BMI 計算機

What BMI is

Body Mass Index is a single number that compresses your weight and height into a position on a population-level statistical curve. It was first proposed by Belgian astronomer and statistician Adolphe Quetelet in 1832 and was originally called the Quetelet Index. The current “BMI” name was popularized by Ancel Keys in 1972 as a simple, cheap proxy for body fatness across large populations.

The math is almost embarrassingly simple:

Metric:    BMI = weight(kg) / [height(m)]²
Imperial:  BMI = 703 × weight(lb) / [height(in)]²

That’s it. There are no coefficients tuned per person, no sex factor, no age correction. That simplicity is both BMI’s strength — it works in a clinic with no equipment beyond a scale and a wall — and its weakness, which we get to below.

The WHO category cutoffs

The World Health Organization (WHO) groups BMI into five bands for adults aged 18 and over:

BMI	Category	Population-level note
< 18.5	Underweight	Higher risk of nutritional deficiency, osteoporosis, fertility issues
18.5 – 24.9	Healthy	Lowest mortality risk in most large epidemiological studies
25.0 – 29.9	Overweight	Modestly elevated cardiometabolic risk on average
30.0 – 34.9	Obese (Class I)	Clearly elevated risk for type 2 diabetes, hypertension, sleep apnea
35.0 – 39.9	Obese (Class II)	Substantially elevated risk
≥ 40.0	Obese (Class III)	“Severe obesity”; significant excess mortality across most studies

These are population-level associations, not personal verdicts. Two people at BMI 27 can have vastly different metabolic profiles depending on body composition, fat distribution, and fitness level.

A worked example

A 75-kg adult, 170 cm tall:

• Metric: 75 / 1.70² = 75 / 2.89 = BMI 25.95 → Overweight (just barely)
• The healthy range (18.5 – 24.9) at that height: 53.5 kg – 71.9 kg

A 180-cm adult who weighs 100 kg:

• BMI = 100 / 1.80² = 100 / 3.24 = BMI 30.86 → Obese Class I
• The healthy range at 180 cm: 59.9 kg – 80.6 kg

If those numbers feel surprising, you’re not alone. BMI assumes an average body composition. A 100 kg lifter at 180 cm with 18% body fat carries 82 kg of lean mass and 18 kg of fat — which is a markedly different body than 100 kg at 36% body fat (64 kg lean, 36 kg fat). BMI cannot tell those apart.

How to use this calculator

1. Toggle to metric or imperial.
2. Enter your height and current weight.
3. Read your BMI, your WHO category, and the healthy weight range for your height.
4. If you’re a strength athlete, also run the body fat calculator — BMI alone will likely flag you as overweight when you’re not.

Where BMI fails

BMI is a population statistic dressed up as personal feedback. It systematically misclassifies several groups:

• Strength athletes and bodybuilders. Higher lean mass inflates BMI without adding fat. Many in-shape lifters score BMI 27–30.
• Endurance athletes. Often healthy at low BMI; the underweight cutoff overflags them.
• Older adults (65+). Lean-mass loss with age means a BMI of 25–28 is often associated with better survival in this group than a “healthy” BMI of 22 — the so-called “obesity paradox.”
• Very tall and very short adults. The squared-height assumption breaks at the population extremes; tall people get inflated BMIs and very short people get deflated ones.
• Pregnant and postpartum women. Use pregnancy-specific weight-gain charts, not BMI.

For a single-number alternative that fixes most of BMI’s blind spots, waist-to-height ratio (your waist circumference divided by your height) is increasingly favored: a ratio above 0.5 flags central adiposity regardless of muscle mass. We don’t show that in this calculator, but it’s worth measuring with a tape measure once a quarter.

What to do with the result

• You’re in the healthy range. Great — BMI agrees with the simple metric. Pair it with a body-composition check every few months and move on.
• You’re overweight by BMI. If you lift hard and look lean, your BMI is reflecting lean mass; check body fat. If you don’t lift and the number surprises you, treat it as a soft prompt to look at trend data over the next 3–6 months.
• You’re obese by BMI. Talk to a clinician. BMI by itself is not a diagnosis, but at this level it’s reliable enough as a screening tool to warrant an honest conversation with a healthcare professional.
• You’re underweight by BMI. Look at intake (chronically under-eating?), training (running too much, lifting too little?), and bloodwork. Underweight at any age is rarely benign.

BMI in one sentence

A useful, free, instant population-level screening number — and a terrible verdict about any individual lifter.