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HOW TO

How to Calculate Your BMI

Calculate your BMI step by step — metric and imperial formulas, what your result means, BMI categories with chart, and check your result with a free BMI calculator.

Updated 2026-06-26

Free calculators used in this guide

BMI CalculatorIdeal Weight CalculatorBody Fat CalculatorTDEE Calculator

BMI — Body Mass Index — is a number calculated from your height and weight that indicates whether you fall in an underweight, normal, overweight, or obese range. It is one of the most widely used screening tools in clinical and public health settings because it requires only two measurements, no equipment, and no blood tests. It is not a diagnostic tool: it cannot tell you how much body fat you have, where that fat sits, or whether you have any specific disease. But as a first filter, it is fast, free, and reasonably informative.

This article walks you through the exact calculation steps in both metric and imperial units, explains what your result means, and flags the situations where you should look beyond BMI.

What You Need

  • Your weight in kilograms (kg) or pounds (lbs)
  • Your height in centimetres (cm), metres (m), or feet and inches

A standard bathroom scale and a wall-mounted measuring tape are enough. You do not need a doctor's appointment to get these numbers. If you find manual calculation error-prone, the BMI Calculator does the arithmetic instantly.

Step 1: Measure and Convert Your Height (Metric)

The metric formula requires height in metres, not centimetres. Most people measure their height in centimetres, so divide by 100.

Height (cm) Height (m)
150 cm 1.50 m
160 cm 1.60 m
170 cm 1.70 m
175 cm 1.75 m
180 cm 1.80 m

Measure height without shoes, standing straight against a wall, with heels together and eyes level.

Step 2: Apply the Metric Formula

BMI = weight (kg) ÷ height (m)²

Square your height in metres first, then divide your weight by that number.

Worked example:

  • Weight: 75 kg
  • Height: 175 cm = 1.75 m
  • Height squared: 1.75 × 1.75 = 3.0625
  • BMI = 75 ÷ 3.0625 = 24.5

A BMI of 24.5 falls in the Normal weight category.

Common mistake: using 175 in the formula instead of 1.75. Dividing 75 by 175² gives 0.00245 — an obviously wrong result. Always convert centimetres to metres before squaring.

Step 3: Convert Your Height to Inches (Imperial)

If you measure in feet and inches, convert to total inches first.

Formula: total inches = (feet × 12) + remaining inches

Height Total Inches
5 ft 0 in 60 in
5 ft 4 in 64 in
5 ft 7 in 67 in
5 ft 9 in 69 in
6 ft 0 in 72 in

Step 4: Apply the Imperial Formula

BMI = (weight (lbs) ÷ height (inches)²) × 703

The factor 703 converts the result from imperial units to the same BMI scale as the metric formula.

Worked example:

  • Weight: 165 lbs
  • Height: 5 ft 9 in = 69 inches
  • Height squared: 69 × 69 = 4,761
  • 165 ÷ 4,761 = 0.03466
  • 0.03466 × 703 = 24.4

The result matches the metric formula closely for the same person (rounding differences are normal).

Step 5: Read Your BMI Category

Global (WHO) BMI Categories

BMI Range Category
Below 18.5 Underweight
18.5 – 24.9 Normal weight
25.0 – 29.9 Overweight
30.0 – 34.9 Obese — Class I
35.0 – 39.9 Obese — Class II
40.0 and above Obese — Class III

Adjusted Cutoffs for South Asians (Including India)

Research published in the Lancet and adopted by India's National Institute of Nutrition shows that South Asians develop metabolic complications — insulin resistance, dyslipidaemia, hypertension — at lower BMI values than Western populations. The adjusted thresholds recommended by multiple Indian health bodies are:

BMI Range Category (South Asian)
Below 18.5 Underweight
18.5 – 22.9 Normal weight
23.0 – 27.4 Overweight
27.5 and above Obese

If you are of South Asian descent, use these adjusted values when interpreting your result. The BMI Calculator notes this distinction.

Step 6: Verify and Explore Related Metrics

Once you have your BMI:

  1. Cross-check with the BMI Calculator — enter your measurements to confirm your manual calculation and see your category in context.
  2. Find your healthy weight range — the Ideal Weight Calculator uses multiple formulas (Devine, Robinson, Miller, Hamwi) to suggest a target weight range for your height and sex.
  3. Estimate your body fat — the Body Fat Calculator uses waist, hip, and neck circumference to estimate fat percentage, giving a more complete picture than BMI alone.
  4. Calculate calorie needs — if your BMI suggests you should lose or gain weight, the TDEE Calculator shows your Total Daily Energy Expenditure at your current activity level, which you can adjust to plan a calorie deficit or surplus.

