BMI Calculator
A BMI calculator divides your weight by your height squared to give your body mass index, the standard screening number for weight status. A healthy adult BMI is 18.5 to 24.9. The free tool below computes your BMI instantly, plus your healthy weight range, BMI Prime, Ponderal Index, body-fat estimate, BMR and calorie needs, waist ratios, and ideal weight.
Free BMI Calculator
How to Use This BMI Calculator
Using this BMI calculator takes less than 30 seconds. The tool updates live as you type, so there is no need to press anything, though the Calculate button is there if you want it. Everything runs in your browser, and nothing you enter is sent anywhere.
Enter your measurements
- Type your height and weight. Use whichever tab feels natural: metric (cm and kg), imperial (feet, inches and pounds), or UK (feet, inches, stones and pounds).
- Add your age and sex. These unlock the body-fat estimate, BMR and calorie targets, and switch children under 20 to the CDC percentile view automatically.
- Optional: enter your waist and hip measurements for waist-to-height and waist-to-hip ratios, and set a target BMI if you have a goal in mind.
Choose your units and mode
The three unit tabs convert everything behind the scenes, so 70 kg and 154 lb give the same result. Below the inputs you will find four modes: athlete, pregnancy, 65+, and Asia-Pacific cutoffs. Tick any that apply and the tool adjusts its notes and, where relevant, its category thresholds. The Asia-Pacific toggle switches to the 23 and 27.5 cutoffs recommended for Asian populations.
Read your results dashboard
Your BMI appears under a color gauge with a marker showing exactly where you sit. Below that, the dashboard lists your healthy weight range for your exact height, how far you are from it, a target-weight plan for any BMI you choose, BMI Prime, Ponderal Index, estimated body fat, BMR and TDEE with calorie targets, waist ratios, ideal weight, and a comparison with US adults. The what-if table shows your BMI at weights a few kilos or pounds up and down. Save readings to plot your own trend line over time.
What Is BMI?
Body mass index (BMI) is a number calculated from your weight and height that screens for weight categories which may lead to health problems. It was devised in the early 1830s by the Belgian mathematician Adolphe Quetelet as the Quetelet Index, and renamed "body mass index" by researcher Ancel Keys in 1972. Today it is the standard first screen used by the World Health Organization (WHO), the US Centers for Disease Control and Prevention (CDC), and the National Institutes of Health (NIH).
BMI is a screening measure, not a diagnosis. It does not measure body fat directly, and it cannot tell muscle from fat. What it does well is sort large groups of people into weight categories that track closely with health risk, which is why doctors still start with it before ordering more specific tests.
Imperial version: BMI = 703 × weight (lb) ÷ height (in)2
The BMI Formula: Metric and Imperial
The metric formula is the clean one: divide your weight in kilograms by your height in meters squared. If you are 70 kg and 1.70 m tall, that is 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2. A handy shortcut when your height is in centimeters: BMI = 10,000 × weight (kg) ÷ height (cm)2. For the same person: 10,000 × 70 ÷ 28,900 = 24.2.
The imperial formula needs the constant 703, and there is a real reason for it. Converting kg/m2 into lb/in2 gives a factor of about 703.07, because one kilogram equals 2.20462 pounds and one meter equals 39.3701 inches. So the imperial formula is BMI = 703 × weight (lb) ÷ height (in)2. Worked example: 160 lb at 5 ft 7 in (67 in) gives 703 × 160 ÷ 4,489 = 25.1, right at the top edge of the healthy range.
Both formulas give the same answer for the same body. Mixing units mid-calculation is the most common hand-calculation mistake, which is exactly why the calculator above converts everything for you.
BMI Categories for Adults
The WHO defines nine adult BMI bands, including three grades of thinness and three classes of obesity. The CDC groups the same numbers into four headline categories. Both use identical cutoffs.
| BMI | WHO category | CDC grouping |
|---|---|---|
| Below 16.0 | Severe thinness | Underweight |
| 16.0 – 16.9 | Moderate thinness | Underweight |
| 17.0 – 18.4 | Mild thinness | Underweight |
| 18.5 – 24.9 | Normal range | Healthy weight |
| 25.0 – 29.9 | Pre-obesity | Overweight |
| 30.0 – 34.9 | Obesity class I | Obesity |
| 35.0 – 39.9 | Obesity class II | Obesity |
| 40.0 and above | Obesity class III | Obesity |
Note the boundaries: 18.5, 25, 30, 35 and 40 are the numbers that change your category. A BMI of 24.9 is healthy weight; 25.0 is overweight. That hard edge is one reason to treat the number as a guide rather than a verdict, especially if you sit close to a line.
