BMI Calculator:
Calculate Body Mass Index Free

What BMI Actually Measures

BMI is not a direct measure of body fat. It is a ratio of weight to height squared. The formula is: BMI = weight (kg) / height (m)^2. In imperial units: BMI = 703 x weight (lbs) / height (in)^2. This ratio was found to correlate reasonably well with body fat percentage in population studies. Taller people naturally weigh more, so squaring the height accounts for this scaling effect.

Why Squared Height?

Quetelet discovered that weight scales with the square of height, not the cube (which would be true for perfect geometric scaling). This is because humans are not perfectly proportional — taller people tend to be leaner relative to their height. The square of height was empirically found to give the best correlation with body fat across populations. This is why BMI uses height squared, not height cubed or height alone.

Population vs Individual

BMI was designed for populations, not individuals. When you measure BMI across thousands of people, the average BMI correlates strongly with average body fat. For any single person, BMI can be misleading — especially if you are very muscular, very short, very tall, elderly, or from certain ethnic backgrounds. This is the most important limitation to understand: BMI is a population tool that we use as a proxy for individuals.

BMI Is Not Diagnostic

A high BMI does not mean you are unhealthy. A normal BMI does not mean you are healthy. BMI is a screening tool, not a diagnostic test. It identifies people who might need further assessment. A doctor would never diagnose obesity based on BMI alone — they would also consider waist circumference, body fat percentage, blood pressure, cholesterol, blood sugar, family history, and lifestyle factors. Use BMI as a starting point for conversation, not a final verdict.

Metric Formula (kg and m)

BMI = weight (kg) / [height (m)]^2

Example: A person weighs 70 kg and is 1.75 m tall.

BMI = 70 / (1.75 x 1.75) = 70 / 3.0625 = 22.9

This falls in the "normal weight" category (18.5 to 24.9).

Imperial Formula (lbs and inches)

BMI = 703 x weight (lbs) / [height (in)]^2

Example: A person weighs 154 lbs and is 69 inches tall.

BMI = 703 x 154 / (69 x 69) = 108,262 / 4,761 = 22.7

The 703 factor converts imperial units to metric equivalents. This also falls in the "normal weight" category.

Using the Online Calculator

Online BMI calculators do the math for you. Simply enter your weight and height, and the tool instantly calculates your BMI. Most calculators support both metric and imperial units. The AFFLIGO BMI Calculator goes further: it also computes the Oxford (New) BMI (more accurate for tall or short people), your BMR (daily calorie burn at rest), and TDEE (total daily calorie need including activity). This is the fastest and most error-free method — manual calculation is prone to unit conversion errors and arithmetic mistakes.

Common Calculation Mistakes

Wrong: Using height in cm instead of m (e.g., 175 instead of 1.75). Wrong: Forgetting to square the height. Wrong: Mixing metric weight with imperial height. Wrong: Using feet instead of inches (5'9" = 69 inches, not 5.9). Wrong: Not accounting for rounding (BMI should be calculated to one decimal place). Always double-check your units before calculating.

Set Your Height

Use the height unit toggle to switch between centimetres and feet/inches. If you choose feet/inches, enter both the feet and inches fields (e.g., 5 ft 9 in). Stand barefoot against a wall for the most accurate measurement. Measure in the morning before gravity compresses your spine, and do not wear shoes — they add 2-3 cm and skew results.

Set Your Weight

Use the weight unit toggle to switch between kg and lbs. Weigh yourself on a hard, flat surface in the morning, after using the bathroom, before eating. Use the same scale consistently — different scales can vary by 1-2 kg. Record to the nearest 0.1 kg or 0.2 lb for best accuracy.

Enter Age, Gender, and Activity Level

Unlike basic calculators, this tool also needs your age (for BMR), gender (male or female — BMR formulas differ), and activity level (sedentary, lightly active, moderately active, very active, or elite athlete). These inputs allow the tool to calculate your Basal Metabolic Rate (BMR) and Total Daily Energy Expenditure (TDEE) — two metrics that tell you how many calories your body actually needs.

Click Calculate and Read the Gauge

Hit the calculate button and the tool instantly computes your results. A colour-coded gauge with a rotating needle shows where your BMI falls — blue for underweight, green for normal, yellow for overweight, and red for obese. The large number in the centre is your Standard BMI. Below it, the category label updates in real time.

Review All Seven Metrics

The results panel shows seven calculations:
Standard BMI — the classic weight/height² formula.
Oxford (New) BMI — a refined formula (1.3 × weight × height^2.5) more accurate for people above 180 cm or below 155 cm.
BMI Prime — your BMI divided by 25, showing how far you are from the upper normal limit.
BMR — calories your body burns at rest (Mifflin-St Jeor equation).
TDEE — BMR × activity multiplier, your total daily calorie need.
Body Fat % — estimated body fat percentage (Deurenberg formula).
Lean Mass — your estimated lean body mass in kg.

Check Your Healthy Weight Range

Below the seven metrics, the tool shows your ideal weight range for your height (18.5-24.9 BMI band) and a personalised advice line: how many kg to gain or lose to reach the healthy zone, or confirmation that you are already within it. Use this as a practical target, not a strict rule.

Copy Your Report

Click the Copy button to copy your full results — BMI, category, BMR, TDEE, and healthy range — to your clipboard. Paste it into a notes app, diet plan, or share it with your doctor. All calculations happen in your browser; nothing is stored on any server.

Does Not Distinguish Muscle from Fat

A bodybuilder with 8% body fat and a sedentary person with 30% body fat can have the same BMI if they weigh the same and are the same height. Muscle is denser than fat, so muscular individuals often have "overweight" or "obese" BMIs despite being very healthy. This is the most common and most valid criticism of BMI.

Does Not Account for Body Fat Distribution

BMI treats all weight equally, but where fat is stored matters enormously. Visceral fat (around organs) is far more dangerous than subcutaneous fat (under the skin). A person with a normal BMI but a large waist may have higher health risks than a person with a slightly higher BMI but a small waist. Waist circumference and waist-to-hip ratio are better indicators of metabolic health.

Ethnic and Population Differences

BMI categories were developed using data from European populations. For Asian populations, health risks begin at lower BMI values. For Pacific Islanders and some African populations, higher BMI may be normal. The standard cutoffs do not apply universally. The WHO has proposed different cutoffs for Asian populations (overweight at 23, obesity at 25).

Not Accurate for the Elderly

For adults over 65, a slightly higher BMI (25-27) may actually be protective. Older adults naturally lose muscle mass, and some extra weight can provide reserves during illness. The standard BMI categories may overestimate health risks in this population. Geriatricians often use adjusted BMI ranges for patients over 65.

Not Accurate for Pregnant Women

Pregnancy naturally increases weight, and BMI calculations do not account for the growing fetus, placenta, amniotic fluid, and increased blood volume. BMI is not used to assess weight during pregnancy. Instead, doctors track weight gain against pregnancy-specific guidelines.

Does Not Consider Sex Differences

Women naturally have higher body fat percentages than men at the same BMI. A healthy woman typically has 20-30% body fat, while a healthy man has 10-20% body fat. Yet BMI categories are the same for both sexes. This means BMI may underestimate obesity in women and overestimate it in men.

Type 2 Diabetes

The risk of type 2 diabetes increases significantly with BMI. A BMI over 30 increases risk by 3-5 times compared to normal weight. A BMI over 35 increases risk by 7-12 times. Abdominal fat is particularly linked to insulin resistance. Weight loss of 5-10% can reduce diabetes risk by 50% in high-risk individuals. This is one of the strongest BMI-disease correlations in medicine.

Cardiovascular Disease

Higher BMI is associated with increased risk of heart disease, stroke, and hypertension. For every 5-point increase in BMI above 25, the risk of heart disease increases by about 30%. However, fitness level matters more than BMI alone. A fit person with a BMI of 28 may have lower cardiovascular risk than an unfit person with a BMI of 24. Exercise independently reduces cardiovascular risk regardless of weight.

Sleep Apnea

Obesity is the strongest risk factor for obstructive sleep apnea. A BMI over 30 increases sleep apnea risk by 4-10 times. Excess weight around the neck narrows the airway during sleep. Weight loss of 10-15% can significantly reduce or eliminate sleep apnea symptoms in many patients. Sleep apnea itself increases risk of heart disease, stroke, and diabetes.

Certain Cancers

Higher BMI is linked to increased risk of 13 types of cancer: esophageal, gastric, colorectal, liver, gallbladder, pancreatic, breast, uterine, ovarian, kidney, prostate, thyroid, and multiple myeloma. The mechanisms are complex and include chronic inflammation, insulin resistance, and hormonal changes. Even a 5% weight loss can reduce cancer risk factors.

Why Athletes Have Higher BMI

Muscle is denser than fat. A pound of muscle takes up less space than a pound of fat but weighs the same. A muscular person can be very lean but weigh more than a sedentary person of the same height. This pushes their BMI into the "overweight" or even "obese" category despite low body fat. NFL players, bodybuilders, and Olympic sprinters often have BMIs in the 28-35 range with 6-15% body fat.

Better Metrics for Athletes

Instead of relying on BMI, athletes should use: Body fat percentage (measured by DEXA, calipers, or bioelectrical impedance). Waist circumference (under 94 cm for men, under 80 cm for women). Performance metrics (VO2 max, strength, endurance). Visual assessment (can you see muscle definition? are abs visible?). Health markers (blood pressure, cholesterol, blood sugar). These give a far more accurate picture than BMI.

When BMI Is Still Useful for Athletes

Even for athletes, BMI has value as a tracking tool over time. If your BMI increases while your body fat stays the same, you are gaining muscle. If your BMI increases and your body fat also increases, you are gaining fat. If your BMI decreases while body fat stays constant, you are losing muscle. Use BMI as one data point in a dashboard that includes body fat, performance, and health markers.

The "Fat but Fit" Debate

Research shows that cardiovascular fitness is more important than BMI for mortality risk. A fit person with obesity has lower mortality risk than an unfit person with normal weight. However, this does not mean obesity is harmless. The combination of high BMI and low fitness carries the highest risk. The best strategy is to pursue both: maintain a healthy weight AND stay physically active.

Measure Monthly, Not Daily

Weigh yourself once per month, at the same time of day, under the same conditions. Daily weight fluctuations (2-3 lbs) are caused by water, food, and salt intake, not actual fat changes. Monthly measurements smooth out noise and reveal true trends. Use a spreadsheet or health app to log each measurement with the date.

Track Multiple Metrics

BMI is one metric in a dashboard. Track these together: Weight (kg or lbs), Waist circumference (cm or inches), BMI, Resting heart rate (beats per minute), Blood pressure (if available), Body fat % (if measured). Together, these metrics give a complete picture that BMI alone cannot provide.

Set Realistic Goals

Do not aim for a specific BMI number. Aim for a healthier trend. A 5% weight loss over 6 months is a realistic and sustainable goal. A 1-2 kg weight loss per month is healthy. Crash diets that lose 5 kg in a week are mostly water loss and are not sustainable. Focus on habits, not numbers: eat more vegetables, walk 30 minutes daily, sleep 7-8 hours, reduce sugar.

Celebrate Non-Scale Victories

BMI does not capture everything. Track these improvements too: Energy levels (do you feel more energetic?), Sleep quality (do you sleep better?), Strength (can you lift more?), Endurance (can you walk/run farther?), Clothing fit (do your clothes fit better?), Mood (do you feel better mentally?). These are often more important than the number on the scale.

My BMI Says I am Obese But I Am Fit

This is the most common BMI complaint. If you have high muscle mass, BMI is not the right metric for you. Measure your body fat percentage and waist circumference. If body fat is under 20% (men) or 30% (women) and waist is under 94 cm (men) or 80 cm (women), you are likely healthy regardless of BMI. Focus on performance metrics and health markers instead.

I Lost Weight But My BMI Did Not Change

BMI depends on both weight and height. If you lost fat but gained muscle, your weight may stay the same. Check your waist circumference and body fat percentage — these may have improved significantly even if BMI did not. Also ensure you are calculating correctly: weight in kg divided by height in meters squared.

My BMI Is Normal But I Have Belly Fat

This is called "normal weight obesity" or "skinny fat." You can have a normal BMI but high body fat percentage, especially if you are sedentary. Check your waist circumference. If it is over 94 cm (men) or 80 cm (women), you may have visceral fat even with a normal BMI. Focus on strength training and reducing processed foods rather than just weight loss.

My BMI Seems Too Low or Too High

Double-check your units. Make sure you entered weight in kg (not grams) and height in meters (not centimeters). In the imperial formula, ensure height is in inches (not feet). A common error is entering 175 cm as 175 meters, which gives a BMI of about 0.002. Also verify you are using the adult calculator, not the child percentile version.

What is a good BMI for my age?

For adults 20-64, the healthy range is 18.5 to 24.9 regardless of age. For adults over 65, a slightly higher BMI (25-27) may be protective. For children, BMI is interpreted by age and sex percentiles, not fixed categories. There is no single "good BMI" — it depends on your body composition, health markers, and individual circumstances.

Can BMI be wrong?

Yes, BMI can misclassify individuals. It does not distinguish muscle from fat, account for body fat distribution, or consider ethnicity, age, or pregnancy. For muscular individuals, athletes, and some ethnic groups, BMI may overestimate or underestimate health risks. BMI is a screening tool, not a diagnostic test. Use it alongside other metrics like waist circumference and body fat percentage.

How often should I calculate my BMI?

Once per month is sufficient for most people. Daily weighing is not recommended because normal weight fluctuations (water, food, salt) can cause unnecessary anxiety. Monthly measurements reveal true trends. If you are actively trying to lose or gain weight, weekly measurements may be helpful, but focus on monthly averages rather than individual readings.

Is BMI the same for men and women?

The BMI formula and categories are identical for men and women. However, women naturally have higher body fat percentages than men at the same BMI. A BMI of 24 in a woman typically corresponds to more body fat than a BMI of 24 in a man. Some researchers suggest sex-specific BMI cutoffs, but the standard categories remain the same for both sexes.

What is the ideal BMI?

Population studies suggest the lowest mortality risk is at a BMI of around 20-22. However, the "ideal" BMI varies by individual. For athletes, a higher BMI with low body fat may be optimal. For older adults, a BMI of 25-27 may be protective. The ideal BMI is one where you feel good, have energy, maintain good health markers, and can sustain it long-term.

Can I trust online BMI calculators?

Yes, if they use the standard formula. The math is simple and universal. However, some calculators may have unit conversion errors or use non-standard categories. Always use a reputable calculator like the one from the CDC, NHS, or AFFLIGO. Verify the result with a manual calculation if the number seems surprising.

Does muscle weigh more than fat?

A pound of muscle and a pound of fat weigh the same — a pound. However, muscle is denser and takes up less space than fat. This means a muscular person can be lean but heavy, which pushes their BMI higher. This is why BMI is not accurate for very muscular individuals. A person with 20% body fat and lots of muscle may have a higher BMI than a person with 30% body fat and little muscle.

What BMI is considered skinny?

A BMI below 18.5 is classified as underweight. However, "skinny" is subjective. Some people with a BMI of 18 are perfectly healthy with good nutrition and muscle mass. Others with the same BMI may be malnourished. If your BMI is below 18.5 and you are concerned, consult a healthcare provider. Do not self-diagnose based on BMI alone.

Can BMI predict my life expectancy?

At the population level, BMI correlates with mortality risk. People with BMIs in the 20-25 range tend to have the lowest mortality. However, at the individual level, BMI is a poor predictor of lifespan. Fitness, diet, genetics, smoking status, and mental health all play larger roles. A fit person with a BMI of 28 will likely outlive an unfit person with a BMI of 22.

Why do doctors still use BMI if it is flawed?

BMI is flawed for individuals but useful for populations. It is also simple, free, and requires no special equipment. For most patients, it provides a reasonable first-line assessment. Doctors use BMI alongside other metrics (waist circumference, blood pressure, blood tests) to build a complete picture. No single metric is perfect — BMI is one tool in a comprehensive health assessment.

How can I lower my BMI?

The only way to lower BMI is to reduce weight or increase height (height is fixed in adults). To reduce weight safely: create a modest calorie deficit (300-500 calories per day), eat more protein and vegetables, reduce processed foods and sugar, drink more water, sleep 7-8 hours, and exercise regularly (both cardio and strength training). Aim for 0.5-1 kg weight loss per week. Crash diets are not sustainable and often lead to rebound weight gain.

Is BMI used in other countries?

Yes, BMI is the global standard used by the WHO, CDC, NHS, and health organizations in virtually every country. However, some countries use adjusted cutoffs. For example, Japan and China use lower thresholds (overweight at BMI 23, obesity at BMI 25) because health risks begin at lower BMI values in Asian populations. The formula itself is the same everywhere — only the interpretation cutoffs vary.