BMI Calculator

The BMI Formula Explained

How Body Mass Index is Calculated

BMI (Body Mass Index) is a simple calculation using your weight and height to estimate body fat. The formula differs slightly based on your measurement system:

Metric: BMI = weight (kg) / height² (m²)

Imperial: BMI = [weight (lbs) / height² (inches²)] × 703

The 703 is a conversion factor to make imperial
units match metric results.

Step-by-Step Calculation Example

Let's calculate BMI for someone who is 5'9" (69 inches) and weighs 170 pounds:

Square your height: 69 × 69 = 4,761
Divide weight by height squared: 170 ÷ 4,761 = 0.0357
Multiply by 703: 0.0357 × 703 = 25.1
Result: BMI of 25.1 (borderline overweight)

BMI Categories and Health Ranges

Underweight: Below 18.5 (may indicate malnutrition or health issues)
Normal weight: 18.5-24.9 (associated with lowest health risks)
Overweight: 25.0-29.9 (increased risk for certain health conditions)
Obese Class I: 30.0-34.9 (high risk)
Obese Class II: 35.0-39.9 (very high risk)
Obese Class III: 40.0+ (extremely high risk)

How to Use This BMI Calculator

Choose your unit system: Select imperial (pounds/feet/inches) or metric (kilograms/centimeters)
Enter your weight: Use your current weight, not your goal weight
Enter your height: Be accurate—even 1 inch difference changes your BMI by 1-2 points
Click Calculate: See your BMI, category, healthy weight range, and how much to lose/gain to reach normal range

Pro tip: Measure your height and weight in the morning before eating for most accurate results. Weight can fluctuate 2-5 pounds during the day due to food, water, and bathroom use.

Understanding Your BMI Results

Normal Weight (BMI 18.5-24.9)

If your BMI falls in this range, you're at a statistically healthy weight for your height. This range is associated with the lowest risk of weight-related diseases like type 2 diabetes, heart disease, and certain cancers. Maintain this through balanced nutrition and regular physical activity (150+ minutes of moderate exercise per week).

Underweight (BMI Below 18.5)

Being underweight can be as risky as being overweight. Risks include weakened immune system (more frequent illnesses), osteoporosis (brittle bones), anemia (fatigue, weakness), and fertility problems. If you're underweight, consult a doctor to rule out underlying conditions like thyroid issues, digestive problems, or eating disorders. Healthy weight gain focuses on nutrient-dense foods and strength training to build muscle, not just fat.

Overweight (BMI 25-29.9)

Being overweight increases your risk for high blood pressure, type 2 diabetes, heart disease, stroke, and sleep apnea. The good news: losing just 5-10% of your body weight (10-20 pounds for a 200-pound person) significantly improves health markers like blood pressure, cholesterol, and blood sugar. Focus on sustainable lifestyle changes rather than crash diets.

Obese (BMI 30+)

Obesity significantly increases risk for serious conditions including heart disease, type 2 diabetes (60-90% of type 2 diabetes cases are weight-related), certain cancers, osteoarthritis, and sleep apnea. If you're in this range, work with healthcare professionals to create a sustainable weight loss plan. Even 5-10% weight loss provides major health benefits. Medical interventions like medication or bariatric surgery may be appropriate for BMI 35+ with health conditions or BMI 40+.

Important Limitations of BMI

BMI is a useful screening tool but has significant limitations. It doesn't measure body fat directly or show where fat is located (belly fat is more dangerous than fat elsewhere). Here's when BMI can be misleading:

Athletes and bodybuilders: High muscle mass shows as "overweight" or "obese" despite low body fat. A muscular athlete at 6' and 220 lbs has BMI 29.8 (overweight) but might have 10% body fat.
Elderly individuals: May show "normal" BMI despite low muscle mass and higher body fat due to age-related muscle loss.
Pregnant women: BMI isn't applicable during pregnancy due to natural weight gain from baby, placenta, and fluids.
Children and teens: Require age and sex-specific BMI percentiles, not adult BMI ranges.
Different ethnicities: Asian populations have higher health risks at lower BMI (25 for Asians equals 30 for others). Pacific Islanders naturally have more muscle mass.

