⚖️ BMI Calculator
Work out your BMI, and the measurement current NICE guidance asks for alongside it: waist-to-height ratio, which sees where the weight sits when BMI cannot.
Find the bottom of your ribs and the top of your hips, and measure midway between them — just above the navel — breathing out naturally. That is the method NICE publishes, and it turns two numbers into a measure of where the weight actually sits, which BMI cannot see.
What BMI Calculator Does
Body Mass Index expresses your weight relative to the square of your height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet as a way to describe populations, not individuals — which is exactly why it works well as a first-pass screening number and poorly as a diagnosis. A single value places you on a scale that public health bodies have mapped to broad weight-status bands.
This calculator applies the World Health Organization classification for adults aged 20 and over, in both metric and US customary units. It reports your BMI, the band that value falls in, and the weight range that would put you in the healthy band at your current height — the last of which is usually the more actionable number.
BMI is a ratio of mass to height, so it says nothing about what that mass is made of. A powerlifter and a sedentary person of identical height and weight receive identical scores. Read the limitations section below before treating the result as a verdict on your health.
How to Use BMI Calculator
- Select your preferred unit system (Metric or Imperial)
- Enter your weight in kilograms or pounds
- Enter your height in centimeters or feet/inches
- Click "Calculate BMI" to see your result
- View your BMI category and health interpretation
Formula Used by BMI Calculator
Metric formula (kilograms and meters)
BMI = weight(kg) ÷ height(m)²
- weight
- Body mass in kilograms
- height
- Standing height in meters — note this is meters, not centimeters, so 178 cm becomes 1.78
- BMI
- Result in kg/m²
Worked example
A person weighing 72 kg with a height of 178 cm.
- Convert height to meters: 178 ÷ 100 = 1.78 m
- Square the height: 1.78 × 1.78 = 3.1684
- Divide mass by that square: 72 ÷ 3.1684 = 22.72
Result: BMI = 22.7 kg/m² — inside the 18.5–24.9 healthy band.
US customary formula (pounds and inches)
BMI = 703 × weight(lb) ÷ height(in)²
- 703
- Unit conversion constant that reconciles lb/in² with kg/m²
- weight
- Body mass in pounds
- height
- Standing height in total inches — 5 ft 10 in is 70 in, not 5.10
Worked example
A person weighing 160 lb at 5 ft 10 in (70 inches).
- Convert height to inches: (5 × 12) + 10 = 70 in
- Square the height: 70 × 70 = 4900
- Divide and scale: (160 ÷ 4900) × 703 = 22.95
Result: BMI = 23.0 kg/m² — inside the healthy band.
WHO BMI Categories for Adults
The World Health Organization classification, applied to adults aged 20 and over. The same cut-off points are used for men and women.
| Classification | BMI range (kg/m²) |
|---|---|
| Severe thinness | Below 16.0 |
| Moderate thinness | 16.0 – 16.9 |
| Mild thinness | 17.0 – 18.4 |
| Healthy weight | 18.5 – 24.9 |
| Overweight (pre-obese) | 25.0 – 29.9 |
| Obesity class I | 30.0 – 34.9 |
| Obesity class II | 35.0 – 39.9 |
| Obesity class III | 40.0 and above |
Healthy Weight Range by Height
The weight span that corresponds to a BMI of 18.5–24.9 at each height. Derived by rearranging the metric formula: weight = BMI × height².
| Height | Healthy weight (kg) | Healthy weight (lb) |
|---|---|---|
| 5 ft 0 in / 152 cm | 42.8 – 57.5 | 94 – 127 |
| 5 ft 4 in / 163 cm | 49.2 – 66.2 | 108 – 146 |
| 5 ft 8 in / 173 cm | 55.4 – 74.5 | 122 – 164 |
| 6 ft 0 in / 183 cm | 62.0 – 83.4 | 137 – 184 |
| 6 ft 4 in / 193 cm | 68.9 – 92.8 | 152 – 205 |
Alternative Cut-off Points for Asian Populations
A WHO expert consultation concluded that many Asian populations show elevated cardiovascular and type 2 diabetes risk at BMI values below the standard 25 threshold, and proposed lower public health action points.
| Classification | Standard BMI | Proposed Asian cut-off |
|---|---|---|
| Overweight / increased risk | 25.0 | 23.0 |
| High risk | 30.0 | 27.5 |
Source: WHO expert consultation, The Lancet (2004) — PubMed 14726171
How to Read Your Result
Below 18.5 — underweight
Being underweight carries its own risks, including reduced bone density, impaired immune function, anaemia and, in some cases, fertility problems. A low BMI can also be a marker of an underlying condition rather than a cause in itself. If your BMI is below 18.5 and you have not deliberately lost weight, it is worth raising with a doctor.
18.5 to 24.9 — healthy weight
This band is associated with the lowest all-cause mortality across large population studies. Sitting inside it does not by itself indicate good health — waist circumference, blood pressure, blood lipids and activity level all matter independently of BMI.
25.0 to 29.9 — overweight
Population-level risk of hypertension, type 2 diabetes and cardiovascular disease begins to rise across this band. For an individual, the number matters far less than body composition and where fat is distributed: central (abdominal) fat is more strongly associated with metabolic risk than the same mass carried on hips and thighs.
30.0 and above — obesity
Divided into three classes by severity. Associated risks include type 2 diabetes, coronary heart disease, stroke, several cancers, sleep apnoea and osteoarthritis. This is a screening signal for clinical follow-up, not a diagnosis — a clinician will weigh it alongside history, examination and laboratory findings.
Limitations & Accuracy Notes
- BMI measures excess weight, not excess fat. It cannot distinguish muscle from adipose tissue, so muscular people are routinely classified as overweight while carrying low body fat.
- It ignores fat distribution. Two people with identical BMI carry different metabolic risk depending on whether that mass is visceral or subcutaneous. Waist circumference and waist-to-height ratio capture this better.
- The adult thresholds do not apply to anyone under 20. Children and teenagers are assessed on BMI-for-age percentiles against growth reference charts, because healthy BMI changes substantially with age and sex during development.
- It is not valid during pregnancy, and is unreliable for people with significant oedema, amputations, or very short or very tall stature — the height-squared term systematically over-reads for tall people and under-reads for short people.
- Risk thresholds vary by ancestry. WHO has proposed lower action points for several Asian populations, and evidence also suggests differences for Black and Polynesian populations at the same BMI.
- BMI is a screening number, not a diagnosis, and nothing on this page is medical advice. Discuss your result with a qualified healthcare professional before acting on it.
Frequently Asked Questions
What is BMI?
How is BMI calculated?
Is BMI accurate for everyone?
Is this BMI calculator free?
Is BMI accurate for an individual?
Does BMI work the same for everyone?
Is BMI valid for children?
What should I look at instead?
Why is BMI still used at all?
Is my data stored?
References & Further Reading
- CDC — Adult BMI Calculator — US reference implementation and category definitions for adults 20 and older
- WHO — Global Health Observatory: Body mass index — International classification and cut-off points
- NHLBI (NIH) — Calculate Your BMI — US National Heart, Lung, and Blood Institute standard calculator
- CDC — Growth Charts — BMI-for-age percentile charts used for children and teens aged 2–19
- NHS — Calculate your body mass index — UK guidance, including the lower thresholds applied to some ethnic groups