How to Calculate BMI
BMI — Body Mass Index — is a simple number that gives a rough estimate of whether your weight is in a healthy range for your height. It's not a perfect measure of health, but it's a fast and widely used screening tool that doctors and health organizations have relied on for decades.
You can calculate your BMI in seconds using our free BMI Calculator, or you can do it manually using the formulas below.
The BMI Formula
There are two versions of the formula depending on whether you use metric or imperial measurements.
Metric Formula (kg and cm)
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BMI = weight (kg) ÷ height (m)²
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Example: A person who weighs 70 kg and is 1.75 m tall:
- Height squared: 1.75 × 1.75 = 3.0625
- BMI: 70 ÷ 3.0625 = 22.9
Imperial Formula (pounds and inches)
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BMI = (weight (lbs) × 703) ÷ height (inches)²
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Example: A person who weighs 154 lbs and is 5'9" (69 inches) tall:
- Height squared: 69 × 69 = 4,761
- Weight × 703: 154 × 703 = 108,262
- BMI: 108,262 ÷ 4,761 = 22.7
The multiplier 703 converts the imperial units into the same scale as the metric formula.
BMI Categories — What Your Number Means
| BMI Range | Category | What It Means | |---|---|---| | Below 18.5 | Underweight | May indicate nutritional deficiency | | 18.5 – 24.9 | Normal weight | Generally considered healthy range | | 25.0 – 29.9 | Overweight | Some increased health risk | | 30.0 – 34.9 | Obese (Class I) | Significant health risk | | 35.0 – 39.9 | Obese (Class II) | High health risk | | 40.0 and above | Obese (Class III) | Very high health risk |
These categories are defined by the World Health Organization and are used by most healthcare systems worldwide.
How to Calculate BMI Step by Step
Using the metric system:
1. Measure your height in meters (e.g., 1.68 m) 2. Square that number (1.68 × 1.68 = 2.8224) 3. Divide your weight in kilograms by that result (65 ÷ 2.8224 = 23.0) 4. Your BMI is 23.0 — Normal weight
Using feet and pounds:
1. Convert your height to inches (5 feet 6 inches = 66 inches) 2. Square that number (66 × 66 = 4,356) 3. Multiply your weight by 703 (145 lbs × 703 = 101,935) 4. Divide the result (101,935 ÷ 4,356 = 23.4) 5. Your BMI is 23.4 — Normal weight
Or skip all of this and use our BMI Calculator — enter your height and weight, get your BMI instantly.
BMI for Men vs. Women
The standard BMI formula is the same for men and women, but the interpretation can differ slightly:
- Men typically have more muscle mass, which is denser than fat. A muscular man may have a "high" BMI while being very lean.
- Women typically have a higher percentage of body fat at the same BMI, which is normal and healthy.
- Some researchers suggest slightly different BMI ranges for men and women, but the WHO standard categories are still the most widely used.
BMI Limitations — What It Doesn't Tell You
BMI is a useful screening tool, but it has real limitations:
- It doesn't measure body fat directly. A highly muscular athlete may have a high BMI but very low body fat.
- It doesn't account for age. Older adults naturally lose muscle mass, so BMI can underestimate health risks in seniors.
- It doesn't consider where fat is stored. Belly fat (visceral fat) is more dangerous than fat stored in the hips and thighs, but BMI doesn't distinguish between the two.
- Ethnic differences matter. Research shows that people of Asian descent may face higher health risks at lower BMI values, leading some health organizations to recommend lower cutoff points for this group.
Always discuss your BMI with a healthcare professional who can consider your full picture.
Healthy BMI by Age Group
| Age Group | Healthy BMI Range | Notes | |---|---|---| | Children (2–19) | Varies by age/sex | Pediatric growth charts used | | Adults (20–65) | 18.5 – 24.9 | Standard WHO range | | Older adults (65+) | 22 – 27 | Slightly higher may be protective |
For children and teenagers, BMI is plotted on age- and sex-specific growth charts rather than using fixed categories. Our adult BMI Calculator is designed for people 20 and older.
Tips for a Healthy BMI
If your BMI is outside the healthy range, the goal isn't to hit a specific number — it's to improve overall health:
- For weight loss: Focus on sustainable habits — regular movement, balanced eating, adequate sleep
- For weight gain: Work with a nutritionist; increasing protein and calorie intake in a controlled way is key
- Track trends, not just one number: BMI can fluctuate with water, clothing, and time of day — track it monthly, not daily
- Combine BMI with other measures: Waist circumference, body fat percentage, and blood work give a more complete picture
Frequently Asked Questions
What is a good BMI for my age? For adults aged 20–65, a BMI between 18.5 and 24.9 is considered normal weight. For adults over 65, a slightly higher range (22–27) is often considered healthy because some extra weight can be protective in older age. Use our BMI Calculator to get your number instantly.
Is BMI accurate for athletes? Not always. Athletes with significant muscle mass often have a high BMI despite having low body fat. In these cases, BMI overestimates health risk. Body fat percentage measurements are more accurate for athletic individuals.
How often should I check my BMI? For most adults, checking BMI every 3–6 months is sufficient. Daily or weekly tracking isn't necessary because BMI changes slowly. Focus on long-term trends rather than short-term fluctuations.
Can I have a normal BMI but still be unhealthy? Yes. A person with a normal BMI can still have high blood pressure, high cholesterol, or high blood sugar. BMI is one screening tool, not a complete health assessment. Regular check-ups with your doctor are the best way to understand your overall health.