For a complete health picture, combine BMI with waist circumference (over 40" for men or 35" for women increases risk), body fat percentage (DEXA scan or bioelectrical impedance), and overall fitness level.

How to Reach a Healthy BMI

For Weight Loss (BMI Above 25)

Create moderate calorie deficit: Reduce intake by 500-750 calories/day to lose 1-1.5 lbs/week (safe, sustainable pace)
Focus on whole foods: Vegetables, fruits, lean proteins, whole grains (higher satiety, fewer calories than processed foods)
Strength train 2-3x per week: Builds muscle, which burns more calories at rest (muscle burns 6 cal/lb/day vs. 2 cal/lb for fat)
Walk 30-60 minutes daily: Burns 200-400 calories, low-impact, improves cardiovascular health
Track progress weekly: Weigh yourself same day/time each week, track measurements, take photos

For Weight Gain (BMI Below 18.5)

Increase calorie intake by 300-500/day: Add nutrient-dense foods, not junk food
Eat more frequently: 5-6 smaller meals instead of 3 large ones
Include healthy fats: Nuts, nut butters, avocados, olive oil (high calories, nutritious)
Strength training: Build muscle mass, not just fat, for healthy weight gain
Protein at every meal: Aim for 0.7-1g per pound of goal body weight to support muscle growth

Frequently Asked Questions

Is BMI accurate for everyone?

No. BMI is a good screening tool for most adults but has limitations. It's less accurate for athletes (high muscle mass shows as overweight), elderly (low muscle mass shows as normal), bodybuilders, pregnant women, and certain ethnic groups. For example, an NFL linebacker at 6'3" and 250 lbs has BMI 31.2 (obese) but likely has 8-12% body fat. Use BMI as one indicator among many—also consider waist circumference, body fat percentage, and how you feel.

What's a healthy BMI for my age?

For adults 20-65, the healthy range is BMI 18.5-24.9 regardless of age. However, some research suggests slightly higher BMI (25-27) may be protective for adults over 65 due to better resilience during illness. Children and teens use different percentile-based charts since they're still growing. Very elderly individuals (85+) may benefit from BMI 23-28 as some weight cushion helps recovery from illness or surgery.

How quickly can I change my BMI safely?

Safe weight loss is 1-2 pounds per week, which equals a BMI change of about 0.3-0.6 points per month for most people. For a 5'9" person (170 lbs, BMI 25.1), losing 1.5 lbs/week would reach BMI 24.9 (normal range) in about 2-3 weeks. Losing 15 pounds total (BMI 22.8) would take 8-15 weeks. Faster weight loss often leads to muscle loss, nutritional deficiencies, and weight regain. Slow and steady wins.

Does muscle weigh more than fat?

Muscle is denser than fat (muscle is about 18% denser), so 1 pound of muscle takes up less space than 1 pound of fat. A pound is a pound, but a muscular person at 180 lbs looks much leaner than an out-of-shape person at 180 lbs. This is why BMI can be misleading for muscular individuals and why you should also track body measurements and how clothes fit, not just scale weight.

Can I be overweight but still healthy?

It's possible to have elevated BMI (25-30) but excellent health markers if you exercise regularly, don't smoke, eat well, and have good bloodwork (normal blood pressure, cholesterol, blood sugar). This is sometimes called "metabolically healthy overweight." However, research shows this group still has slightly elevated long-term risk. Conversely, people with normal BMI but sedentary lifestyle and poor diet can be "metabolically unhealthy" despite normal weight.

Medical Disclaimer: This BMI calculator is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. BMI is a screening tool and should not be used as a diagnostic tool. Individual health assessment requires considering multiple factors beyond BMI. Always consult with your physician or qualified healthcare provider before starting any weight loss or weight gain program.