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Health · 7 min read · Updated 2026
健康 · 阅读约 7 分钟 · 更新于 2026

How to Calculate BMI Accurately (and Why It Matters)

如何准确计算 BMI（以及它为什么重要）

Body Mass Index (BMI) is one of the simplest and most widely used screening tools for weight categories. It is not a diagnosis, but it is a quick, free, evidence-based way to check whether your weight falls in a healthy range. In this guide we walk through the formula, the categories, the common pitfalls, and when to use a more precise method.

身体质量指数（BMI）是最简单、应用最广的体重分类筛查工具之一。它不是诊断，但作为一个免费、循证的方法，可以快速判断你的体重是否在健康范围内。本指南将介绍公式、分类、常见误区，以及何时需要使用更精确的方法。

The BMI formula

BMI 公式

For metric units the formula is:

公制下的公式为：

BMI = weight (kg) ÷ height (m)²

For imperial units (pounds and inches) the formula uses a conversion factor of 703:

英制下（磅和英寸）使用 703 这个换算系数：

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

For example, a person who is 170 cm tall and weighs 65 kg has a BMI of 65 ÷ (1.7 × 1.7) ≈ 22.5, which falls in the "normal weight" category. You can compute this for yourself instantly with our BMI calculator.

例如，一个身高 170 厘米、体重 65 公斤的人，BMI 为 65 ÷ (1.7 × 1.7) ≈ 22.5，属于"正常"范围。你可以用我们的 BMI 计算器 即时计算自己的值。

The four BMI categories

四类 BMI 分级

The World Health Organization (WHO) defines four adult BMI categories. They apply to most adults, but the cutoffs are not universal — some Asian health authorities use lower thresholds (23 for overweight, 27.5 for obese) because metabolic risk rises at lower BMIs in those populations.

世界卫生组织（WHO）将成人 BMI 划分为四类。这一划分适用于大多数成年人，但并非全球通用 —— 部分亚洲国家采用更低的阈值（超重 23，肥胖 27.5），因为这些人群在较低 BMI 时代谢风险就已经升高。

  • Below 18.5 — Underweight
  • 低于 18.5 —— 偏瘦
  • 18.5 – 24.9 — Normal weight
  • 18.5 – 24.9 —— 正常
  • 25.0 – 29.9 — Overweight
  • 25.0 – 29.9 —— 超重
  • 30.0 and above — Obese (often further split into Class I, II, III)
  • 30.0 及以上 —— 肥胖（常进一步分为 I、II、III 级）

Where BMI gets it wrong

BMI 的局限

BMI is a ratio of weight to height squared, which is convenient but ignores two important variables: body composition and fat distribution. A trained athlete with significant muscle mass may register as "overweight" or even "obese" while having very low body fat. Conversely, a sedentary person with low muscle mass may have a "normal" BMI but high visceral fat — the kind that surrounds organs and is strongly linked to metabolic disease.

BMI 只用体重和身高的平方做比值，虽然方便，却忽略了两个关键变量：身体成分和脂肪分布。肌肉量大的运动员可能"超重"甚至"肥胖"，但体脂率极低。反过来，久坐少动、肌肉量低的人可能 BMI 正常，却有大量内脏脂肪 —— 这种围绕器官的脂肪与代谢疾病高度相关。

BMI also does not distinguish fat from muscle, water, or bone. A pregnant person, a child, an elderly person with sarcopenia, or someone with edema will get a misleading number. For these groups, BMI is not the right tool.

BMI 也无法区分脂肪、肌肉、水分和骨骼。孕妇、儿童、患有肌少症的老年人，或水肿患者都会得到误导性的数值。对这些人群，BMI 不是合适的工具。

When to use a more precise method

何时使用更精确的方法

If you need a number that reflects body fat, consider one of the following:

如果你需要一个真正反映体脂的数值，可以考虑以下方法之一：

  • Waist circumference — simple and predictive of metabolic risk. Above 102 cm (40 in) for men or 88 cm (35 in) for women signals elevated risk.
  • 腰围 —— 简单且能预测代谢风险。男性超过 102 厘米（40 英寸）、女性超过 88 厘米（35 英寸）即为风险升高。
  • Waist-to-height ratio — waist ÷ height. A value under 0.5 is generally considered healthy.
  • 腰高比 —— 腰围 ÷ 身高。低于 0.5 通常视为健康。
  • DEXA scan — the gold standard for body composition, available at hospitals and some clinics.
  • DEXA 扫描 —— 身体成分检测的金标准，可在医院或部分诊所进行。
  • Bioelectrical impedance — scales and handheld devices; less accurate but useful for tracking trends.
  • 生物电阻抗 —— 体脂秤或手持设备，精度较低，但适合追踪趋势。

Practical tips

实用建议

  1. Weigh yourself in the morning, after using the bathroom, before eating or drinking.
  2. 在早晨、如厕之后、吃喝之前称重。
  3. Measure height without shoes, ideally in the morning when you are slightly taller.
  4. 不穿鞋测量身高，最好在早晨（人会略高一些）。
  5. Track BMI over weeks and months, not days. Daily weight fluctuates due to water, food, and sodium.
  6. 以周和月为单位追踪 BMI，不要看日变化。日间体重因水分、食物和钠摄入而波动。
  7. If your BMI is outside the normal range, talk to a clinician. They can interpret it alongside your medical history, bloodwork, and waist measurement.
  8. 如果 BMI 不在正常范围内，请咨询医生，他们会结合病史、血液检查和腰围来综合判断。

Bottom line

总结

BMI is fast, free, and useful as a first-pass screen. Use it to identify a potential issue, then validate with a more specific measurement if needed. The BMI calculator on ToolHub runs entirely in your browser — your measurements never leave your device.

BMI 快速、免费，是很好的初筛工具。先用它来发现潜在问题，再用更精确的指标验证。ToolHub 上的 BMI 计算器 完全在浏览器中运行 —— 你的数据不会离开设备。