BMI and Health Metrics Guide

"Is a BMI of 23 normal?" "How many calories should I eat per day?"

We live in an age where health is measured by numbers. But without understanding what these numbers mean, they can be misleading.

This guide explains how to calculate health metrics and interpret them correctly.

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BMI (Body Mass Index)

Formula

BMI = Weight(kg) / Height(m)²

Example

Height 170cm, Weight 68kg

BMI = 68 / (1.70)²
    = 68 / 2.89
    = 23.5

WHO Standards

Asian Standards (WHO Western Pacific)

Asians have higher metabolic disease risk at the same BMI, hence different standards.

Limitations of BMI

BMI cannot distinguish muscle mass.

Athlete A: Height 180cm, Weight 90kg, muscular
→ BMI = 27.8 (overweight)

Person B: Height 180cm, Weight 90kg, sedentary
→ BMI = 27.8 (overweight)

Same BMI but completely different health status

BMI is just a reference indicator; it should be considered alongside body fat percentage.

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Body Fat Percentage

Estimation Formula (US Navy Method)

Men:

Body Fat % = 495 / (1.0324 - 0.19077 × log(waist - neck) + 0.15456 × log(height)) - 450

Women:

Body Fat % = 495 / (1.29579 - 0.35004 × log(waist + hip - neck) + 0.22100 × log(height)) - 450

Normal Ranges

Measurement Methods

Use formula calculations for reference only.

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Basal Metabolic Rate (BMR)

BMR: Minimum energy needed for survival (breathing, body temperature, etc.)

Mifflin-St Jeor Formula (Recommended)

Men:

BMR = (10 × weight kg) + (6.25 × height cm) - (5 × age) + 5

Women:

BMR = (10 × weight kg) + (6.25 × height cm) - (5 × age) - 161

Example

Male, 30 years old, 175cm tall, 70kg

BMR = (10 × 70) + (6.25 × 175) - (5 × 30) + 5
    = 700 + 1093.75 - 150 + 5
    = 1,648 kcal/day

This person burns 1,648 kcal per day even lying in bed all day.

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Total Daily Energy Expenditure (TDEE)

TDEE = BMR × Activity Factor

Activity Factors

Example

BMR = 1,648 kcal
Activity level: Moderate (exercise 3-5 times/week)

TDEE = 1,648 × 1.55 = 2,554 kcal/day

Calories by Goal

Person with TDEE of 2,554 kcal wants to diet?
→ 2,554 - 500 = 2,054 kcal/day target

Caution: Below 1,200 kcal (women) / 1,500 kcal (men) is unhealthy.

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Ideal Weight Calculation

Lorentz Formula

Men:

Ideal weight = (height cm - 100) - (height cm - 150) / 4

Women:

Ideal weight = (height cm - 100) - (height cm - 150) / 2

Example

Male, 175cm

Ideal weight = (175 - 100) - (175 - 150) / 4
             = 75 - 6.25
             = 68.75kg

BMI-based Ideal Weight

Ideal weight = 22 × (height m)²

Height 175cm

Ideal weight = 22 × 1.75²
             = 22 × 3.0625
             = 67.4kg

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Important Considerations

1. Don't Obsess Over Numbers

The health difference between BMI 24.9 and 25.0 is negligible. Don't be too sensitive about boundary values.

2. Consider Individual Differences

• High muscle mass can mean healthy despite high BMI
• Same body fat percentage can have different risks based on distribution
• Standards vary by age, gender, and ethnicity

3. Watch the Trends

Single measurement: Insufficient information
Weekly measurement: Can track trends
Daily measurement: Causes stress

Measure every 1-2 weeks under the same conditions and watch the trends.

4. Make Comprehensive Judgments

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FAQ

Q: My BMI is normal but I have belly fat

A: This is abdominal obesity (skinny fat). Check your waist circumference.

• Men: Concern at 90cm or more
• Women: Concern at 85cm or more

Q: Different scales show different body fat percentages

A: Bioelectrical impedance (BIA) method is affected by hydration levels. Measure at the same time (e.g., morning before eating) on the same device.

Q: I gained weight after starting exercise

A: Initial weight gain can occur from muscle growth and water retention. Focus on body fat percentage and waist circumference changes rather than weight.

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Summary

Key point: Don't rely on a single number; make judgments based on multiple metrics combined.

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