🧮 How to Calculate BMI: Complete Guide with Examples (2025)
Quick Answer: BMI = Weight (kg) ÷ [Height (m)]² for metric units, or
[Weight (lbs) ÷ Height (inches)²] × 703 for imperial units. A healthy BMI
ranges from 18.5 to 24.9 for most adults.
You've probably seen BMI mentioned at doctor's appointments, gym assessments, or
health apps. But do you actually know how to calculate your BMI and what it
means for your health? Body Mass Index (BMI) is one of the most widely used
health metrics worldwide, yet it's also one of the most misunderstood.
🧮 Interactive BMI Calculator
BMI Calculator
Calculate your Body Mass Index instantly
💡 Use the calculator above to instantly calculate your BMI and get
personalized health recommendations based on your results!
📋 What You'll Learn in This Guide
✅ How to calculate BMI (metric & imperial formulas)
✅ BMI categories and what they mean for your health
✅ Step-by-step examples with detailed solutions
✅ Why BMI is flawed (but still useful)
✅ Better alternatives to BMI for tracking health
✅ Common mistakes to avoid when calculating BMI
🤔 What is BMI? (Simple Definition)
Body Mass Index (BMI) is a numerical value calculated from your height and
weight. It's used as a screening tool to categorize individuals as underweight,
normal weight, overweight, or obese.
⚠️ Important Disclaimer
BMI is NOT a direct measure of body fat, muscle mass, or overall health.
It's a quick screening tool that doctors use as a starting point.
👨⚕️ Why Do Doctors Use BMI?
ReasonExplanation⚡ Quick and EasyNo special equipment needed🌍 StandardizedSame formula worldwide📊 Population ScreeningUseful for tracking public health trends🔬 Research-BackedCorrelates with certain health risks
📐 How to Calculate BMI: The Formulas
🟢 Formula 1: Metric Units (kg and cm)
┌─────────────────────────────────────────┐
│ BMI = Weight (kg) ÷ [Height (m)]² │
│ │
│ Or: Weight (kg) ÷ Height (m) ÷ Height │
└─────────────────────────────────────────┘
Where:
📏 Weight is in kilograms (kg)
📏 Height is in meters (m)
🔵 Formula 2: Imperial Units (pounds and inches)
┌──────────────────────────────────────────────────┐
│ BMI = [Weight (lbs) ÷ Height (inches)²] × 703 │
│ │
│ Or: [Weight ÷ Height ÷ Height] × 703 │
└──────────────────────────────────────────────────┘
Where:
📏 Weight is in pounds (lbs)
📏 Height is in inches
🔢 703 is the conversion factor
🎯 Step-by-Step: How to Calculate Your BMI
📝 Example 1: Calculate BMI Using Metric Units
❓ Question: You weigh 70 kg and your height is 175 cm. What's your BMI?
💡 Solution:
StepActionCalculationResult1️⃣Convert height to meters175 cm ÷ 1001.75 m2️⃣Square your height1.75 × 1.753.06 m²3️⃣Divide weight by height²70 ÷ 3.0622.9
✅ Answer: Your BMI is 22.9 (Normal weight)
📝 Example 2: Calculate BMI Using Imperial Units
❓ Question: You weigh 154 lbs and your height is 5 feet 7 inches. What's
your BMI?
💡 Solution:
StepActionCalculationResult1️⃣Convert to total inches(5 × 12) + 767 inches2️⃣Square your height67 × 674,4893️⃣Divide weight by height²154 ÷ 4,4890.03434️⃣Multiply by 7030.0343 × 70324.1
✅ Answer: Your BMI is 24.1 (Normal weight)
📝 Example 3: Tall Person BMI Calculation
❓ Question: You weigh 90 kg and your height is 190 cm. What's your BMI?
💡 Solution:
Step 1: Convert to meters
190 cm = 1.90 m
Step 2: Square the height
1.90 × 1.90 = 3.61 m²
Step 3: Divide weight by height squared
BMI = 90 ÷ 3.61 = 24.9
✅ Answer: Your BMI is 24.9 (Normal weight, upper range)
📝 Example 4: Short Person BMI Calculation
❓ Question: You weigh 50 kg and your height is 155 cm. What's your BMI?
