健康・フィットネス

BMI計算機

体格指数と健康的な体重範囲を計算

使い方

Select your preferred unit system: メートル法 (kg, cm) or ヤード法 (lb, ft/in).
Enter your height and weight.
Enter your age and sex for a more accurate healthy weight range.
BMIが自動計算され、カテゴリーと健康体重範囲が表示されます。

Note: BMI is a screening tool, not a diagnostic tool. Consult a healthcare provider for a complete assessment.

計算式

BMI = weight(kg) / height²(m)

ヤード法：BMI = 703 × 体重(lb) / 身長²(in)

Underweight< 18.5

Normal weight18.5 – 24.9

Overweight25 – 29.9

Obese (Class I)30 – 34.9

Obese (Class II)35 – 39.9

Obese (Class III)≥ 40

BMIとは何ですか?なぜそれが重要なのでしょうか?

Body Mass Index (BMI) is a numerical value derived from your height and weight that serves as a widely adopted screening metric for weight-related health risks. Developed in the early 19th century by Belgian mathematician Adolphe Quetelet and later popularized in clinical medicine, BMI has become the global standard for classifying adults into weight categories—underweight, healthy weight, overweight, and obese. Today, it is endorsed by the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) as an affordable, non-invasive first-line assessment tool.

While BMI does not directly measure body fat percentage, research published in journals such as the International Journal of Obesity consistently shows that BMI is moderately correlated with more precise measures like dual-energy X-ray absorptiometry (DEXA) scans. For most adults, a BMI between 18.5 and 24.9 kg/m² is considered the healthy range associated with the lowest risk of weight-related disease.

BMIはどのように計算されますか?数式の背後にある科学

The BMI formula is elegantly simple: your body weight in kilograms divided by the square of your height in metres. In imperial units, the same result is achieved by multiplying 703 by your weight in pounds divided by height in inches squared. Despite its simplicity, the formula encapsulates a useful relationship between body size and health risk for population-level screening.

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Metric: BMI = weight (kg) ÷ height² (m)
Imperial: BMI = 703 × weight (lb) ÷ height² (in)

Beyond raw BMI, our calculator also computes BMIプライム—the ratio of your BMI to the upper limit of normal (25 kg/m²). A BMI Prime below 1.0 indicates you are within or below the healthy range, while values above 1.0 signal overweight or obesity. The Wikipedia article on BMI Prime provides an in-depth discussion of this metric and its cross-population utility. The calculator additionally shows the ポンデラル指数 (kg/m³), which scales more appropriately for very tall or very short individuals.

BMI カテゴリーの説明: 低体重から重度の肥満まで

The WHO classifies BMI into six distinct categories for adults aged 20 and older. Each bracket carries different health implications and recommended clinical actions:

< 18.5

低体重

栄養失調、ホルモンの不均衡、または摂食障害を示している可能性があります。医療提供者に相談してください。

18.5 – 24.9

健康的な体重

体重に関連する慢性疾患のリスクが最も低いことに関連しています。バランスの取れた食事と活動によって維持しましょう。

25 – 29.9

過体重

2 型糖尿病、高血圧、心血管疾患のリスクの上昇。生活習慣の改善をお勧めします。

30 – 34.9

肥満（クラスI）

リスクが高い。医学的評価と体系化された体重管理プログラムが推奨されます。

35 – 39.9

肥満（クラスII）

非常にリスクが高い。「重度の肥満」と呼ばれることが多いです。専門家によるケアを強くお勧めします。

≥ 40

肥満（クラスIII）

併存疾患の極度のリスク。以前は「病的肥満」と呼ばれていました。即時の臨床介入が重要です。

Children & Teens (ages 2–20): BMI is interpreted differently. The CDC uses age- and sex-specific percentiles rather than fixed cutoffs. Healthy weight falls between the 5th and 85th percentile; a BMI at or above the 95th percentile is classified as obese. See the CDC's Child & Teen BMI calculator for age-adjusted results.

異常なBMIによる健康リスク: 研究結果によると

⚠️ Risks of High BMI (Overweight & Obesity)

A BMI above 25 kg/m² has been linked in meta-analyses to a significantly elevated risk of developing serious chronic conditions. The Wikipedia overview of obesity health effects summarises decades of epidemiological evidence:

Type 2 diabetes — up to 7× higher lifetime risk
Coronary artery disease & stroke
Hypertension (high blood pressure)
Obstructive sleep apnea
Non-alcoholic fatty liver disease (NAFLD)
Certain cancers (breast, colon, endometrial, kidney)
Osteoarthritis of the knee and hip
Reduced psychological wellbeing & quality of life

❄️ 低BMI（低体重）のリスク

Being underweight (BMI < 18.5) carries its own set of serious health consequences, often overlooked compared to obesity-related discourse:

Malnutrition and micronutrient deficiencies
Anemia and impaired immune function
Osteoporosis and increased fracture risk
Delayed wound healing
Fertility issues and menstrual irregularities
Muscle wasting (sarcopenia)
Greater surgical complication rates
All-cause mortality risk comparable to Class I obesity

BMI の制限: この数値からは分からないこと

BMI is a population-level screening metric, not a personalised diagnostic tool. Understanding its limitations helps you put your result in the right context:

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Athletes & Highly Muscular Individuals — Muscle is denser than fat. A trained athlete may have a BMI in the "overweight" range while carrying very little body fat. Use our 体脂肪率計算機 for a more accurate picture.

