How is BMI calculated?

Divide weight in kilograms by height in metres squared: BMI = kg ÷ m². Example: 70 kg ÷ (1.75 × 1.75) = 22.9. In imperial units: 703 × lb ÷ in².

Is BMI accurate for muscular people?

Not on its own. Muscle is denser than fat, so athletes often land in Overweight without excess fat. If you lift regularly, add waist circumference and body-fat estimates.

What is the BMI for women vs men?

The formula and WHO bands are the same for adult men and women. What differs is interpretation — women typically carry more essential fat at the same BMI, so clinicians weigh waist and health history alongside.

BMI ≥ 30 is classified as obesity (Class I 30–34.9, Class II 35–39.9, Class III ≥ 40). It signals higher average risk, not a personal diagnosis — discuss screening with your clinician.

How often should I check BMI?

Monthly or quarterly is plenty. Daily checks mostly capture water and food. Compare like-for-like conditions and watch the 3-month trend.

Does age affect BMI interpretation?

Yes. After ~65, slightly higher BMI (22–27) is often associated with lower frailty risk. Muscle loss (sarcopenia) also makes BMI less informative, so grip strength and balance matter more.

Health · Free tool

BMI Calculator

Know your Body Mass Index in seconds — with healthy-range context.

Body Mass Index (BMI) is the world's most widely used screening measure for weight status. It takes just two numbers — your weight and your height — and places you on a standardised scale used by the World Health Organization (WHO), the CDC, and clinicians in more than 100 countries. Our BMI calculator above does that maths instantly, shows your WHO category, and tells you the healthy weight range for your exact height.

BMI is not a diagnosis and it cannot see muscle, bone density, pregnancy, or where fat is stored — but it remains the fastest, cheapest first check. A 2023 Lancet review found BMI still correctly flags elevated health risk in roughly 8 in 10 adults when combined with waist measurement. That is why doctors use it as a conversation starter, not a verdict. On this page you will learn the formula, work through a full example, see who should (and should not) rely on BMI, and get practical next steps whatever your result.

Enter your weight in kilograms and height in centimetres — or switch to pounds, feet and inches. You will get your BMI to one decimal place, your category (Underweight, Healthy, Overweight, Obesity), and the weight band that would put you in the healthy 18.5–24.9 range. Everything is calculated in your browser; nothing is uploaded or stored.

Updated 2026-09-01 · 7-min read · Formula + steps included

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BMI formula

BMI = weight (kg) ÷ height (m)²

BMI divides weight by the square of height so that taller people are not unfairly penalised. Squaring height approximates how body volume scales with stature. The result is expressed in kg/m². Imperial users can use BMI = 703 × weight (lb) ÷ height (in)² — the 703 is simply the unit-conversion constant that makes the two formulas identical.

weight: Your body weight in kilograms (1 kg = 2.20462 lb)
height: Your standing height in metres (cm ÷ 100). Measure without shoes, heels together.
BMI: Index in kg/m². WHO bands: <18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, ≥30 obesity.

Worked example: 70 kg at 175 cm

Priya weighs 70 kg and is 175 cm tall. Here is exactly what the calculator does:

Convert height to metres: 175 ÷ 100 = 1.75 m.
Square the height: 1.75 × 1.75 = 3.0625 m².
Divide weight by squared height: 70 ÷ 3.0625 = 22.857…
Round to one decimal: BMI = 22.9 — Healthy range.
Healthy band for 1.75 m: 18.5 × 3.0625 = 56.7 kg to 24.9 × 3.0625 = 76.3 kg. Priya sits comfortably inside it.

Result: BMI 22.9 — Healthy. Healthy weight for this height: 56.7–76.3 kg.

How to use this BMI calculator

Three steps, under 30 seconds. For best accuracy, weigh yourself in the morning before eating.

Step 1: Enter weight

Use kilograms (e.g. 68.5). If you only know pounds, divide by 2.20462 or switch the unit toggle — the maths is identical.

Step 2: Enter height

Use centimetres measured without shoes. Stand against a wall, heels together, look straight ahead, and mark the top of your head.

Step 3: Press Calculate

Read your BMI, colour-coded category, and personal healthy range. Use Reset to clear and try another person or unit system.

Step 4: Interpret with context

Pair BMI with waist circumference, activity, and how you feel. Athletes, pregnant people, and over-65s should read the limitations below.

When a BMI check actually helps

BMI is most useful as a low-friction screening step in these real situations:

Annual health MOTs

GPs record BMI yearly because a 2–3 point drift often appears 12–18 months before blood-pressure or blood-sugar changes. Tracking your own number quarterly gives the same early warning.

Fitness programmes

If you are starting strength training, BMI plus waist and progress photos tells you whether weight gain is muscle or fat. Muscle gain with a stable waist is usually positive.

Fertility & pregnancy planning

Both low (<18.5) and high (≥30) pre-conception BMI are linked with longer time-to-pregnancy. Clinicians use the number to tailor folic-acid dose and screening.

Insurance & visa medicals

Many insurers and immigration medicals still ask for BMI bands. Knowing yours avoids surprises and lets you bring supporting context like body-fat %.

Weight-loss goal setting

Instead of picking an arbitrary target, use the top of your healthy band as an evidence-based first goal, then reassess with your clinician.

Pro tips for an accurate reading

Small measurement errors move BMI more than people expect — 2 cm of height error shifts BMI by ~0.5.

Weigh on a hard floor, not carpet, with the scale zeroed. Weigh twice and average.
Measure height in the morning — spinal compression makes you ~1 cm shorter by evening.
Wear light clothing or adjust: subtract ~0.5–1 kg for everyday indoor clothes.
Track the trend, not the day: weigh weekly under the same conditions and compare 4-week averages.
Add waist circumference: <80 cm (women) / <94 cm (men) alongside a healthy BMI is the strongest low-risk signal.

Common BMI mistakes

Avoid these four errors that make BMI misleading:

Using it as a body-fat test

BMI cannot distinguish muscle from fat. A rugby player at BMI 29 may have 12% body fat; a sedentary adult at BMI 23 may have 32%. Pair it with waist or body-fat %.

Guessing height

Most adults overstate height by 2–3 cm, which understates BMI by nearly a full point. Measure properly once a year.

Applying adult bands to children

Under-18s need age-and-sex BMI percentiles, not 18.5–24.9. Use a paediatric chart instead.

Panicking over 0.2 points

Hydration, meals and scales swing weight ±1 kg daily. Only sustained moves across a boundary matter.

Disclaimer: This calculator is for general information only and is not medical advice. BMI has known limitations. If you are concerned about weight, pregnant, under 18, an athlete, or managing a condition, speak with a qualified health professional.

FAQs

Frequently asked questions

For most adults, 18.5–24.9 kg/m² is the WHO healthy range. Risk rises gradually from 25 and more steeply from 30, but waist size, blood pressure, cholesterol and fitness modify what your number means for you.