BMI Calculator: Calculate Your Body Mass Index
Table of Contents - Bmi
- How to Use This Calculator
- The Core Principle: Weight-to-Height Ratio
- How to Calculate BMI Manually
- Real-World Applications
- Scenarios People Actually Run Into
- Trade-Offs and Decisions People Underestimate
- Common Mistakes and How to Recover
- Related Topics
- How This Calculator Works
- FAQs
How to Use This Calculator - Bmi
The calculator provides two input sections. For weight, enter your value and select units from the dropdown—either kg (metric) or lbs (imperial). When you select imperial, the height input changes accordingly.
For metric mode, enter your height as a single value in centimeters. For imperial mode, two fields appear for feet and inches.
Click "Calculate BMI" to see your results. A "Clear" button resets all fields.
Results appear in two highlighted boxes: your BMI number (like 24.3) with "Your BMI" labeled beneath, and your weight category (like "Normal weight") with color coding—blue for underweight, green for normal, yellow for overweight, red for obese.
Below the results, a "BMI Categories" reference shows the standard thresholds: Underweight below 18.5, Normal weight 18.5-24.9, Overweight 25-29.9, Obese 30 and above.
Additional information panels cover what BMI measures (body fat based on height and weight), BMI limitations (doesn't distinguish muscle from fat), age considerations (different standards for children), and health assessment recommendations.
The Core Principle: Weight-to-Height Ratio
BMI is a simple ratio that compares your body weight to your height squared. The formula produces a single number that places you in a weight category, making it useful as a quick screening tool.
The squaring of height is key. A person who is twice as tall as another isn't expected to weigh twice as much—they're expected to weigh roughly four times as much, because body volume scales with the cube of linear dimensions. Squaring height (rather than cubing) is a compromise that works reasonably well for typical adult body proportions.
BMI doesn't measure body fat directly. It doesn't distinguish between weight from muscle, bone, water, or fat. A muscular athlete and an inactive person of the same height and weight have the same BMI but very different body compositions.
Despite these limitations, BMI correlates with health outcomes at the population level. People with BMIs in the "normal" range statistically have lower rates of certain diseases than those at either extreme. This is why BMI remains a standard screening tool, even as its limitations are well understood.
How to Calculate BMI Manually
Metric formula:
BMI = weight (kg) ÷ height (m)²
Example: 70 kg, 1.75 m tall Height squared: 1.75 × 1.75 = 3.0625 m² BMI: 70 ÷ 3.0625 = 22.9
Imperial formula:
BMI = [weight (lb) ÷ height (in)²] × 703
The 703 is a conversion factor to make imperial units produce the same BMI as metric.
Example: 154 lbs, 5'9" (69 inches) Height squared: 69 × 69 = 4,761 in² Division: 154 ÷ 4,761 = 0.03235 Multiply: 0.03235 × 703 = 22.7
Quick reference for converting height to inches:
- 5'0" = 60 inches
- 5'6" = 66 inches
- 6'0" = 72 inches
- 6'6" = 78 inches
Standard BMI categories (WHO):
- Underweight: below 18.5
- Normal weight: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obesity Class I: 30.0 to 34.9
- Obesity Class II: 35.0 to 39.9
- Obesity Class III: 40.0 and above
Real-World Applications
Clinical screening. Healthcare providers use BMI as an initial assessment during checkups. It's quick, requires only a scale and height measurement, and flags patients who might benefit from lifestyle discussions or further evaluation.
Public health monitoring. Population-level BMI statistics track obesity trends. Health departments compare BMI distributions across regions, demographics, and time periods to identify public health priorities.
Insurance risk assessment. Some life and health insurance policies consider BMI in underwriting. While controversial, BMI provides a standardized metric for population-level risk assessment.
Personal health tracking. Tracking BMI over time shows the general direction of weight relative to height. For adults whose height is stable, BMI changes reflect weight changes.
Research enrollment criteria. Many medical studies use BMI to define eligible populations. "Normal weight participants" or "participants with BMI over 30" are common enrollment criteria.
Scenarios People Actually Run Into
The fit person with a high BMI. A person who strength trains regularly might have a BMI of 27 (overweight category) despite having visible abs and excellent health markers. Their weight comes largely from muscle, which BMI can't distinguish from fat.
The "normal BMI" with concerning body composition. Conversely, someone with a BMI of 22 might carry excessive abdominal fat and little muscle—sometimes called "skinny fat." Their BMI looks fine, but their body composition suggests metabolic risk.
The height measurement dispute. You're 5'10" at the doctor's office but measured 5'11" somewhere else. That inch difference changes your BMI from 25.1 to 24.8—crossing a category boundary. Measure height consistently (same time of day, proper posture) for meaningful comparisons.
The morning versus evening weight. Body weight fluctuates 2-5 pounds during the day due to food, water, and bathroom visits. Weigh at a consistent time (morning, after using the bathroom, before eating) for meaningful tracking.
The slow creep. BMI increases by 0.5 over a year, still within "normal." Easy to dismiss. But 0.5 per year for 10 years means going from 22 to 27. Tracking helps you notice trends before they become concerning.
Trade-Offs and Decisions People Underestimate
Individual versus population utility. BMI works well for population-level health assessment but poorly for individual diagnosis. Your BMI tells you where you fall on a statistical distribution, not whether you specifically are healthy.