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Older Adults — Ageing naturally shifts body composition toward higher fat and lower muscle mass, even at the same BMI. Seniors often need additional assessments such as waist circumference or grip strength tests.

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Ethnic Differences — Research shows that people of South, East, and Southeast Asian descent face elevated metabolic risk at lower BMI thresholds. The WHO suggests using a cutoff of 23 kg/m² for overweight and 27.5 kg/m² for obesity in these populations.

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Sex & Age Differences — Women tend to have higher body fat at the same BMI as men. Children and teens require age- and sex-specific percentile charts, not adult cutoffs.

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Pregnancy — BMI is not meaningful during pregnancy as weight gain is expected and healthy. Pre-pregnancy BMI may be used as a reference point for gestational weight gain recommendations.

For a deeper dive into the science and controversy surrounding BMI, the Wikipedia section on BMI limitations and this New England Journal of Medicine perspective on BMI and obesity offer balanced, evidence-based perspectives.

健康的なBMIを達成し、維持する方法

Whether your goal is to lower an elevated BMI or build mass from an underweight baseline, sustainable lifestyle changes consistently outperform short-term diets. Key evidence-based strategies include:

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バランスの取れた栄養

Focus on whole foods, adequate protein (1.6–2.2 g/kg body weight for muscle retention), and a moderate caloric deficit or surplus. Use our カロリー計算機 to find your target intake.

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定期的な身体活動

The WHO recommends at least 150–300 minutes of moderate-intensity aerobic activity per week, plus muscle-strengthening exercises on 2+ days. Check our 1日の総消費カロリー計算機 to plan your energy expenditure.

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質の高い睡眠

Chronic sleep deprivation elevates hunger hormones (ghrelin) and suppresses satiety signals (leptin), making weight management significantly harder. Aim for 7–9 hours per night.

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ストレス管理

Sustained high cortisol promotes fat accumulation—particularly visceral abdominal fat—independent of caloric intake. Mindfulness, yoga, and adequate social support all help.

Combining BMI tracking with complementary tools gives a fuller picture of your health trajectory. We recommend also monitoring your Basal Metabolic Rate (BMR), body fat percentage, and ideal weight range to set realistic, personalised goals.

BMIに関するよくある質問

BMI25は危険ですか？

A BMI of exactly 25 kg/m² sits at the boundary between "healthy weight" and "overweight" categories. By itself, this value is not dangerous, but it signals the point at which statistical health risk begins to increase. Context matters enormously: waist circumference, blood pressure, blood glucose, and cholesterol levels paint a more complete picture than BMI alone.

女性と男性の健康的な BMI はどれくらいですか?

The WHO applies the same BMI cutoffs (18.5–24.9 for healthy weight) to both sexes. However, at any given BMI, women typically carry 5–10% more body fat than men due to hormonal and reproductive factors. This is physiologically normal and does not inherently imply elevated health risk.

BMI が正常であっても不健康な状態になることはありますか?

Yes. The concept of "metabolically obese, normal weight" (MONW) describes individuals with a normal BMI who carry excess visceral fat and have metabolic risk factors such as insulin resistance, dyslipidemia, or hypertension. Conversely, some individuals with a BMI just above 25 are metabolically healthy. This underscores the importance of comprehensive health assessments beyond BMI alone.

どのくらいの頻度でBMIをチェックすればよいですか?

For most adults, checking BMI every 3–6 months is sufficient to track meaningful trends. Daily fluctuations in body weight (due to hydration, meals, clothing) can mask true changes, so tracking weekly averages is more informative than single readings.

BMIプライムとは何ですか?なぜそれが役立つのですか?

BMI Prime is your BMI divided by 25 (the upper limit of normal). A value of 1.00 means you are exactly at the boundary; values below 1.00 indicate healthy or underweight status, while values above 1.00 indicate overweight or obesity. BMI Prime is particularly useful for communicating deviation from the healthy range in a simple, percentage-based format.

Related Health Calculators

Tracking your BMI alongside other health metrics provides a comprehensive view of your overall wellbeing. Explore our suite of free health tools:

References & Further Reading

World Health Organization. Obesity and Overweight Fact Sheet. WHO, 2024.
Centers for Disease Control and Prevention. About Body Mass Index. CDC, 2024.
Wikipedia contributors. Body Mass Index. Wikipedia, The Free Encyclopedia.
Wikipedia contributors. Obesity — Health Effects and Epidemiology. Wikipedia, The Free Encyclopedia.
Flegal KM, et al. Association of All-Cause Mortality with Overweight and Obesity Using Standard BMI Categories. JAMA, 2013.
Nuttall FQ. Body Mass Index: Obesity, BMI, and Health — A Critical Review. Nutrition Today, 2015.
WHO Expert Consultation. Appropriate body-mass index for Asian populations. The Lancet, 2004.