BMI by Weight at 170 cm — Quick Reference Table

Weight (kg) BMI Category (Global) Category (South Asian)
50 kg 17.3 Underweight Underweight
55 kg 19.0 Normal weight Normal weight
65 kg 22.5 Normal weight Normal weight
67 kg 23.2 Normal weight Overweight
73 kg 25.3 Overweight Overweight
80 kg 27.7 Overweight Obese
87 kg 30.1 Obese Class I Obese
100 kg 34.6 Obese Class I Obese
115 kg 39.8 Obese Class II Obese

BMI Limitations

BMI is a useful screening tool but has well-documented blind spots. Knowing these helps you interpret your result correctly.

It Does Not Distinguish Muscle from Fat

Weight is weight to the BMI formula. A competitive rugby player weighing 95 kg at 178 cm has a BMI of 30 — technically obese — but may carry only 10–12% body fat. Conversely, a sedentary person of the same weight and height has the same BMI but potentially 28–30% body fat. The health implications are entirely different. Use the Body Fat Calculator if you have above-average muscle mass.

It Misses Sarcopenic Obesity

Sarcopenic obesity occurs when muscle mass has declined (sarcopenia) and fat mass has increased, but total weight — and therefore BMI — remains in the normal range. This is particularly common in adults over 60. The person appears "normal weight" on a scale but has metabolically harmful excess fat and insufficient muscle for functional health. BMI will not detect this condition.

More Accurate Alternatives

  • Body fat percentage: the Body Fat Calculator uses circumference measurements to estimate fat versus lean mass — far more informative than BMI for body composition.
  • Waist-to-height ratio: divide your waist circumference by your height (both in the same unit). A ratio above 0.5 signals abdominal obesity risk regardless of BMI. For example, at 170 cm height, a waist above 85 cm is a warning sign.
  • Waist circumference alone: above 80 cm for women and 90 cm for men (Asian cutoffs) indicates elevated cardiometabolic risk.

BMI for Children

Children and teenagers (ages 2–19) should not be assessed using adult BMI cutoffs. The same BMI value means different things at different ages and sexes during growth and development.

For children, BMI is calculated using the same formula but then plotted on age- and sex-specific growth charts to produce a percentile:

Percentile Weight Status
Below 5th Underweight
5th to 84th Healthy weight
85th to 94th Overweight
95th and above Obese

A 12-year-old boy with a BMI of 22 may be perfectly normal for his age and size — or may be overweight — depending on how he compares to other boys his age. Never use the adult table (18.5, 25, 30) for children. Always consult a paediatrician for children's weight assessment.

Common Mistakes to Avoid

Not converting centimetres to metres. In the metric formula, height must be in metres. Using 175 instead of 1.75 produces a nonsensical result. Divide your height in cm by 100 before squaring.

Treating BMI as a diagnosis. BMI is a screening number. A high BMI does not mean you have a disease; it means further assessment may be warranted. Only a clinician can diagnose obesity-related conditions.

Using adult BMI for children. See the section above — percentile charts are required for anyone under 19.

Ignoring population-specific cutoffs. If you are South Asian, the standard 25/30 thresholds overstate your healthy BMI range. Apply the 23/27.5 thresholds instead.

Weighing yourself at different times of day. Body weight can vary by 1–3 kg across a single day depending on hydration, meals, and digestion. Weigh yourself first thing in the morning, after using the toilet, without clothes, for the most consistent reading.

Key Terms

  • BMI — Body Mass Index: weight (kg) divided by height in metres squared; a population-level screening number for weight status.
  • Body Fat Percentage: the proportion of total body weight that is fat tissue, as opposed to muscle, bone, water, and organs.
  • Obesity: a chronic condition defined by excess body fat that impairs health; BMI ≥ 30 globally, ≥ 27.5 for South Asians.
  • BMR — Basal Metabolic Rate: the number of calories your body burns at rest to maintain basic physiological functions.
  • Sarcopenic Obesity: a condition combining low muscle mass and excess fat, often with a normal BMI, common in older adults.

