How to Calculate BMI

The formula is simple. The classification varies dramatically by country — here's the complete picture.

QUICK ANSWER

BMI = weight(kg) ÷ height(m)². Example: 70 kg at 1.75 m → 70 ÷ 3.0625 = BMI 22.9. That's "Normal" by WHO — but the same BMI could be classified differently in Japan or by Asia-Pacific guidelines. The formula is universal; the category thresholds are not.

The BMI Formula

Metric: BMI = weight (kg) ÷ height (m)²

Imperial: BMI = 703 × weight (lb) ÷ height (in)²

1
Convert height to metres (metric)

Divide centimetres by 100. 175 cm = 1.75 m. Then square it: 1.75 × 1.75 = 3.0625 m².

2
Divide weight by height-squared

70 kg ÷ 3.0625 = 22.86. Round to 1 decimal place: BMI = 22.9.

3
Look up your category (see table below)

WHO says 22.9 = Normal weight. But Japan's JASSO sets obesity at ≥25 — so BMI 22.9 is Normal there too, but the line between "OK" and "obese" is closer than in the West.

BMI Classification Thresholds by Standard

Category 🌍 WHO (global) 🌏 Asia-Pacific 🇯🇵 Japan (JASSO) 🇨🇳 China (WGOC)
Underweight <18.5 <18.5 <18.5 <18.5
Normal 18.5–24.9 18.5–22.9 18.5–22.9 18.5–23.9
Overweight 25–29.9 23–27.4 23–24.9 24–27.9
Obese ≥30 ≥27.5 ≥25 ≥28

Key insight: A BMI of 26 is "Overweight" by WHO but "Obese Grade I" by Japan's JASSO standard. The same number, different meaning.

Frequently Asked Questions

What is a healthy BMI? ▾
By WHO standards, 18.5–24.9 is considered normal/healthy weight. However, "healthy" is context-dependent: a muscular athlete with BMI 27 is healthier than a sedentary person with BMI 22 who has high visceral fat. BMI is a population screening tool, not a personal health verdict.
Why does Japan use a lower BMI threshold for obesity? ▾
Research shows Asian populations develop obesity-related diseases (diabetes, cardiovascular disease, metabolic syndrome) at lower BMI values than Western populations — due to differences in body composition and fat distribution. Japan's JASSO sets obesity at ≥25 (vs WHO's ≥30) to reflect this earlier health risk.
How do I convert my height from feet/inches to metres? ▾
Multiply feet by 0.3048, then add inches multiplied by 0.0254. Example: 5 feet 9 inches = (5 × 0.3048) + (9 × 0.0254) = 1.524 + 0.2286 = 1.7526 metres. Or use the shortcut: total inches × 0.0254. 5'9" = 69 inches × 0.0254 = 1.7526 m.
Does BMI work the same way for children? ▾
No. For children and teenagers (ages 2–19), the adult BMI thresholds do not apply. Instead, BMI is plotted on age- and sex-specific percentile charts (such as the CDC or WHO growth charts). A child is considered overweight at or above the 85th percentile and obese at or above the 95th percentile for their age and sex, because healthy body composition changes naturally as children grow.
What is the difference between BMI and body fat percentage? ▾
BMI only uses height and weight, so it cannot tell muscle from fat. Body fat percentage measures the actual proportion of your body that is fat tissue (typically 10–20% for men and 18–28% for women in a healthy range). A muscular athlete can have a high BMI but a low body fat percentage. BMI is a quick screening number; body fat percentage is a more direct measure of body composition but requires tools like calipers, bioelectrical impedance, or a DEXA scan.

Calculate Your BMI

Open BMI Calculator (WHO + Japan + Asia-Pacific) →