descent, where cardiometabolic risk rises at lower values. The band is a population guide rather than an individual target.
- Why does my BMI say overweight when I am fit?
- BMI uses only height and weight, so it counts muscle exactly as it counts fat. Someone who trains regularly carries more lean mass at the same height and reads higher without any of the risk the category implies. If your waist measurement is well within range and your fitness markers are good, the BMI figure is the misleading one.
- Is BMI accurate for older adults?
- Less so. Muscle mass declines with age while fat often increases, so an older adult can sit in the healthy band while carrying excess body fat and too little muscle. Height loss inflates the figure further. Waist circumference, grip strength and how easily you rise from a chair say more about health at that stage.
- What should I use instead of BMI?
- Use it alongside other measures rather than instead. Waist circumference captures visceral fat, waist-to-height ratio works across body types, and body fat percentage from calipers or a DEXA scan measures composition directly. Blood pressure, resting heart rate, lipids and blood glucose carry information no body measurement provides.
- Does BMI apply to children?
- Not in the adult form. Children and adolescents are assessed against age and sex specific percentile charts, because body composition changes substantially through growth. A child's raw BMI number is meaningless without the percentile, and interpretation belongs with a paediatrician or child health nurse.
Use this with
BMI Calculator answers one part of the question. These pick up where it stops: Body Fat Percentage Calculator, Ideal Weight Calculator, BMR Calculator and TDEE Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- World Health Organization: obesity and overweight. BMI categories and health risk
- NIH: assessing your weight and health risk. Waist circumference and risk thresholds
BMI Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.