What Is a Healthy BMI?
A healthy BMI for adults is 18.5 to 24.9, according to both the WHO and the CDC. The same range applies to adult men and women of all ages. Many clinicians consider the low 20s, roughly 21 to 23, a comfortable middle of the healthy band, but anywhere inside 18.5 to 24.9 counts as healthy.
"Good BMI" questions usually mean one of two things: am I in the healthy range, and where inside it should I aim? If you are inside the range, there is no evidence that chasing a specific number like 22 buys extra health. If you are outside it, the goal that matters most is moving toward the range, not hitting a perfect figure. Even a 5 to 10 percent weight loss measurably improves blood pressure and blood sugar in people above the healthy range.
One more citable fact: about 26 percent of US adults fall in the healthy BMI range, while roughly 72 percent have a BMI of 25 or more (CDC NHANES data). So a "normal" BMI is, statistically, less common than an above-range one.
Your Results Explained: Beyond the Number
Most free BMI tools stop at one number and a label. This BMI calculator goes further, because a single number rarely tells the whole story. Here is what each extra result means and why it is worth a look.
BMI Prime
BMI Prime expresses your BMI as a fraction of 25, the top of the healthy range. A BMI Prime of 1.0 means you are exactly at the healthy limit; 0.8 means you sit at 80 percent of it. The scale is simple: below 0.74 is underweight, 0.74 to 0.99 is healthy, 1.00 to 1.19 is overweight, and 1.20 and above is obese. It is a dimensionless number, which makes it handy for comparing results across unit systems.
Ponderal Index
The Ponderal Index, also called the Corpulence Index, is weight in kilograms divided by height in meters cubed. Typical values run from about 11 to 15. Because it uses height cubed instead of squared, it treats very tall and very short people more fairly than BMI does: BMI tends to overestimate heaviness in tall people and underestimate it in short people. If your height is unusual, glance at this number alongside your BMI.
Estimated body fat percentage
Using the Deurenberg formula (1.2 × BMI + 0.23 × age − 10.8 × sex − 5.4), the tool estimates your body-fat percentage. Treat it as a rough guide, usually within 3 to 8 percentage points of lab methods. Typical healthy ranges are about 8 to 20 percent for younger men and 21 to 33 percent for younger women, rising with age. If your BMI is high but your estimated body fat is not, muscle may be doing the heavy lifting.
BMR and TDEE
BMR (basal metabolic rate) is the energy your body burns at complete rest, estimated here with the Mifflin-St Jeor equation, the most validated prediction formula in clinical use. TDEE (total daily energy expenditure) multiplies BMR by your activity level. The calorie targets follow the standard safe guidance: roughly 500 kcal below TDEE to lose about half a kilo a week, TDEE to maintain, and about 300 kcal above to gain. Never drop below 1,200 kcal a day (women) or 1,500 (men) without medical supervision.
Waist-to-height ratio
Divide your waist by your height, in the same units. Keep it below 0.5: your waist should be less than half your height. This simple rule catches risky belly fat that BMI can miss, and many researchers consider it a better single predictor of cardiometabolic risk than BMI alone. The companion waist-to-hip ratio uses WHO cutoffs of 0.90 for men and 0.85 for women.
Ideal weight formulas
The tool averages four classic ideal-weight formulas: Devine, Robinson, Miller and Hamwi. Each estimates a healthy weight from height and sex, originally developed for medication dosing. Averaging the four smooths out their differences. They only apply at 5 feet (152 cm) and above.
Healthy Weight by Height
Your healthy weight range is simply the 18.5 to 24.9 BMI band translated into kilograms for your height. Find your height below for a quick lookup, or use the calculator for your exact figure.
| Height | Healthy weight range | Obesity starts at |
|---|---|---|
| 150 cm | 41.6 - 56.0 kg | 67.5 kg |
| 155 cm | 44.4 - 59.8 kg | 72.1 kg |
| 160 cm | 47.4 - 63.7 kg | 76.8 kg |
| 165 cm | 50.4 - 67.8 kg | 81.7 kg |
| 170 cm | 53.5 - 72.0 kg | 86.7 kg |
| 175 cm | 56.7 - 76.3 kg | 91.9 kg |
| 180 cm | 59.9 - 80.7 kg | 97.2 kg |
| 185 cm | 63.3 - 85.2 kg | 102.7 kg |
| 190 cm | 66.8 - 89.9 kg | 108.3 kg |
| 195 cm | 70.3 - 94.7 kg | 114.1 kg |
How Your BMI Compares With US Adults
A number means more with context. Measured CDC NHANES data from 2021 to 2023 puts the median US adult BMI at 28.2, and 72.4 percent of US adults have a BMI of 25 or more. The table below shows roughly what share of US adults sits below each BMI.
| Your BMI | Approximate percentile among US adults |
|---|---|
| 18.5 | 8th |
| 20 | 11th |
| 22 | 18th |
| 25 | 32nd |
| 28.2 | 50th (median) |
| 30 | 60th |
| 35 | 84th |
| 40 | 96th |
The calculator above gives your personal percentile line with one click. It is an approximation from population data, not a medical judgment: being above average is common, and health depends on far more than where you rank.
Is This BMI Calculator Accurate? Limitations You Should Know
Is BMI accurate? For most adults, BMI is a reasonably good screening tool: it correctly flags weight-related risk most of the time, but it misclassifies roughly one in four adults when checked against actual body fat. Here are the situations where you should read it with extra care.
Athletes and muscle
Muscle is denser than fat, so a trained lifter can land in the overweight or obese BMI band with low body fat. The classic example is a 210 lb bodybuilder at 5 ft 9 in with a BMI near 31. If you train hard, pair your BMI with the waist-to-height ratio and the body-fat estimate above.
Older adults and sarcopenia
After about 65, muscle loss (sarcopenia) can hide fat gain: the scale stays put while body composition worsens. Research by Winter and colleagues (2014) found the lowest mortality in older adults around BMI 23 to 33, higher than the standard healthy band. For seniors, strength and waist size matter more than chasing 18.5 to 24.9.
Where fat sits matters
Two people can share a BMI of 27 with very different risk. Fat around the belly and organs (visceral fat) drives cardiometabolic risk far more than fat on hips and thighs. This is why the calculator includes waist ratios: a healthy BMI with a waist-to-height ratio above 0.5 still deserves attention.
Sex and ethnicity differences
At the same BMI, women typically carry more body fat than men, and older adults carry more than younger ones. People of Asian descent also tend to develop metabolic risk at lower BMIs, which is why the WHO suggested lower action cutoffs for Asian populations in 2004: overweight from 23, obese from 27.5. The Asia-Pacific mode in the tool applies these cutoffs.
Children and teens need percentiles
Adult cutoffs never apply to growing bodies. For ages 2 to 19, BMI is plotted as a percentile for age and sex on CDC growth charts: under the 5th percentile is underweight, 5th to 85th is healthy, 85th to 95th is overweight, and the 95th and above is obesity. The calculator switches to this CDC percentile logic automatically when you enter an age under 20.
Pregnancy
BMI categories pause during pregnancy. The Institute of Medicine suggests total pregnancy weight gain of 11.5 to 16 kg for a healthy pre-pregnancy BMI, 12.5 to 18 kg for underweight, 7 to 11.5 kg for overweight, and 5 to 9 kg for obesity. Tick pregnancy mode and the tool swaps category labels for this guidance.
BMI and Your Health
BMI earned its place because the categories track real outcomes. Carrying extra weight raises the likelihood of several conditions, and being underweight carries its own risks. These are associations from large population studies, not certainties for any one person.
Risks linked with overweight and obesity (CDC, NHLBI): high blood pressure, type 2 diabetes, coronary heart disease, stroke, sleep apnea, osteoarthritis, gallbladder disease, fatty liver disease, and several cancers including breast and colon cancer. Risks linked with being underweight: weakened immunity, fragile bones, fertility problems, anemia, and slower wound healing.
The encouraging part: you do not need a "perfect" BMI to gain most of the benefit. Losing 5 to 10 percent of body weight measurably improves blood pressure, blood sugar and cholesterol (Wing et al., 2011). The American Heart Association recommends at least 150 minutes of moderate activity a week, and keeping waist under 35 inches for women and 40 inches for men (NHLBI). One practical note: some health and life insurers use BMI bands in underwriting, so the number can occasionally affect premiums as well as health advice.
What To Do Next: An Action Plan by Category
A result without a next step is just trivia. Pick your category below for a concrete starting checklist, and see a doctor before any big change if you have a medical condition.
If your BMI is below 18.5
- Rule out causes first: unintentional weight loss deserves a doctor visit, not a diet plan.
- Eat in a modest surplus built on nutrient-dense food: nuts, dairy or alternatives, eggs, legumes, whole grains.
- Add strength training two to three times a week so gains become muscle, not just fat.
- Recheck monthly. If the trend keeps falling without explanation, get checked promptly.
If your BMI is 18.5 to 24.9
- Hold the line: weigh yourself weekly under the same conditions (morning, after the bathroom).
- Keep moving: 150 minutes of moderate activity a week plus two strength sessions.