💡 Solution:
Step 1: Convert to meters
155 cm = 1.55 m
Step 2: Square the height
1.55 × 1.55 = 2.40 m²
Step 3: Divide weight by height squared
BMI = 50 ÷ 2.40 = 20.8
✅ Answer: Your BMI is 20.8 (Normal weight)
📊 BMI Categories: What Your Number Means
According to the World Health Organization (WHO) and CDC:
BMI RangeCategoryHealth RiskIndicator< 18.5🔵 UnderweightMalnutrition risk⚠️18.5 - 24.9🟢 Normal WeightLowest risk✅25.0 - 29.9🟡 OverweightIncreased risk⚠️30.0 - 34.9🟠 Obese Class IHigh risk⚠️⚠️35.0 - 39.9🔴 Obese Class IIVery high risk⚠️⚠️⚠️40.0+🟣 Obese Class IIIExtremely high risk🚨
📈 Visual BMI Scale
─────────────────────────────────────────────────────
<18.5 │ 18.5-24.9 │ 25-29.9 │ 30-34.9 │ 35-39.9 │ 40+
─────────────────────────────────────────────────────
↓ ↓ ↓ ↓ ↓ ↓
Under Normal Over Obese I Obese II Obese III
weight Weight weight
⚠️ Why BMI is Flawed (But Still Useful)
❌ Problems with BMI
1. 💪 Doesn't Distinguish Muscle from Fat
Bodybuilders have "overweight" BMIs
Muscle weighs more than fat
Same BMI ≠ same body composition
2. 🎯 Ignores Body Composition
Two people, same BMI, different fat %
No visceral vs. subcutaneous fat info
Missing the full health picture
3. 🧓 Doesn't Consider Age
Older adults lose muscle naturally
No age-related adjustments
Same range for 25 and 75-year-olds
4. ⚧️ Ignores Gender Differences
Women have higher body fat naturally
Same BMI means different things
No hormonal considerations
5. 🌍 Not Adjusted for Ethnicity
Asian populations: higher risks at lower BMIs
Different ethnic groups carry weight differently
One-size-fits-all approach doesn't work
✅ Why Doctors Still Use It
Despite its flaws, BMI is:
AdvantageWhy It Matters⚡ Quick screening toolTakes seconds to calculate🔧 No equipment neededJust height and weight📊 Population-level accuracyWorks well for large groups🎯 Starting pointCombined with other measurements
💡 Bottom Line: BMI is a screening tool, NOT a diagnostic tool. Always
combine it with other health metrics.
🎯 Better Alternatives to BMI
1. 📏 Waist Circumference
Measures: Abdominal fat (most dangerous type)
GenderHealthy Range👨 MenLess than 40 inches (102 cm)👩 WomenLess than 35 inches (88 cm)
2. 📐 Waist-to-Hip Ratio
Formula: Waist circumference ÷ Hip circumference
GenderHealthy Range👨 MenBelow 0.90👩 WomenBelow 0.85
3. 💪 Body Fat Percentage
Measures: Actual fat vs. muscle
GenderHealthy Range👨 Men10-20%👩 Women18-28%
4. 📊 Waist-to-Height Ratio
Formula: Waist circumference ÷ Height
Rule: Should be less than 0.5
🔍 Real-World Examples: When BMI Misleads
🏋️ Case 1: The Athlete
┌─────────────────────────────────┐
│ Profile: │
│ • Weight: 90 kg │
│ • Height: 180 cm │
│ • BMI: 27.8 (Overweight) ⚠️ │
│ • Body Fat: 12% (Athletic) ✅ │
│ │
│ Reality: Muscular athlete, │
│ very healthy despite │
│ "overweight" BMI │
└─────────────────────────────────┘
🪑 Case 2: The Sedentary Person
┌─────────────────────────────────┐
│ Profile: │
│ • Weight: 70 kg │
│ • Height: 175 cm │
│ • BMI: 22.9 (Normal) ✅ │
│ • Body Fat: 30% (High) ⚠️ │
│ │
│ Reality: "Skinny fat" - │
│ normal BMI but high body fat │
│ and low muscle mass │
└─────────────────────────────────┘
👴 Case 3: The Elderly Person
┌─────────────────────────────────┐
│ Profile: │
│ • Age: 70 years │
│ • Weight: 60 kg │
│ • Height: 170 cm │
│ • BMI: 20.8 (Normal) ✅ │
│ │
│ Reality: May have lost │
│ significant muscle mass │
│ (sarcopenia) despite normal BMI │
└─────────────────────────────────┘
👶 BMI for Children and Teens
⚠️ Important: BMI is calculated differently for those under 20 years old!
Children's BMI uses percentiles that compare them to other children of the
same age and sex:
CategoryPercentile Range🔵 UnderweightBelow 5th percentile🟢 Normal5th to 84th percentile🟡 Overweight85th to 94th percentile🔴 Obese95th percentile or above
👨⚕️ Always consult a pediatrician for children's BMI interpretation.
