A BMI of 24 is comfortably "normal weight" on the chart most Western health sites publish. Look up the same number on the BMI guidelines used by health ministries in India, China, Japan, or Singapore, and 24 is already overweight. Nobody's height or weight changed — the chart did. Here's the actual formula, why two "standard" charts disagree, and the bigger limitation that applies no matter which chart you use.
The Formula (Metric and Imperial Are Genuinely Different Equations)
Metric:
BMI = weight (kg) / [height (m)]²
Imperial:
BMI = 703 × weight (lb) / [height (in)]²
These aren't the same formula with unit conversion baked in loosely — the 703 constant exists specifically to correct for inches-and-pounds so the imperial version lands on the same BMI value as the metric one for the same person. Plugging pounds and inches into the metric formula (or kg/m² math into the imperial one) without converting units first is the single most common manual-calculation error, and it produces a wildly wrong number rather than a slightly-off one.
Worked Example
A person weighing 70 kg at 1.75 m tall:
BMI = 70 / (1.75)² = 70 / 3.0625 ≈ 22.9
The same person in imperial units — 154 lb at 68.9 inches:
BMI = 703 × 154 / (68.9)² = 108,262 / 4,747.21 ≈ 22.8
Both land at roughly the same BMI (small gap is rounding from the imperial conversion), confirming the two formulas measure the same thing — just be sure to keep weight and height in matching unit pairs, never mixed.
Two Different "Normal" Ranges
WHO / standard chart (most common globally, used across most of the West):
| Category | BMI Range |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese | 30.0 and above |
Asian BMI cutoffs (used by health authorities in India, China, Japan, Singapore, and recommended by the WHO's own Asian-specific guidance):
| Category | BMI Range |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 22.9 |
| Overweight | 23.0 – 27.4 |
| Obese | 27.5 and above |
The reason for the lower cutoffs: population studies found that people of Asian descent tend to develop weight-related health risks (like type 2 diabetes and cardiovascular issues) at lower BMI values than the original WHO chart assumed, because body fat percentage at a given BMI tends to run higher in Asian populations compared to the European populations the original chart was calibrated on. A BMI of 24 sitting just inside "normal" on the standard chart is already "overweight" on the Asian chart — same number, different risk interpretation, because the chart itself was recalibrated for a different population's risk curve.
Where BMI Breaks Down Regardless of Chart
BMI only uses weight and height — it has no way to distinguish muscle from fat. A muscular athlete can land in the "overweight" or even "obese" range on either chart while carrying very low body fat, because muscle is denser than fat and adds weight without adding the health risk BMI is trying to estimate. The reverse also happens: someone with "normal" BMI but low muscle mass and high body fat (sometimes called "skinny fat") can carry more metabolic risk than their BMI number suggests.
This is why BMI is used as a population-level screening tool, not an individual diagnostic — it's cheap and fast to calculate for large groups, but a single person's BMI number should be read alongside waist circumference, body fat percentage, or a doctor's assessment, not treated as a standalone verdict.
Quick FAQ
Does BMI account for age or sex? No — the standard adult BMI formula and both charts above apply the same cutoffs regardless of age or sex, which is itself a limitation, since body composition norms differ between men and women and change with age. Children and teens use a completely different age-and-sex-adjusted percentile system, not these adult cutoffs.
Which chart should I use — WHO standard or Asian cutoffs? Use whichever one your doctor or local health authority references, since that's the chart your risk assessment and any related guidance will be based on. If you're comparing your BMI to global health content by default, check whether it's using the standard or Asian chart before assuming the category applies to you.
Is a "normal" BMI enough to confirm good health? No — BMI doesn't measure body fat percentage, muscle mass, fat distribution, or metabolic markers like blood sugar and cholesterol, all of which affect health risk independently of BMI.
Why do some BMI calculators give slightly different results for the same inputs? Almost always rounding — some tools round height/weight inputs or intermediate values before the final calculation, producing a BMI that differs by a decimal point or two from a calculator that carries full precision through.
Calculate Yours
Rather than converting units and squaring height by hand, the BMI Calculator computes it instantly in either metric or imperial units and shows the health category alongside the number.
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