Category thresholds are arbitrary. Why is 24.9 "normal" but 25.0 "overweight"? These cutoffs are based on statistical risk curves, but the curves are smooth—there's no magic transition at exactly 25. Someone at 24.8 isn't meaningfully different from someone at 25.2.
Ethnic variations in risk. BMI cutoffs were developed primarily on European populations. Research suggests that some ethnic groups (particularly Asian populations) may face elevated health risks at lower BMI values. Some guidelines recommend lower thresholds for Asian populations.
Age adjustments. The standard BMI categories apply to adults 20+. For children and teens, BMI is compared to growth charts as percentiles. For older adults, slightly higher BMI may actually be associated with better outcomes—the "obesity paradox" in elderly populations.
Trend versus snapshot. A single BMI reading is a snapshot. The trend over time is often more informative. A BMI of 26 that's been stable for years suggests different things than a BMI of 26 that was 22 two years ago.
Common Mistakes and How to Recover
Mixing unit systems. You enter weight in pounds but height in centimeters, or forget to convert feet-and-inches to total inches. The calculator handles unit selection through dropdowns to prevent this, but if doing manual calculation, ensure consistency.
Using height in centimeters with the metric formula expecting meters. The formula uses meters squared. If you're 175 cm tall, you need to convert to 1.75 m before squaring, or you'll get a BMI that's 10,000× too small.
Obsessing over decimal places. A BMI of 24.87 versus 24.91 is meaningless noise—your weight fluctuates by more than that difference throughout the day. Focus on whole numbers and trends, not precise decimals.
Treating BMI as a fitness metric. BMI doesn't measure fitness. An athlete can have a "normal" BMI but be in exceptional shape, or have an "overweight" BMI with excellent cardiovascular fitness. Use actual fitness tests for fitness assessment.
Ignoring context. A very muscular person, a pregnant woman, an elderly person, and a sedentary young adult might all have the same BMI but vastly different health profiles. BMI is a starting point, not a conclusion.
Related Topics
Waist circumference. Where you carry fat matters. Abdominal fat (measured by waist circumference) is associated with higher health risk than fat on hips and thighs. Some guidelines combine BMI with waist circumference for better risk assessment.
Body fat percentage. Direct measurement of fat versus lean mass provides more information than BMI. Methods include DEXA scans, bioelectrical impedance, skinfold calipers, and underwater weighing. Each has different accuracy and accessibility.
Waist-to-hip ratio. This measures fat distribution. A ratio above 0.85 for women or 0.90 for men suggests excess abdominal fat and increased health risk, regardless of BMI.
BMR and metabolism. BMI is about body size; BMR (Basal Metabolic Rate) is about energy expenditure. Two people with the same BMI can have very different metabolic rates depending on muscle mass and other factors.
Metabolic health markers. Blood pressure, blood glucose, cholesterol levels, and inflammatory markers tell you more about health than BMI alone. Someone with a high BMI but excellent blood work may be metabolically healthy.
How This Calculator Works
The calculator takes weight and height inputs in either metric or imperial units. If imperial is selected, it converts feet and inches to total inches, then to centimeters internally.
The BMI calculation follows the standard formula: weight in kilograms divided by height in meters squared. For imperial input, the calculator first converts pounds to kilograms (divide by 2.205) and inches to centimeters (multiply by 2.54), then applies the metric formula.
The result is compared against WHO category thresholds to determine the weight category label and color coding:
- Below 18.5: "Underweight" (blue)
- 18.5-24.9: "Normal weight" (green)
- 25.0-29.9: "Overweight" (yellow)
- 30.0+: "Obese" (red)
The calculator rounds the BMI to one decimal place for display.
All calculations happen locally in your browser.
FAQs
Is BMI accurate for athletes?
Often not. BMI can't distinguish muscle from fat. A heavily muscular person will have an elevated BMI despite low body fat. For athletes, body fat percentage or performance metrics are more relevant than BMI.
How does age affect BMI interpretation?
For adults, the standard categories apply regardless of age, though some research suggests slightly higher BMI may be acceptable or even beneficial in older adults. For children and teens, BMI is interpreted using age- and sex-specific percentile charts rather than fixed cutoffs.
Should I aim for a specific BMI number?
Aiming for a healthy weight range makes more sense than fixating on a specific number. Within the "normal" range (18.5-24.9), there's no evidence that lower is better. Other factors—fitness, diet quality, sleep, stress—matter as much or more.
Why is BMI still used if it has so many limitations?
It's free, fast, and requires no special equipment. For population health screening, its limitations average out. For individuals, it's a starting point that should be combined with other assessments.
Does BMI apply to pregnant women?
BMI before pregnancy helps assess healthy weight gain targets during pregnancy, but BMI during pregnancy isn't interpreted the same way—weight gain is expected and necessary.
Can medications affect my BMI?
Medications affect weight, which affects BMI. Some medications cause weight gain (certain antidepressants, steroids, antipsychotics); others cause weight loss. If your weight changes significantly on medication, discuss with your healthcare provider.
What's more important: BMI or waist circumference?
Both contribute useful information. Waist circumference specifically indicates abdominal fat, which is associated with metabolic risk. Many health guidelines now recommend measuring both.
How often should I check my BMI?
For general health monitoring, monthly is sufficient. More frequently introduces noise from normal weight fluctuations. The goal is to track trends over months and years, not daily variations.