Core answer: whether a BMI calculator is "accurate" depends first on its standard — Chinese adult cutoffs are 24/28 (overweight/obese), WHO cutoffs 25/30. A good tool shows both, outputs a healthy weight range, and warns that BMI cannot tell muscle from fat. Sanity test: 175cm, 70kg → BMI = 70 ÷ 1.75² = 22.9.
China vs WHO table
| Grade | China (WS/T 428) | WHO |
|---|---|---|
| Underweight | <18.5 | <18.5 |
| Normal | 18.5-23.9 | 18.5-24.9 |
| Overweight | 24.0-27.9 | 25.0-29.9 |
| Obese | ≥28.0 | ≥30.0 |
The gap is the 24-25 band: overweight in China, normal by WHO. China’s lower line reflects visceral-fat risk appearing at lower BMI in East Asian populations.
Sanity checks and formula
BMI = weight(kg) ÷ height(m)². Three self-checks: 175cm/70kg → 22.9; 160cm/60kg → 23.4; 180cm/90kg → 27.8. Watch the units: height must be in metres (175cm = 1.75m), and it is the height that gets squared.
| Height | China normal range | Overweight from | Obese from |
|---|---|---|---|
| 160cm | 47.4-61.1 kg | 61.4 kg | 71.7 kg |
| 170cm | 53.5-69.1 kg | 69.4 kg | 80.9 kg |
| 175cm | 56.7-73.2 kg | 73.5 kg | 85.8 kg |
| 180cm | 59.9-77.4 kg | 77.8 kg | 90.7 kg |
BMI’s three blind spots
- Muscle vs fat: muscle is about 1.2× denser — athletes get flagged "overweight"
- No fat distribution: visceral fat (waistline) matters more; normal BMI can still be central obesity
- Special groups: pregnancy, minors, seniors 65+, athletes all need dedicated standards
The full check is a trio: BMI for grade, waist (men ≥90cm / women ≥85cm = central obesity) for distribution, body-fat % (men >25% / women >30% high) for composition.
Example: two fates at BMI 24.5
170cm, 70.8kg → BMI 24.5: "normal" by WHO, "overweight" by China. The right move: follow the Chinese standard (health-commission recommended), then measure the waist. At 88cm (male) the case is "overweight, near central obesity" — aim to cut 3-5kg; at 80cm with 20% body fat, risk is low — maintain.
Example: the muscular “overweight”
A lifter at 178cm/82kg returns BMI 25.9 — "overweight" by China cutoffs. But at 14% body fat and a 78cm waist, the excess is muscle. BMI grading simply does not apply; body-fat percentage and waist-to-height ratio (<0.5) are the right primary metrics. This is exactly why a good BMI tool must carry the limitation note and links to the linked metrics.
Common mistakes
- "Lower is always better": below 18.5 brings immunity and bone-density risks; slightly higher values can be protective for seniors.
- "One standard fits the world": Asian populations carry more visceral fat at a given BMI — using WHO cutoffs under-flags risk in China.
- "Normal BMI means done": roughly one in five normal-BMI adults has an oversized waist ("hidden obesity") with real metabolic risk.
- "Kids use adult cutoffs": minors need age-sex percentile curves; adult 24/28 cutoffs badly misjudge children.