How to use the BMI calculator
- Choose your units. US units (feet, inches and pounds) are selected by default; switch to metric if you think in centimeters and kilograms. The form relabels itself and the results come back in the same system.
- Enter your height, weight and age. Waist circumference is optional, but adding it unlocks the waist-to-height ratio, which many clinicians consider a useful companion to BMI.
- Pick your sex and typical activity level. They do not change your BMI, which depends only on height and weight; they feed the daily energy estimate.
- Press Calculate BMI. You get the number, the WHO category, the weight range that corresponds to a BMI between 18.5 and 24.9 at your height, and a rough daily calorie estimate. Use Copy result to paste a summary into a note or message.
This calculator is for general information. It is not medical advice, it cannot diagnose anything and it does not recommend a diet, a supplement or a medication. If a result worries you, talk to a doctor or a registered dietitian.
What is body mass index?
Body mass index, usually shortened to BMI, is a simple ratio of weight to height: your weight in kilograms divided by your height in meters squared. A person who is 1.75 m tall and weighs 70 kg has a BMI of 70 / (1.75 × 1.75) = 22.9. In pounds and inches the formula becomes 703 × weight / height squared, which gives the same answer. The idea goes back to the Belgian mathematician Adolphe Quetelet in the 1830s, who was studying the average build of populations rather than the health of individuals. The name "body mass index" was only coined in 1972, when the physiologist Ancel Keys argued that this ratio was the best quick predictor of body fatness across large groups.
That history matters, because it explains both the strengths and the weaknesses of the number. BMI is cheap, instant and needs no equipment, which is why doctors, insurers, researchers and public health agencies use it as a first screening step. It is also blunt. It treats every kilogram the same, whether it is muscle, bone, water or fat, and it ignores age, sex and ethnicity apart from the cut-offs chosen for the category labels.
Adult BMI categories
The World Health Organization and the U.S. Centers for Disease Control and Prevention use the same cut-offs for adults. The table below shows the ranges built into the calculator.
| BMI range | Category |
|---|
Some health authorities use lower cut-offs for people of South Asian, East Asian or other Asian backgrounds, because health risks can appear at a lower BMI in those groups. If that applies to you, ask your doctor which thresholds are appropriate rather than relying on the generic table.
What the extra results mean
The weight range for BMI 18.5 to 24.9
This is simply the formula run backwards. For your height, it shows the lowest and highest weights that land inside the "normal weight" band. It is a reference range, not a target. Plenty of healthy people sit slightly outside it, and a person can sit inside it and still have health problems.
Basal metabolic rate and daily energy
Basal metabolic rate (BMR) estimates the calories your body burns at complete rest. The calculator uses the Mifflin-St Jeor equation, which dietitians widely prefer to older formulas: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5 for men, or − 161 for women. Multiplying BMR by an activity factor, from 1.2 for a desk-bound routine to 1.9 for very heavy labor, gives a rough estimate of the calories needed to maintain your current weight. Treat it as a starting point; real needs can differ by several hundred calories a day.
Waist-to-height ratio
If you enter your waist, the tool divides it by your height. A ratio of 0.50 or more is commonly used as a warning sign that fat is concentrated around the middle, which is linked to higher cardiometabolic risk than the same BMI with fat stored elsewhere. The tool also compares your waist with the thresholds published by the WHO: 94 cm (37 in) and 102 cm (40 in) for men, and 80 cm (31.5 in) and 88 cm (35 in) for women.
Limits of BMI you should know about
- Muscle counts as weight. Athletes, lifters and people with physically demanding jobs can land in the overweight range while having low body fat. A rugby player and an office worker with the same height and weight get the same BMI.
- Fat distribution is invisible. Two people with a BMI of 27 can have very different health profiles if one carries fat around the abdomen and the other on the hips and thighs.
- Age changes the picture. Older adults lose muscle and height, so the same BMI can hide more body fat. Some studies suggest that a BMI slightly above 25 is not harmful, or even protective, in people over 65, but this is a topic for your doctor, not a calculator.
