To calculate BMI, divide your weight in kilograms by your height in meters squared. For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 22.9 because 70 ÷ (1.75 × 1.75) = 22.9. This falls in the healthy range (18.5–24.9).
This guide covers the metric and imperial formulas, three worked examples, a full BMI chart, and the situations where BMI is misleading.
What Is BMI?
BMI stands for Body Mass Index. It's a simple screening measure that uses your height and weight to estimate whether your weight falls in a healthy range. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and adopted globally by the WHO in the 20th century.
BMI is a screening tool — not a diagnostic one. It doesn't measure body fat directly, and it doesn't account for muscle mass, bone density, or fat distribution. But it's cheap, fast, and useful for population-level health estimates.
The BMI Formula
Metric Formula (kg and meters)
Imperial Formula (pounds and inches)
Example 1: Metric Calculation
A person weighs 68 kg and is 1.72 m tall. What is their BMI?
Example 2: Imperial Calculation
A person weighs 154 lb and is 5 ft 8 in tall. What is their BMI?
Example 3: A High BMI Case
A person weighs 105 kg and is 1.70 m tall. What is their BMI?
Complete BMI Chart
The World Health Organization defines these categories for adults (age 20+):
| Category | BMI Range |
|---|---|
| Severe thinness | Below 16.0 |
| Moderate thinness | 16.0 – 16.9 |
| Mild thinness | 17.0 – 18.4 |
| Normal weight | 18.5 – 24.9 |
| Overweight (pre-obese) | 25.0 – 29.9 |
| Obese Class I | 30.0 – 34.9 |
| Obese Class II | 35.0 – 39.9 |
| Obese Class III | 40.0 and above |
Healthy range: For most adults, BMI between 18.5 and 24.9 is considered healthy. This is the range where weight-related health risks are lowest in population studies.
How to Calculate Your Healthy Weight Range
If you want to know what weight is healthy for your height, work the formula backwards. Solve for weight using BMI 18.5 and 24.9 at your height:
Max kg = 24.9 × height (m)²
When BMI Is Misleading
BMI is a rough estimate. It can be wrong in either direction for these groups:
- Athletes and bodybuilders: Muscle weighs more than fat. A muscular person can have a BMI of 28 without having excess body fat.
- Older adults: Muscle mass declines with age while fat increases. Normal-BMI older adults may still have unhealthy body fat levels.
- Pregnant women: BMI doesn't apply during pregnancy. Different weight guidelines are used by healthcare providers.
- Children and teens: BMI uses age- and sex-specific percentiles, not the adult categories above.
- Very tall or short people: BMI can overestimate fat in very tall people and underestimate it in very short people.
- Ethnic differences: Some populations (South Asian, Chinese) have higher health risks at lower BMIs. WHO recommends adjusted thresholds for these groups.
Better measures exist: If you want a more accurate picture of body composition, consider body fat percentage, waist-to-hip ratio, or waist circumference (men: <40 in / 102 cm; women: <35 in / 88 cm). These capture fat distribution, which BMI cannot.
BMI Limitations You Should Know
- Doesn't measure fat location. Visceral fat (around organs) is more dangerous than subcutaneous fat — and BMI can't tell them apart.
- Doesn't reflect body composition. Two people with the same BMI can have very different body fat percentages.
- Doesn't predict individual health. A person with a BMI of 27 who exercises and eats well may be healthier than someone with a BMI of 22 who doesn't.
- Race and ethnicity matter. The standard thresholds are based on European populations and may not fit everyone equally.
How to Use BMI Correctly
BMI works best as one data point among many. Use it to:
- Screen for potential weight-related health risks
- Track weight changes over time (same person, same formula)
- Compare against population averages
Don't use it to:
- Diagnose health conditions
- Judge athletic performance
- Set precise weight goals without other context
Skip the Math — Use a Calculator
If you want an instant BMI with the healthy range, category, and BMI Prime, use our free calculator. It supports both metric and imperial units and shows everything at once.
⚖️ Free BMI Calculator
Enter your height and weight — get BMI, category, and healthy weight range instantly.
Open BMI Calculator →Related: Daily Calorie Needs
Once you know your BMI, the next useful number is your daily calorie target. Our calorie calculator uses your BMI inputs plus your goal (lose, maintain, gain) to give you a specific daily calorie number and macro breakdown.
🔥 Free Calorie Calculator
Get your daily calorie target, BMR, TDEE, and macros.
Open Calorie Calculator →Frequently Asked Questions
How do I calculate BMI?
Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = 703 × weight (lb) ÷ height (in)². Example: 70 kg at 1.75 m = 70 ÷ (1.75 × 1.75) = 22.9.
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese.
Is BMI accurate for everyone?
No. BMI doesn't distinguish muscle from fat. Athletes and very muscular people may be classified as overweight despite low body fat. BMI is also less accurate for children, pregnant women, and older adults.
What's the difference between BMI and body fat percentage?
BMI is a rough estimate based on height and weight only. Body fat percentage is a direct measurement of fat mass, which requires tools like a DEXA scan, hydrostatic weighing, or a smart scale.
Does BMI differ for men and women?
The BMI formula is identical for men and women. However, healthy ranges can vary slightly because men and women tend to carry fat differently. Some clinicians use sex-adjusted interpretations.
What BMI is considered obese?
A BMI of 30 or above is classified as obese. Obesity is divided into three classes: Class I (30–34.9), Class II (35–39.9), and Class III (40+).
What is BMI Prime?
BMI Prime is your BMI divided by 25 (the upper limit of the healthy range). A BMI Prime below 1.0 means you're within the healthy range; above 1.0 means you're above it. It's a quick way to see how far you are from the top of the healthy band.
How much should I weigh for my height?
Healthy weight ranges are roughly: 5'0" (152 cm) → 47–63 kg; 5'6" (168 cm) → 58–78 kg; 6'0" (183 cm) → 68–90 kg. Use our BMI calculator for your exact range.