BMI Calculator
Calculate body mass index and see where you fall in the WHO weight categories.
About BMI Calculator
Body Mass Index estimates body fat from your weight relative to height. It's a useful population screening tool but doesn't distinguish muscle from fat, so athletes may read high.
Formulas
Metric — BMI = kg/m²
BMI = weight (kg) ÷ height (m)²
- BMI
- Body Mass Index
- m_kg
- Body weight in kilograms
- h_m
- Height in metres (cm ÷ 100)
Imperial — BMI = 703 × lb / in²
BMI = 703 × weight (lb) ÷ height (in)²
- 703
- Unit conversion constant
- BMI
- Body Mass Index
- m_lb
- Body weight in pounds
- h_in
- Height in inches
Worked example
With the default values below, BMI Calculator returns Your BMI of 22.9. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Unitsone of Metric (kg, cm), Imperial (lb, in)
- Metric (kg, cm)
- Weightaccepts 1–700 kg / lb
- 70 kg / lb
- Heightaccepts 30–260 cm / in
- 175 cm / in
Results
- Your BMI
- 22.9
- Category
- Normal weight
Step by step
- 1Convert height to metres= 1.75 m
- 2Apply BMI = kg ÷ m²= 22.9
- 3Look up WHO category= Normal weight
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- This is an estimate and not medical advice. BMI is a screening tool, not a diagnostic measure. Consult a healthcare professional for health decisions.
- This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
BMI Calculator — full guide
What BMI is, and what it is not
Body Mass Index is weight divided by height squared. That is the whole of it. It was devised in the 1830s by Adolphe Quetelet as a way to describe populations, not to diagnose individuals, and it is still most reliable used that way.
The standard WHO categories are:
| Category | BMI |
|---|---|
| Underweight | below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese class I | 30.0 – 34.9 |
| Obese class II | 35.0 – 39.9 |
| Obese class III | 40.0 and above |
Where BMI is misleading
BMI does not know what your weight is made of. That produces predictable errors:
- Muscular people are frequently classified as overweight or obese despite
low body fat. A rugby forward and a sedentary person of the same height and weight get the same BMI.
- Older adults lose muscle and gain fat at a stable weight, so BMI can look
unchanged while body composition worsens.
- Height extremes are distorted because the formula squares height, which
under-reads very tall people and over-reads very short people.
- Ethnicity matters. Health risk rises at lower BMI in South Asian and East
Asian populations, and several health bodies use a lower overweight threshold of around 23 for these groups.
BMI is a screening signal, not a diagnosis. A number in the overweight band is
a reason to look further, not a conclusion on its own.
Better questions than "what is my BMI"
- Waist circumference is a stronger predictor of metabolic risk than BMI
because it reflects abdominal fat specifically. Above roughly 94 cm for men and 80 cm for women, risk begins to rise.
- Waist-to-height ratio is simpler still: keep your waist under half your
height.
- Body fat percentage measures what BMI is trying to approximate — see the
Using this sensibly
Track the trend rather than the single reading. A BMI moving steadily in one direction over months tells you something real; a reading that differs by 0.4 from last week tells you about hydration and what you last ate.
If you want a figure that drives actual decisions, the TDEE calculator and calorie calculator are more actionable, because they output something you can change.
Guides that use this calculator
This is an estimate and not medical advice. BMI is a screening tool, not a diagnostic measure. Consult a healthcare professional for health decisions.
Frequently Asked Questions
Is BMI accurate for everyone?
BMI is a general screening measure. It can misclassify very muscular people and doesn't account for age, sex, or fat distribution. Treat it as a starting point, not a diagnosis.
What is a healthy BMI?
The WHO considers 18.5–24.9 the normal range for most adults. Discuss your individual healthy weight with a healthcare provider.
Switching from metric to imperial moved my BMI slightly — why?
Rounding in the inputs, not the formula. 70 kg and 175 cm give 22.9, while the nearest round imperial equivalents of 154 lb and 69 in give 22.7, because 154 lb is 69.85 kg and 69 in is 175.3 cm. Enter decimals in whichever unit you measured in and the two agree.
I am sitting at 24.9 — does one kilogram either side really change anything?
Only the label. At 1.75 m, 76.0 kg reads 24.8 and 76.6 kg reads 25.0, flipping the category from normal weight to overweight. The WHO bands are reporting conventions drawn across large populations; nothing about your body changes as you cross one, so watch the trend rather than the boundary.
Can I use this for my children's weight checks?
No. This page applies the adult WHO bands, which are fixed numbers. Children and teenagers are assessed on age- and sex-specific percentile charts, because healthy BMI changes year by year through growth. A 12-year-old reading 19 may be entirely typical, while the adult bands would call it normal for different reasons.
Where does the 703 in the imperial formula come from?
It is a unit conversion packed into one constant. Dividing pounds by inches squared needs scaling by 0.453592 kg/lb ÷ 0.0254² m²/in², which works out to 703.07 — rounded to 703 by convention. It carries no biological meaning, which is why the metric form, kg ÷ m², needs no constant at all.