BMI Calculator

What Is BMI and How Is It Calculated?

Body Mass Index (BMI) is a quick screening measure of body fat based on height and weight, used worldwide by the WHO.

BMI category reference (Chinese adult standard)

Tips for a healthy weight

• Aim for 150+ minutes of moderate aerobic exercise per week
• Keep protein adequate and limit sugary drinks
• Sleep 7–9 hours — poor sleep is linked to weight gain
• Re-measure monthly rather than daily; weight fluctuates naturally

What is the normal BMI range?

The normal BMI range differs by standard. Chinese adults use a stricter cut-off because Asian populations carry more visceral fat at lower body weights, which increases metabolic disease risk:

• Underweight: BMI < 18.5
• Normal: BMI 18.5 – 23.9
• Overweight: BMI 24.0 – 27.9
• Obese: BMI ≥ 28.0

The WHO international standard is slightly looser: normal 18.5–24.9, overweight 25–29.9, obese ≥30. If you are comparing yourself to international sources, you may see different thresholds — use the Chinese standard for a more accurate health assessment.

BMI formula — step by step

The BMI formula is simple division:

Step 1: Measure your weight in kilograms (e.g. 65 kg).
Step 2: Measure your height in meters (e.g. 1.70 m).
Step 3: Square the height: 1.70 × 1.70 = 2.89.
Step 4: Divide weight by squared height: 65 ÷ 2.89 = 22.5.

A BMI of 22.5 falls in the normal range (18.5–23.9). This is considered the healthiest weight zone with the lowest risk of metabolic disease.

How to calculate your own BMI — practical guide

For the most accurate BMI, follow these tips:

1. Weigh yourself in the morning, after using the bathroom and before eating or drinking. This is your most consistent weight baseline.
2. Measure height without shoes, standing straight against a wall. Have someone place a flat object on your head and mark the wall.
3. Convert height to meters. If you measured 170 cm, that is 1.70 m. If you measured 5'7" (67 inches), multiply by 0.0254 → 1.702 m.
4. Use the calculator above — enter height in cm and weight in kg, click Calculate, and instantly see your BMI and category.
5. Track monthly, not daily. Weight naturally fluctuates 1–2 kg due to water, salt and food volume. Monthly trends are more meaningful.

BMI limitations and alternatives

BMI is a screening tool, not a diagnostic test. Its key limitation: it cannot distinguish muscle from fat. This means:

• Athletes and bodybuilders often score "overweight" despite having low body fat, because muscle is denser than fat.
• Older adults may have a "normal" BMI but high body fat (sarcopenic obesity) — muscle loss masks the risk.
• Body shape matters. Two people with the same BMI can have very different health risks if one carries fat around the waist (visceral fat) and the other around the hips (subcutaneous fat).

Better risk assessment combines BMI with:

• Waist circumference — ≥90 cm for men or ≥85 cm for women (Chinese standard) signals elevated metabolic risk regardless of BMI.
• Waist-to-hip ratio — >0.90 (men) or >0.85 (women) suggests central obesity.
• Body fat percentage — measured by bioimpedance scales, DEXA scans or skinfold calipers. Healthy range: men 10–20%, women 18–28%.

BMI standards for different populations

BMI cut-offs vary by ethnicity because body composition and disease risk differ across populations:

Asian populations face higher cardiovascular and diabetes risk at lower BMIs, which is why the cut-offs are stricter. Always use the standard that matches your ethnicity for the most relevant assessment.