- Pregnancy, illness and medication. Pregnant people, those with fluid retention, and anyone recovering from illness should not interpret BMI the usual way.
- Children and teens. For people under 18, BMI is compared with percentile charts that depend on age and sex, so this tool refuses to calculate for them and says why.
Tips for getting a reliable number
Measure your height without shoes, standing straight against a wall, and round to the nearest half inch or centimeter. Weigh yourself at a consistent time, ideally in the morning after using the bathroom and before eating, in light clothing. Daily weight fluctuates by a pound or two from water alone, so look at the trend over several weeks instead of one reading. To measure your waist, breathe out normally and place a flexible tape around your middle at the level of your navel, snug but not squeezing the skin.
If you are tracking a health goal, BMI works best as one data point beside blood pressure, blood tests, fitness, sleep, energy levels and how your clothes fit. A doctor can combine these with your history in a way no online calculator can.
Common questions behind the number
Many visitors arrive with the same worries: "Is my BMI too high?", "What should I weigh for my height?" or "Why does my doctor say something different from this chart?". The honest answer is that BMI is a screening tool, and screening tools are designed to flag who might need a closer look, not to deliver a verdict. A reading in the overweight band is a prompt to check blood pressure, blood sugar and cholesterol, and to think about movement and food habits. A reading in the normal band is not a guarantee of good health, and a reading slightly outside it is not a reason for alarm.
If you decide to change your habits, small and sustainable steps usually beat dramatic ones. Regular walking, adequate sleep, enough protein and vegetables, and fewer sugary drinks make a measurable difference for many people. Anyone with a medical condition, who is pregnant, or who has a history of disordered eating should plan changes together with a health professional.
Private by design
Your height, weight, age and waist are processed by JavaScript running inside your own browser tab. Nothing is uploaded, logged or stored, there are no cookies for tracking and no account to create. The page also keeps working offline after it has loaded.
Frequently asked questions
How is BMI calculated?
Divide your weight in kilograms by your height in meters squared. In US units, multiply your weight in pounds by 703 and divide by your height in inches squared. A person who is 5 ft 9 in (69 in) and weighs 155 lb has a BMI of 703 × 155 / 69² = 22.9.
What is a healthy BMI range for adults?
The WHO and the CDC label a BMI from 18.5 to 24.9 as normal weight, 25.0 to 29.9 as overweight and 30.0 or above as obesity. These are screening categories, not a diagnosis, and individual health also depends on fitness, diet, family history and other factors.
Is BMI accurate for athletes and muscular people?
Not always. BMI cannot tell muscle from fat, so a muscular person can land in the overweight range with low body fat. In that case measures such as waist circumference, waist-to-height ratio or a body-composition scan, interpreted by a professional, give a better picture.
Why does the calculator refuse to work for people under 18?
For children and teenagers, a healthy BMI changes with age and differs between boys and girls, so doctors compare it with percentile growth charts. A single adult cut-off would give misleading answers, so the tool asks you to consult a pediatrician instead.
How accurate is the daily calorie estimate?
It is a rough estimate based on the Mifflin-St Jeor equation and the activity multiplier you select. Real energy needs vary with muscle mass, genetics, hormones, medication and day-to-day movement, often by several hundred calories. Use it as a starting point, not a prescription.
What does the waist-to-height ratio tell me?
It compares your waist circumference with your height. A value of 0.50 or higher is often used as a flag for extra abdominal fat, which is associated with higher heart and metabolic risk. It is a screening hint, not a diagnosis, and it works for most adults regardless of height.
Should I use different BMI cut-offs if I have Asian ancestry?
Several health bodies, including the WHO expert consultation, note that risks rise at a lower BMI in many Asian populations and suggest lower action points, commonly 23 for overweight and 27.5 for obesity. This calculator shows the standard WHO table, so ask your doctor which thresholds fit you.
Is my data sent anywhere, and is the tool free?
No. The calculation runs in your browser and none of the values you enter are uploaded or stored. The tool is free with no sign-up, no daily limit and no premium tier.
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