BMI Calculator

Free BMI Calculator (Body Mass Index)

This BMI calculator works out your Body Mass Index from your height and weight and tells you which standard weight category you fall into — underweight, normal weight, overweight, or obese — along with a healthy weight range worked out for your own height. Switch between metric (kg, cm) and imperial (lb, ft/in) units, and the result updates instantly as you type.

BMI is the most widely used weight-screening number in the world because it needs only two measurements and gives a repeatable result you can track over months or years. It is a starting point, not a full health check. Use it alongside how you feel, your fitness level, and — where it matters — a conversation with a doctor.

BMI Formula: How BMI Is Calculated

In metric units, BMI = weight in kilograms ÷ (height in metres)². So someone weighing 70 kg at a height of 1.70 m has a BMI of 70 ÷ (1.70 × 1.70) = 24.2. In imperial units, BMI = 703 × weight in pounds ÷ (height in inches)². A 150 lb person who is 5 ft 6 in (66 in) tall has a BMI of 703 × 150 ÷ 66² ≈ 24.2 — the same result, just converted through a different unit system.

The formula was first proposed in the 1830s by Belgian mathematician Adolphe Quetelet, which is why BMI is sometimes still called the Quetelet Index. It was designed to describe body-weight patterns across large populations, not to assess a single person's health in detail — a distinction worth keeping in mind when you read your own number.

Standard BMI Categories (Adults)

For most adults aged 20 and above, the World Health Organization defines four standard categories used worldwide as a screening guide:

• Underweight: BMI below 18.5
• Normal or healthy weight: BMI 18.5 – 24.9
• Overweight: BMI 25.0 – 29.9
• Obesity: BMI 30.0 and above

BMI Cutoffs Differ Around the World

The 18.5–24.9 range above is the WHO's general international standard, but it is not universal. Several Asian health bodies — including in India, China, and Singapore — use lower cutoffs because research links a higher risk of diabetes and heart disease at a lower BMI in South and East Asian populations. Under these Asian BMI standards, overweight typically starts at 23 rather than 25, and obesity starts around 27.5 rather than 30.

This matters in practice: two people with the same BMI of 24 might be classed as "normal weight" under the WHO scale and "overweight" under an Asian-specific scale used by their own national health guidelines. If your doctor or local health authority uses a regional cutoff, follow that guidance over a generic international chart.

What Counts as a Healthy BMI?

A BMI between 18.5 and 24.9 is generally described as the healthy range for adults, and the calculator above converts that range into an estimated healthy weight for your exact height. Treat it as a broad reference zone rather than a single number to hit exactly — two people with the same healthy BMI can look and feel quite different depending on muscle, frame size, and where they carry weight.

If your result falls outside this range, resist the urge to make a sudden or extreme change based on one number. A slow, sustainable adjustment — built around regular meals, activity you enjoy, and enough sleep — is safer and more likely to stick than a crash diet or an intense short-term programme.

Why BMI Can Be Misleading

BMI cannot tell the difference between muscle, fat, bone, and water — it only relates total weight to height. A muscular athlete or someone who trains with weights regularly can register as "overweight" or even "obese" on the BMI scale despite having low body fat and excellent health markers. At the other end, someone can land in the "normal" range while carrying a higher proportion of body fat and having relatively little muscle — a pattern sometimes called "skinny fat".

This is why BMI works best as a population-level screening tool rather than a personal verdict. If your BMI seems out of step with how fit or healthy you feel, a body-fat percentage estimate, a waist measurement, or a conversation with a healthcare professional will tell you far more than the BMI number on its own.

BMI vs. Body Fat Percentage

BMI and body fat percentage answer different questions. BMI compares your total weight to your height; body fat percentage estimates how much of your weight is actually fat versus lean mass. Two people can share an identical BMI and have noticeably different body-fat percentages — which is why doctors, trainers, and dietitians often use both figures together rather than relying on either alone.

If you want a fuller picture, pair this BMI result with a body fat percentage calculator, a waist measurement, and — where relevant — a skinfold or bioimpedance reading from a professional. None of these tools is perfect individually, but together they give a much more complete view than any single number.

BMI for Children, Teens, and Older Adults

This calculator uses the standard adult BMI formula and adult categories, which apply to people aged 20 and over. Children and teenagers grow at different rates by age and sex, so their BMI is plotted on age- and sex-specific growth percentile charts instead — use a dedicated BMI calculator for kids or teens rather than the adult categories shown here.

Older adults are a special case too: a slightly higher BMI later in life is sometimes associated with better outcomes than a very low one, because it can reflect protective muscle and fat reserves. A geriatric-specific BMI calculator accounts for this shifted reference range if you are calculating BMI for someone in their 60s or beyond.

Waist Size and Where You Carry Weight

Where fat is stored matters as much as how much of it there is. Fat carried around the abdomen is more strongly linked to cardiovascular and metabolic risk than fat carried on the hips or thighs, which is why waist circumference is often used alongside BMI. Measure around your waist with a flexible tape, at the midpoint between your lowest rib and the top of your hip bone, without pulling the tape tight or holding your breath.

A rising waist measurement with a stable BMI can be an early sign worth paying attention to, just as a falling waist with stable weight often reflects a favourable shift in body composition — for example, gaining muscle while losing fat during resistance training.

Using Your BMI Result to Set a Goal

The healthy weight range shown with your result comes directly from reversing the BMI formula for your height — it is a reference zone, not a prescription. Choose a goal that fits your starting point, your medical history, your schedule, and what you can realistically sustain. Small, repeatable changes to eating and movement tend to outlast dramatic short-term diets.

If you have a chronic condition such as diabetes, thyroid disease, heart disease, or kidney disease, or you take regular medication, ask a doctor or registered dietitian to help set a personal target rather than relying on the general range above.

How Often Should You Check Your BMI?

Checking once a month is enough for general monitoring. If you are actively working on a weight goal, weekly checks can help, as long as you look at the overall trend rather than reacting to any single reading — day-to-day weight naturally shifts with hydration, food, sodium intake, and sleep. Measure your height once (unless you're still growing) and try to weigh yourself under similar conditions each time, such as first thing in the morning.

Limitations of BMI and When to See a Doctor

BMI is not designed for children, pregnant women, elite athletes, bodybuilders, or people with significant muscle mass or limb differences — all of these groups need a different assessment method. It also does not account for ethnicity-specific risk differences on its own, which is why regional cutoffs like the Asian BMI standard exist.

Speak with a healthcare professional if you experience rapid or unexplained weight change, persistent fatigue, shortness of breath, or concerns about eating or exercise, or if your BMI falls in the underweight or obesity range. This calculator is an educational tool, not a diagnosis, and it does not replace individual medical advice.

Key Takeaway

BMI gives you a fast, consistent way to check whether your weight is broadly in proportion to your height, and to follow that relationship over time. Read the number with context rather than judgement: pair it with a waist measurement or body-fat estimate if you train regularly, use an age- or region-specific chart when one applies to you, and bring your results to a healthcare professional if anything about them concerns you.