Use the free BMI calculator above to find your body mass index in seconds — personalised to you. Enter your height, weight, age and sex, choose metric (cm, kg) or imperial (feet, inches, pounds or stones), and you’ll instantly see your BMI number, your weight category, your healthy weight range, an estimate of your body-fat percentage and your daily calorie needs. The tool works for men, women, teenagers and children, and for kids and teens aged 2–19 it uses official CDC age-and-sex percentiles rather than the adult scale, so the result is genuinely accurate for a growing body.

Most BMI tools online give you a single number and stop there. Your BMI is more useful when you can see what it means — the healthy weight range for your exact height, how far you are from it, and the extra measures that fill in what BMI alone can miss. This calculator shows all of that, and it explains how it reached every figure so you can trust the result and act on it.

What is BMI?

BMI, or body mass index, is a single number that describes your weight relative to your height. It was developed in the 1830s by the Belgian mathematician Adolphe Quetelet — which is why it is sometimes called the Quetelet Index — and it is now the most widely used screening tool for underweight, healthy weight, overweight and obesity. Doctors, gyms and public-health agencies such as the CDC and WHO rely on it because it is quick, free and needs only two measurements you already know: your height and your weight.

BMI is a screening tool, not a diagnosis. It gives a useful first estimate of whether your weight could be affecting your health, but it does not measure body fat directly and it cannot tell muscle from fat. The fullest picture comes from reading BMI alongside other measures such as waist circumference, body-fat percentage and your overall health history — all of which this page helps you with.

How to calculate BMI (the formula)

You don’t have to do any maths — the tool does it for you — but here is exactly how BMI is worked out. Only the units change.

Metric: BMI = weight (kg) ÷ height (m)²
Example: 70 kg ÷ (1.75 × 1.75) = 22.9 → a healthy weight.

Imperial: BMI = 703 × weight (lb) ÷ height (in)²
Example: 703 × 154 ÷ (67 × 67) = 24.1 → a healthy weight.

That’s the whole calculation. The calculator at the top of this page also converts stones, feet and inches automatically, so you can enter your measurements however you normally think about them and still get the same result.

BMI categories chart (adults 20 and over)

For adults, the World Health Organization and CDC use the same fixed cut-off points, regardless of age or sex:

A BMI between 18.5 and 24.9 is generally linked with the lowest risk of weight-related health problems. Above 25, the risk of conditions such as type 2 diabetes, high blood pressure and heart disease tends to rise; below 18.5 can signal undernutrition or an underlying condition worth discussing with a doctor.

BMI calculator for women

The BMI formula is identical for women and men — there is no separate equation and no separate adult category chart. What differs is body composition: at the same BMI, women naturally carry a higher percentage of body fat than men. That’s why the calculator personalises the extras it shows once you choose Female — the body-fat estimate and the daily calorie figures are adjusted for you. If you mainly want to know your ideal weight, use the healthy weight range the tool displays for your exact height. One important note: BMI is not designed for use during pregnancy, so if you’re pregnant, use your pre-pregnancy weight or ask your provider about healthy weight-gain targets instead.

BMI calculator for men

For men, BMI uses the same weight-÷-height² formula, and the same adult categories apply — healthy at 18.5–24.9, overweight at 25–29.9, and so on. The one caveat worth knowing is that very muscular men can register a “high” BMI while carrying very little fat, because muscle is dense and BMI can’t tell muscle from fat. If you lift heavily or you’re an athlete, read your BMI alongside your waist measurement and body-fat estimate — both available in the calculator — rather than treating the number on its own as the final word.

BMI for children and teens (ages 2–19)

For anyone under 20, an adult BMI category is misleading, because a child’s body composition changes constantly as they grow. Instead, a child’s BMI is compared with thousands of others of the same age and sex and expressed as a percentile. This calculator uses the official CDC BMI-for-age growth reference, so the category you see for a child or teen is clinically grounded rather than a guess.

So a 10-year-old boy in the 60th percentile is at a healthy weight, while one in the 96th percentile falls into the obesity category. Because children grow in spurts, a single reading matters far less than the trend over time — always discuss a child’s growth with a paediatrician before drawing any conclusions.

BMI by age

For adults, the healthy BMI range (18.5–24.9) is the same at every age — a 25-year-old and a 70-year-old are read against the same chart. That said, age still matters for interpretation. Older adults tend to lose muscle and gain fat, so BMI can underestimate body fat later in life. The calculator asks for your age so it can tailor the extras — your body-fat estimate and calorie needs both change with age — and, crucially, so it can switch to CDC percentiles automatically for anyone under 20, where age is central to the result.

Your healthy weight range (and reverse BMI)

Plenty of people don’t just want their BMI — they want to know what they should weigh for their height. The calculator answers that directly: it shows the weight band that matches a healthy BMI of 18.5–24.9 for your exact height, and tells you how much you’d need to gain or lose to get there. This “reverse BMI” view turns an abstract number into a concrete, motivating target — for example, at 5 ft 7 in a healthy weight is roughly 118–159 lb (54–72 kg).

