How to Calculate BMI - Formula, Steps & Interpretation

Your doctor says your BMI is 27. You have no idea if that's good, bad, or meaningless. The number sits there like a verdict, but nobody explained the formula, the cutoffs, or why the same BMI means something different for an Indian body versus a European one.

Here is the math, the meaning, and the limitations — all in one place.

The BMI Formula

BMI = Weight (kg) ÷ Height (m)²

That's it. Divide your weight in kilograms by the square of your height in meters.

Step-by-Step Calculation

Example: 70 kg, 170 cm (1.70 m)

1. Convert height to meters: 170 cm = 1.70 m
2. Square the height: 1.70 × 1.70 = 2.89
3. Divide weight by height²: 70 ÷ 2.89 = 24.2

BMI = 24.2

Imperial Formula (if you use pounds and inches)

BMI = (Weight in lbs × 703) ÷ (Height in inches)²

Example: 154 lbs, 5'7" (67 inches)

1. 154 × 703 = 108,262
2. 67 × 67 = 4,489
3. 108,262 ÷ 4,489 = 24.1

Use our BMI Calculator to skip the manual math — it handles both metric and imperial units.

What Your BMI Means: The Categories

International (WHO) Classification

| BMI | Category | |-----|----------| | < 18.5 | Underweight | | 18.5 – 24.9 | Normal weight | | 25.0 – 29.9 | Overweight | | ≥ 30.0 | Obese |

Indian-Specific Cutoffs (ICMR/WHO Asia-Pacific)

Indian bodies carry more fat at lower BMIs than European bodies. The Indian Council of Medical Research uses lower cutoffs:

| BMI | Category (Indian) | |-----|-------------------| | < 18.5 | Underweight | | 18.5 – 22.9 | Normal | | 23.0 – 27.9 | Overweight | | ≥ 28.0 | Obese |

Same person, different verdict: A BMI of 24.2 is "normal" by WHO standards but overweight by Indian standards. This isn't a contradiction — Indian genetics store more visceral fat (belly fat) at the same BMI, which is metabolically more dangerous.

The Trench Truth: If you're Indian and your doctor uses WHO cutoffs (25+ = overweight), they're using the wrong standard. By the time your BMI hits 25, you may already have insulin resistance. The Indian cutoff of 23 exists because metabolic risk starts earlier in South Asian populations. Always ask which standard your doctor is using.

BMI by Age and Gender

BMI interpretation changes with age:

Children and Teens (Age 2-20)

Children use BMI percentiles instead of fixed categories because body composition changes dramatically during growth.

| Percentile | Category | |-----------|----------| | < 5th | Underweight | | 5th – 84th | Normal | | 85th – 94th | Overweight | | ≥ 95th | Obese |

A 10-year-old boy with BMI 20 is at the 95th percentile (obese). A 16-year-old boy with the same BMI 20 is at the 50th percentile (normal). Same number, different meaning.

Older Adults (65+)

For adults over 65, the "healthy" BMI range shifts upward to 23-28. A BMI of 27 in a 70-year-old is associated with better survival outcomes than a BMI of 22. Low body weight in elderly patients is a stronger mortality predictor than moderate overweight.

When BMI Is Misleading

BMI has well-documented limitations:

1. Doesn't Distinguish Fat from Muscle

A bodybuilder at 180 cm, 90 kg has BMI = 27.8 (overweight by WHO, obese by Indian standards). But their body fat might be 12% — extremely lean. BMI can't tell the difference between 90 kg of muscle and 90 kg of fat.

2. Doesn't Measure Fat Distribution

Two people with BMI 26 can have very different health risks:

• Belly fat (visceral): High metabolic risk, insulin resistance, heart disease
• Hip/thigh fat (subcutaneous): Lower metabolic risk

Waist circumference is a better predictor: >90 cm for Indian men, >80 cm for Indian women = high risk, regardless of BMI.

3. Doesn't Account for Ethnicity

The WHO cutoffs were derived primarily from European populations. South Asians, East Asians, and Pacific Islanders all have different fat-to-muscle ratios at the same BMI.

Better Alternatives to BMI

| Measure | What It Measures | Indian Cutoff (Men/Women) | |---------|-----------------|--------------------------| | BMI | Weight vs height | 23 / 23 | | Waist circumference | Belly fat | 90 cm / 80 cm | | Waist-to-hip ratio | Fat distribution | 0.90 / 0.80 | | Body fat % | Actual fat mass | 20-25% / 25-31% |

For the most complete picture, combine BMI with waist circumference. A normal BMI + large waist = "normal weight obesity" — a real and dangerous condition.

Use our BMI Calculator for instant results with Indian-specific categories.

Frequently Asked Questions