Frequently Asked Questions

Standard BMI cutoffs were developed from Western populations and underestimate obesity risk in South Asians. Indian and other South Asian bodies accumulate visceral (abdominal) fat at lower BMI values, increasing the risk of type 2 diabetes, cardiovascular disease, and metabolic syndrome. Adjusted cutoffs recommended for South Asians classify overweight as BMI ≥ 23 and obese as BMI ≥ 27.5, compared to the global thresholds of 25 and 30. Use the [BMI Calculator](/bmi-calculator/) and discuss your result with a doctor who is aware of these population-specific guidelines.
A BMI of 28 falls in the overweight range (25–29.9 globally; obese by South Asian standards at ≥ 27.5), and reducing it is associated with meaningful health benefits. A 5–10% reduction in body weight can improve blood pressure, blood sugar, and cholesterol levels. Use the [TDEE Calculator](/tdee-calculator/) to find your daily calorie needs, then create a modest deficit of 300–500 kcal per day, which produces roughly 0.3–0.5 kg of fat loss per week. Check the [Ideal Weight Calculator](/ideal-weight-calculator/) to set a realistic target weight for your height.
BMI does not distinguish between muscle mass and fat mass — it only uses total weight relative to height. A professional athlete or bodybuilder can have a BMI of 27–30, placing them in the overweight or obese range, despite having very low body fat. For athletes, body fat percentage is a far more meaningful metric; use the [Body Fat Calculator](/body-fat-calculator/) which uses measurements like waist, hip, and neck circumference to estimate fat versus lean mass. If you have above-average muscle mass, treat your BMI result with caution and prioritise direct body composition measurements.
Children and adolescents (ages 2–19) are not assessed using the adult BMI categories. Instead, a child's BMI is plotted on age- and sex-specific growth charts, and the result is expressed as a percentile relative to other children of the same age and sex. Underweight is below the 5th percentile, healthy weight is 5th–84th, overweight is 85th–94th, and obese is at or above the 95th percentile. Never apply the adult thresholds (18.5, 25, 30) to children — always consult a paediatrician who uses validated paediatric growth references.
Waist circumference and waist-to-height ratio capture central (abdominal) fat distribution, which BMI does not. Central obesity is a stronger independent predictor of metabolic disease than overall BMI. A simple rule: your waist circumference should be less than half your height. For example, if you are 170 cm tall, a waist above 85 cm signals elevated risk regardless of your BMI. Using both BMI and waist measurement together gives a more complete picture of health risk than either measure alone.
This is called normal-weight obesity or, in older adults, sarcopenic obesity — a condition where the body has low muscle mass but excess fat, resulting in a normal BMI. It is more common in women and in older adults who have lost muscle without gaining weight. Standard BMI will miss this entirely. The [Body Fat Calculator](/body-fat-calculator/) can flag this situation; a body fat percentage above 25% in men or above 32% in women is generally considered unhealthy, even at a normal BMI.
A DEXA (Dual-Energy X-ray Absorptiometry) scan directly measures bone, muscle, and fat compartments to within a few percentage points of true body composition, and is considered the clinical gold standard. BMI is a population-level screening tool — it correctly identifies weight status at the group level but has significant individual error, particularly for muscular individuals, the elderly, and certain ethnic groups. The [BMI Calculator](/bmi-calculator/) gives you a result in seconds at zero cost; a DEXA scan costs ₹3,000–₹8,000 and requires a radiology appointment. Use BMI for initial screening and DEXA (or at minimum, skinfold callipers with a trained assessor) when precision matters.
The standard WHO BMI cutoffs (18.5–24.9 for normal weight) are the same for adult men and women, but healthy body fat percentages differ significantly by sex. Women naturally carry more essential fat (around 10–13%) due to hormonal and reproductive physiology, so a woman and a man with the same BMI will typically have different body fat percentages — the woman's being higher. This is physiologically normal and does not imply greater health risk. The [Ideal Weight Calculator](/ideal-weight-calculator/) accounts for sex when suggesting a healthy weight range.
The BMI formula itself does not change with age, but the relationship between BMI and health risk does. In older adults (65+), research suggests that a BMI in the 25–27 range may actually be associated with lower mortality than the 18.5–24.9 "normal" range — a phenomenon sometimes called the "obesity paradox." Conversely, body fat percentage tends to increase with age even when weight stays stable, because muscle mass declines (sarcopenia). For older adults, waist circumference and muscle strength are often more clinically relevant than BMI alone.
Use the imperial formula: BMI = (weight in lbs ÷ height in inches²) × 703. First, convert your total height to inches — 5 feet 9 inches is 69 inches (5 × 12 = 60, plus 9 = 69). Then square that: 69² = 4,761. Divide your weight by that number and multiply by 703. For example, 165 lbs at 5 ft 9 in: (165 ÷ 4,761) × 703 = 0.03466 × 703 = 24.4 — Normal weight. The [BMI Calculator](/bmi-calculator/) handles the conversion automatically if you prefer to enter feet, inches, and pounds directly.
No. High BMI can reflect high muscle mass, high bone density, or excess body fat — the formula cannot tell the difference. A 90 kg competitive weightlifter at 175 cm has a BMI of 29.4 (overweight), but may have 12% body fat. