BMI Calculator: Complete Guide to Body Mass Index

Body Mass Index (BMI) is a screening tool that uses your height and weight to estimate whether you're at a healthy weight. A BMI between 18.5 and 24.9 is considered healthy for most adults, while values above 25 indicate overweight and above 30 indicate obesity. Use our free BMI Calculator for an instant result with detailed health insights.

When I first started counseling patients on weight management, I noticed that most people fixated on a single number — the scale. One patient came to me at 192 lbs, convinced she was "obese," when her BMI of 26.1 actually put her just barely into the overweight category. Another patient at 155 lbs assumed she was fine, but at 5'1" her BMI was 29.3 — nearly obese. The numbers tell a different story than our assumptions, and that's exactly why BMI matters as a starting point.

What Is BMI and Why Does It Matter?

Body Mass Index was developed in the 1830s by Belgian mathematician Adolphe Quetelet as a quick way to assess population-level body fat trends. Today, the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) use BMI as a standard screening tool for weight categories that may lead to health problems.

BMI matters because research consistently links higher body weight to increased risks of:

• Heart disease and stroke
• Type 2 diabetes
• High blood pressure
• Certain cancers (breast, colon, kidney)
• Sleep apnea
• Osteoarthritis
• Gallbladder disease

According to the National Institutes of Health (NIH), more than 70% of American adults have a BMI of 25 or higher, placing them in the overweight or obese categories. Understanding where you fall on the BMI scale is the first step toward making informed decisions about your health.

For a deeper dive into what constitutes a healthy weight for your specific body type, see our Ideal Weight: What the Science Actually Says.

How to Calculate Your BMI

BMI uses a straightforward formula. You can calculate it manually or use our BMI Calculator for instant results with personalized recommendations.

BMI Formulas

Metric Formula:

BMI = weight (kg) ÷ height (m)²

Imperial Formula:

BMI = [weight (lbs) ÷ height (inches)²] × 703

The conversion factor of 703 adjusts for the difference between imperial and metric units. Both formulas produce the same result for the same person.

Worked Example 1: Metric Calculation

A woman weighing 70 kg and standing 1.75 m tall:

1. Square the height: 1.75 × 1.75 = 3.0625
2. Divide weight by result: 70 ÷ 3.0625 = 22.9 BMI
3. Category: Healthy Weight (18.5–24.9 range)

This BMI falls in the middle of the healthy range, indicating a good weight-to-height ratio. She would need to reach 76.5 kg before crossing into overweight territory.

Worked Example 2: Imperial Calculation

A man weighing 190 lbs and standing 5'9" (69 inches):

1. Square the height: 69 × 69 = 4,761
2. Divide weight by result: 190 ÷ 4,761 = 0.03991
3. Multiply by 703: 0.03991 × 703 = 28.1 BMI
4. Category: Overweight (25.0–29.9 range)

To reach the top of the healthy range (BMI 24.9), he would need to lose approximately 22 lbs, bringing his weight to 168 lbs. A moderate calorie deficit of 500 calories per day could achieve this in roughly 22 weeks.

Worked Example 3: Borderline Case

A woman weighing 145 lbs at 5'4" (64 inches):

1. Square the height: 64 × 64 = 4,096
2. Divide weight by result: 145 ÷ 4,096 = 0.03540
3. Multiply by 703: 0.03540 × 703 = 24.9 BMI
4. Category: Healthy Weight — but right at the boundary

At exactly 24.9, she's at the upper limit of healthy weight. Even gaining 2 lbs would push her BMI to 25.2 (overweight). This is a good example of why BMI should be tracked over time rather than checked once.

BMI Categories and Health Risk Levels

The WHO and NIH classify adult BMI into the following categories:

What Each Category Means

Underweight (BMI < 18.5): May indicate malnutrition, eating disorders, or underlying health conditions. Risks include weakened immune system, bone loss (osteoporosis), fertility issues, and increased vulnerability to infections.

Healthy Weight (BMI 18.5–24.9): Associated with the lowest risk of weight-related diseases. This is the target range for most adults. Within this range, a BMI closer to 22 is often considered optimal based on large population studies.

Overweight (BMI 25–29.9): Moderately increased risk for chronic conditions. Many people in this range can significantly reduce their risk through lifestyle changes like improved diet and regular exercise — no extreme measures needed.

Obese Class I (BMI 30–34.9): High risk of heart disease, diabetes, and joint problems. Medical consultation is recommended. Lifestyle interventions combined with professional guidance are most effective at this stage.

Obese Class II (BMI 35–39.9): Very high risk. Most physicians recommend structured weight management programs, and medication may be discussed as an option.

Obese Class III (BMI 40+): Extremely high risk of mortality and severe health complications. All treatment options, including bariatric surgery, are typically considered at this level.

BMI Chart by Height and Weight

This reference table shows the weight ranges for each BMI category at common heights:

BMI vs. Other Body Composition Metrics

BMI is just one way to assess body composition. Here's how it compares to other common measurements:

For the most reliable assessment, the American Heart Association (AHA) recommends combining BMI with waist circumference. A healthy BMI combined with a waist circumference under 40 inches (men) or 35 inches (women) indicates the lowest cardiovascular risk.

Use our Body Fat Calculator to estimate your body fat percentage as a complement to your BMI.

BMI Limitations: When It Falls Short

While BMI is a useful population-level screening tool, it has well-documented limitations. The Mayo Clinic notes several situations where BMI can be misleading:

Who BMI May Not Accurately Represent

Real-World Example: A 5'10" bodybuilder weighing 200 lbs has a BMI of 28.7 ("overweight"), yet their body fat percentage might be just 10%. Conversely, a sedentary 5'10" person at the same 200 lbs may carry 30%+ body fat. Same BMI, vastly different health profiles.

Another Example: An 80-year-old woman with a "healthy" BMI of 22 may actually have significant sarcopenia (muscle loss), meaning her body fat percentage is much higher than her BMI suggests. For older adults, body composition matters more than the number on the scale.

The "Obesity Paradox"

Research has shown that for certain conditions — particularly heart failure and in older adults — having a slightly elevated BMI (25–30) may actually be associated with better survival outcomes. This doesn't mean being overweight is healthy, but it highlights that BMI alone is insufficient for individual health assessment.

BMI and Health Conditions

Research shows strong correlations between elevated BMI and specific health conditions:

Risk Increase by BMI Category

These are population averages. Individual risk depends on factors like genetics, diet quality, physical activity level, and where your body stores fat (visceral vs. subcutaneous).

How to Improve Your BMI

If your BMI falls outside the healthy range, evidence-based strategies can help you move toward a healthier weight. The key is gradual, sustainable changes — not crash diets.

Scenario 1: Overweight BMI (25–29.9)

If you're 5'9" and weigh 185 lbs (BMI 27.3):

You need to lose about 17 lbs to reach BMI 24.9. A moderate calorie deficit of 400 calories per day produces safe, sustainable weight loss of about 0.8 lbs per week — reaching your goal in roughly 21 weeks.

Calculate your personal daily calorie needs with our Calorie Calculator, then subtract 300–500 calories for your target intake.

Practical steps:

• Track your food intake for 1 week to establish a baseline
• Reduce portion sizes by 20% rather than eliminating food groups
• Add 150 minutes of moderate exercise per week (brisk walking counts)
• Focus on protein and fiber to stay full on fewer calories — use our Protein Intake Calculator to find your optimal amount
• Weigh yourself weekly at the same time (morning, before eating)

Scenario 2: Obese BMI (30+)

If you're 5'6" and weigh 210 lbs (BMI 33.9):

Your target to reach BMI 24.9 is about 154 lbs — a loss of 56 lbs. This is a longer journey, but a 500-calorie daily deficit achieves about 1 lb per week, reaching the goal in approximately one year.

Practical steps:

• Consult your physician before starting — they may recommend a structured program
• Start with a modest goal: losing 5–10% of body weight (10–21 lbs) delivers significant health benefits even before reaching "healthy" BMI
• Combine diet changes with 3–4 days of exercise per week
• Consider tracking your TDEE (Total Daily Energy Expenditure) to understand your baseline calorie burn
• Set milestone goals: first 10 lbs, then 20, then 30 — celebrate each one

For a complete guide on BMR and how your metabolism affects weight management, see: BMR Explained: What Your Basal Metabolic Rate Means

Scenario 3: Underweight BMI (Below 18.5)

If you're 5'7" and weigh 112 lbs (BMI 17.5):

Gaining weight healthily requires a calorie surplus of 300–500 calories per day, focused on nutrient-dense foods.

Practical steps:

• Eat more frequently (5–6 smaller meals throughout the day)
• Choose calorie-dense whole foods: nuts, avocados, olive oil, whole grains, lean meats
• Add resistance training to build muscle mass, not just fat
• Rule out underlying medical conditions with your doctor (thyroid issues, celiac disease, etc.)
• Track your progress weekly — aim for 0.5–1 lb of weight gain per week

BMI for Different Age Groups

Adults (20–64)

Standard BMI categories apply. The formula and ranges are the same for men and women, though interpretation may differ — women naturally carry a higher body fat percentage than men at the same BMI. A woman with BMI 22 typically has about 25% body fat, while a man at BMI 22 has roughly 18%.

Children and Teens (2–19)

BMI-for-age percentiles are used instead of fixed ranges. A child's BMI is compared to other children of the same age and sex:

The CDC Growth Charts provide age-specific reference data for children. A 10-year-old with a BMI of 21 would be classified differently than an adult with the same number — context matters.

Older Adults (65+)

Some research suggests a slightly higher BMI (23–30) may be protective in older adults. The "obesity paradox" indicates that moderately overweight seniors may have better outcomes during illness and recovery, partly because extra weight provides energy reserves during illness. Discuss your optimal weight range with your doctor.

The Cleveland Clinic recommends that older adults focus on maintaining muscle mass through protein intake and resistance exercise, rather than chasing a lower BMI number.

BMI Across Different Populations

BMI cutoffs were originally developed using primarily Caucasian populations. Research shows that health risks can vary significantly by ethnicity:

The WHO Expert Consultation recommends that Asian populations use lower BMI thresholds because they tend to develop diabetes, hypertension, and cardiovascular disease at lower body weights than Western populations.

This means an Asian person with a BMI of 24 may face similar health risks as a Caucasian person with a BMI of 27. If you're of Asian descent, discuss appropriate thresholds with your healthcare provider.

Frequently Asked Questions

What is BMI and how is it calculated?

BMI (Body Mass Index) is a number calculated from your height and weight. The metric formula is weight (kg) divided by height (m) squared. The imperial formula is weight (lbs) divided by height (inches) squared, multiplied by 703. For example, a 70 kg person who is 1.75 m tall has a BMI of 22.9 (70 ÷ 3.0625 = 22.9). Use our BMI Calculator to calculate yours instantly.

What is a normal BMI for a woman?

The healthy BMI range of 18.5 to 24.9 is the same for both men and women. However, women naturally carry a higher body fat percentage than men at the same BMI. A woman with a BMI of 22 typically has about 25% body fat, while a man at BMI 22 typically has about 18% body fat. Both are considered healthy. For a more tailored assessment, try our Ideal Weight Calculator.

Can you have a high BMI and still be healthy?

Yes. Athletes and people with high muscle mass can have an "overweight" or even "obese" BMI while being metabolically healthy. A 2016 study in the International Journal of Obesity found that nearly half of overweight individuals and 29% of obese individuals were metabolically healthy based on blood pressure, cholesterol, and blood sugar markers. This is why BMI should be combined with other metrics like body fat percentage and waist circumference.

What BMI is considered obese?

A BMI of 30.0 or higher is classified as obese. Obesity is further divided into three classes: Class I (30–34.9), Class II (35–39.9), and Class III (40+). For someone 5'9" tall, obesity begins at approximately 203 lbs. Each class represents progressively higher health risks — Class III obesity carries the highest risk of mortality and complications.

How often should I check my BMI?

Check your BMI once every 3 to 6 months, or whenever you notice significant weight changes. Tracking BMI over time is more useful than a single measurement — a gradual upward trend of even 1–2 BMI points per year may signal the need for lifestyle adjustments before health problems develop.

Is BMI the same as body fat percentage?

No. BMI estimates body fat indirectly using height and weight. Body fat percentage directly measures the proportion of your weight that is fat tissue. A person can have a healthy BMI but high body fat (common in sedentary individuals — sometimes called "skinny fat") or a high BMI but low body fat (common in athletes). Use our Body Fat Calculator to estimate your body fat percentage alongside your BMI.

What is a healthy weight for my height?

Your healthy weight range depends on your height. For 5'0", it's 95–127 lbs; for 5'4", it's 108–145 lbs; for 5'9", it's 125–168 lbs; for 6'0", it's 140–183 lbs. These correspond to BMI values between 18.5 and 24.9. For a more personalized number that accounts for your body frame and age, try our Ideal Weight Calculator.

Does BMI change with age?

The BMI formula doesn't change with age, but your body composition does. After age 30, you lose approximately 3–5% of muscle mass per decade (a condition called sarcopenia). This means your body fat percentage can increase even if your BMI stays the same. Regular exercise — especially strength training — helps maintain muscle mass as you age and is more important than the number on the BMI chart.

Related Calculators

• BMI Calculator — Calculate your body mass index instantly with detailed health insights
• Calorie Calculator — Calculate your daily calorie needs for weight loss, maintenance, or gain
• Ideal Weight Calculator — Find your ideal weight range based on height, age, and body frame
• Body Fat Calculator — Estimate your body fat percentage using multiple methods
• TDEE Calculator — Calculate your Total Daily Energy Expenditure
• Protein Intake Calculator — Find your optimal daily protein intake

Related Articles

• Calorie Deficit: Complete Guide to Losing Weight Safely — Science-backed guide to creating a sustainable calorie deficit for weight loss
• Understanding Your TDEE — How to calculate the total calories you burn each day
• Ideal Weight: What the Science Actually Says — Evidence-based approaches to determining your healthiest weight
• BMR Explained: What Your Basal Metabolic Rate Means — How your resting metabolism affects weight management

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This article provides general health information for educational purposes. BMI is a screening tool, not a medical diagnosis. Consult a qualified healthcare provider for personalized health advice and assessment.