BMI Prime and other quick measures

Alongside your BMI, the calculator shows a few extra numbers that add context: BMI Prime (your BMI ÷ 25 — under 1.00 means a healthy weight); an estimated body-fat percentage from the Deurenberg formula, which factors in your BMI, age and sex; and your BMR and daily calories — the energy your body uses at rest plus a maintenance estimate, using the well-validated Mifflin–St Jeor equation.

BMI vs. waist-to-height ratio and body fat

Because BMI can’t distinguish fat from muscle or show where you carry weight, it helps to pair it with other quick checks. Add your waist measurement in the calculator and it works out your waist-to-height ratio — a strong early-warning sign of health risk, since fat carried around the middle is the most metabolically risky. The simple rule of thumb: keep your waist to less than half your height. A ratio under 0.5 is generally healthy, 0.5–0.6 signals increased risk, and 0.6 or above signals high risk.

Is BMI accurate? Limitations to know

BMI is a fast, useful screen, but it isn’t perfect. It does not measure body fat directly; it can overestimate fat in very muscular or athletic people; it can underestimate fat in older adults who have lost muscle; it isn’t valid during pregnancy; and the adult thresholds don’t fully reflect differences between ethnic groups — for people of South Asian descent, for instance, health risks can begin at a lower BMI. Use BMI as the starting point for a conversation with a healthcare professional, not as the last word on your health.

BMI for the Army and military

The U.S. Army first screens soldiers and recruits with a height-and-weight table; anyone who exceeds the weight limit for their height is then measured for body-fat percentage under the Army Body Composition Program (ABCP), because muscular soldiers routinely exceed a simple weight cut-off despite being lean. This calculator gives you your BMI plus a body-fat estimate as a useful starting point, but note that official Army standards use taped body-fat measurements and the current ABCP tables rather than BMI alone.

How to use this BMI calculator

1. Choose Male or Female.
2. Pick Metric (cm, kg) or US / Imperial (ft, in, lb) units.
3. Enter your age, height and weight (the tool accepts kilograms, pounds or stones).
4. Optionally add your waist for a body-fat and waist-to-height read-out.
5. Press Calculate My BMI to see your BMI, category, healthy weight range, body-fat estimate and daily calories — or, for anyone under 20, your CDC percentile and child-and-teen category.

Frequently asked questions

How do you calculate BMI?

Divide your weight in kilograms by your height in metres squared (BMI = kg ÷ m²). In imperial units, multiply your weight in pounds by 703 and divide by your height in inches squared. For example, 70 kg at 1.75 m gives a BMI of 22.9, and 154 lb at 5 ft 7 in gives 24.1 — both a healthy weight.

What is a healthy BMI?

For adults, a BMI between 18.5 and 24.9 is considered healthy. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above falls into the obesity range. For children and teens, “healthy” is the 5th to 84th percentile for their age and sex.

Is BMI different for men and women?

The formula and the adult categories are the same for both. The difference is body composition — women carry more body fat than men at the same BMI — so use BMI as a general screen and read it alongside body-fat or waist measurements for a fuller picture.

How is BMI calculated for children and teens?

A child’s BMI is worked out with the standard formula, then plotted on the CDC BMI-for-age chart for their exact age and sex and expressed as a percentile. Healthy is the 5th to 84th percentile; 85th–94th is overweight; 95th and above is obesity.

What is the BMI requirement for the Army?

The Army first uses a height-and-weight screening table; those over the limit are then measured for body-fat percentage under the Army Body Composition Program, because muscle can push a fit soldier over a simple weight cut-off. This tool gives your BMI and a body-fat estimate as a starting point, but official standards use taped body-fat measurements.

Can BMI be wrong or inaccurate?

BMI can be misleading for very muscular athletes (reads too high), older adults with less muscle (reads too low), pregnant women, and some ethnic groups. It’s a screening tool, not a diagnosis — treat it as one data point among several.

What is BMI Prime?

BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A value under 1.00 means you’re within a healthy weight; above 1.00 shows how far above it you are.

What should I weigh for my height?

A healthy weight is the range that gives a BMI of 18.5–24.9 for your height. For example, that’s about 108–145 lb at 5 ft 3 in, 118–159 lb at 5 ft 7 in, and 128–174 lb at 5 ft 10 in. The calculator shows your exact healthy weight range and how far you are from it.

Does the calculator work in kg and cm as well as pounds and feet?

Yes. Switch between metric (cm, kg) and imperial (ft, in, lb or stones) at the top of the tool and it converts everything automatically, giving the same BMI either way.

This page is for general education and is not medical advice. BMI is a screening tool, not a diagnosis. Talk to a qualified healthcare provider about what your BMI and related measurements mean for you — especially for children, during pregnancy, or if you have any health concerns.