📋 Quick BMI Calculator Reference
Common Heights & Healthy Weight Ranges (BMI 18.5-24.9)
HeightHealthy Weight (kg)Healthy Weight (lbs)150 cm (4'11")42 - 56 kg92 - 123 lbs160 cm (5'3")47 - 64 kg104 - 141 lbs170 cm (5'7")53 - 72 kg117 - 159 lbs180 cm (5'11")60 - 81 kg132 - 179 lbs190 cm (6'3")67 - 90 kg148 - 198 lbs
⚠️ Common Mistakes When Calculating BMI
❌ Mistake 1: Using Wrong Units
❌ Wrong✅ RightMixing cm with kg in imperial formulaUse consistent units - metric OR imperial
❌ Mistake 2: Not Converting Height to Meters
❌ Wrong✅ RightUsing 175 cm directly in metric formulaConvert to 1.75 m first
❌ Mistake 3: Forgetting the 703 Multiplier
❌ Wrong✅ RightUsing imperial formula without × 703Always multiply by 703 for pounds/inches
❌ Mistake 4: Not Squaring Height
❌ Wrong✅ RightBMI = Weight ÷ HeightBMI = Weight ÷ (Height × Height)
📋 How to Use BMI Correctly
✅ DO:
✅ Use it as a screening tool only
✅ Combine with waist circumference measurements
✅ Consider your muscle mass and fitness level
✅ Track trends over time rather than single measurements
✅ Consult a doctor for a complete health assessment
❌ DON'T:
❌ Don't use BMI as the only health metric
❌ Don't ignore how you feel physically
❌ Don't compare yourself to athletes or bodybuilders
❌ Don't obsess over small daily changes
❌ Don't use for children without pediatric percentile charts
❓ Frequently Asked Questions (FAQs)
1. Is BMI accurate?
Answer: BMI is a useful screening tool for population health but NOT
accurate for individuals. It doesn't account for muscle mass, bone density,
body composition, age, or gender differences. Use it alongside other metrics
like waist circumference and body fat percentage.
2. How do you calculate BMI in kg?
Answer: BMI = Weight (kg) ÷ [Height (m)]². First convert your height from
cm to meters (divide by 100), then square it, then divide your weight by that
number. Example: 70 kg ÷ (1.75 × 1.75) = 22.9
3. What is a healthy BMI?
Answer: A BMI between 18.5 and 24.9 is considered "normal" or healthy
for most adults. However, this range may not apply to athletes, elderly
individuals, or certain ethnic groups who may be healthy outside this range.
4. Can BMI be wrong?
Yes! BMI can be misleading for:
💪 Athletes and bodybuilders (high muscle mass)
🧓 Elderly people (loss of muscle mass)
🤰 Pregnant women
🦴 People with different bone densities
🌏 Certain ethnic groups (Asians have higher risks at lower BMIs)
5. How often should I check my BMI?
Answer: Checking BMI monthly or quarterly is sufficient for tracking
general trends. Daily or weekly checks aren't necessary as normal weight
fluctuations don't significantly change BMI. Focus on long-term trends rather
than daily numbers.
6. What BMI is considered obese?
Answer: A BMI of 30.0 or higher is classified as obese. This is
further divided into:
🟠 Class I (30-34.9)
🔴 Class II (35-39.9)
🟣 Class III (40+)
Remember: BMI alone doesn't diagnose obesity - body composition matters too.
7. Does BMI change with age?
Answer: The standard BMI formula doesn't change with age, but healthy
ranges may vary. Older adults often lose muscle mass, so a slightly higher BMI
(23-28) may be healthier for those over 65. Always consult your doctor for
age-appropriate recommendations.
8. Is BMI the same for men and women?
Answer: Yes, the same formula is used for both, but this is one of BMI's
flaws. Women naturally have 10-12% higher body fat than men, so the same BMI
means different things. Women at BMI 24 may have more body fat than men at
BMI 24.
📝 Summary: Key Takeaways
📐 The Formulas
Metric: BMI = Weight (kg) ÷ [Height (m)]²
Imperial: BMI = [Weight (lbs) ÷ Height (inches)²] × 703
🎯 Healthy BMI Range
18.5 - 24.9 is considered normal for most adults
Range may vary based on age, ethnicity, and fitness level
Use the calculator at the top for instant results
⚠️ Important Limitations
Doesn't account for muscle mass
Ignores overall body composition
Not adjusted for age or gender differences
Different ethnic groups need different ranges
✅ Best Practices
Use BMI as a screening tool, not a diagnosis
Combine with other metrics (waist circumference, body fat %)
Track trends over time, not single measurements
Always consult a healthcare provider for personalized advice
📚 What to Read Next
🎯 Conclusion
Understanding how to calculate BMI is simple - just follow the formulas for
metric or imperial units. But understanding what your BMI means requires
context. While BMI is a quick and useful screening tool, it's not the complete
picture of your health.
🔑 Remember These Important Points:
💪 Athletes with high muscle mass may have "overweight" BMIs but be
perfectly healthy🧓 Elderly individuals may need different BMI ranges due to muscle loss
🏃 Your fitness level and how you feel matter more than a single number
🩺 Combine BMI with other metrics like waist circumference and body fat
percentage👨⚕️ Talk to your doctor for personalized health guidance and
recommendations
Don't let a single number define your health. Use BMI as one tool among
many, focus on how you feel, your energy levels, and your overall wellness.
Try the calculator at the top of this page to get your BMI instantly, then
use it as a starting point for a conversation with your healthcare provider! 🎯