- Watch the waist, not just the scale. A creeping waist-to-height ratio above 0.5 is an early warning.
- Recheck BMI every three to six months. Stable is the goal.
If your BMI is 25 to 29.9
- Aim for 5 to 10 percent weight loss first. For 90 kg, that is 4.5 to 9 kg, enough to move most health markers.
- Eat about 500 kcal below your TDEE (the tool shows your number) and track intake for two weeks to calibrate.
- Walk daily and build toward 150 minutes of moderate activity a week.
- Recheck monthly. If nothing moves after two steady months, talk to your doctor about the plan.
If your BMI is 30 or above
- Book a checkup: blood pressure, blood sugar and cholesterol give the full picture BMI cannot.
- Set process goals, not just scale goals: a daily walk, one sugary drink swapped, a regular bedtime.
- Consider professional support. Structured programs and clinicians beat going it alone at this range.
- Avoid crash diets. Losing half a kilo a week beats losing five and regaining six.
Common BMI Calculator Mistakes
Most wrong BMI results come from five repeatable errors. Run through this list before trusting any number.
- Mixing units. Entering pounds into a kilograms field (or centimeters as inches) is the top error. Pick one tab in the tool and stay in it.
- Using an outdated height. Adults slowly lose height after 40. Measure yearly; two centimeters changes your BMI more than you would guess.
- Weighing at random times. Weight swings 1 to 2 kg through the day. Weigh in the morning, after the bathroom, before breakfast, in similar clothing.
- Putting adult cutoffs on kids. A BMI of 20 is healthy for an adult and overweight for a 10-year-old. Use the CDC percentile view for under-20s.
- Treating BMI as a diagnosis. It is a screening step. Pair it with waist measurement, blood pressure and blood work before drawing conclusions.
Frequently Asked Questions
What is a healthy BMI?
A healthy BMI for adults is 18.5 to 24.9, per the WHO and CDC. The range is the same for men and women of all adult ages. The low 20s (about 21 to 23) sit in the comfortable middle, but any value inside 18.5 to 24.9 counts as healthy weight.
How do I calculate my BMI by hand?
To calculate BMI by hand, use the metric formula: weight in kilograms divided by height in meters squared. Example: 70 kg at 1.70 m is 70 divided by 2.89, which equals 24.2. In imperial units, multiply weight in pounds by 703, then divide by height in inches squared.
Is BMI different for women and men?
No. BMI uses the same formula and the same cutoffs for adult women and men. At an identical BMI, women typically carry a higher body-fat percentage than men, which is why the tool also estimates body fat from age and sex alongside the raw BMI number.
Is BMI accurate?
BMI is a useful screening tool but not a precise health measure. It correctly reflects weight-related risk for most adults, yet misclassifies about one in four people when compared with measured body fat. It works best alongside waist measurement, activity level, and basic blood work.
I am muscular. Does BMI still apply to me?
For very muscular people, BMI often overstates body fat because muscle is dense. A regular lifter can score overweight with a lean waistline. If you train with weights, rely more on your waist-to-height ratio (keep it under 0.5) and the body-fat estimate than on BMI alone.
What is a good BMI for seniors?
The standard 18.5 to 24.9 band was built from younger adults. In people over 65, research finds the lowest mortality around BMI 23 to 33, since some reserve protects against frailty. Seniors should focus on strength, balance, protein intake, and waist size rather than chasing a youthful number.
How is BMI measured for children?
Children use BMI-for-age percentiles, not adult cutoffs. A child's BMI is plotted against CDC growth charts for their exact age and sex: below the 5th percentile is underweight, 5th to 85th is healthy weight, 85th to 95th is overweight, and 95th and above is obesity. This tool computes it automatically for ages 2 to 19.
How often should I recalculate my BMI?
Monthly is plenty for most adults, and every three to six months is fine once you are stable. Weigh weekly if you are actively managing weight, but recalculate BMI monthly since height barely changes. After any big weight change, recheck your healthy range and calorie targets too.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the top of the healthy range. A value of 1.0 means exactly at the healthy limit, 0.8 means 80 percent of it. Below 0.74 is underweight, 0.74 to 0.99 is healthy, 1.00 to 1.19 is overweight, and 1.20 or higher is obese.
Is this BMI calculator free and private?
Yes. This BMI calculator is completely free with no sign-up, and every calculation happens in your browser. Your height, weight, and results never leave your device. Saved history lives only in your own browser storage, which you can clear anytime.
Related Health Tools
Body weight connects to a cluster of related numbers. We are building this health toolkit out over the coming days; each link below opens our tools hub until its own page goes live.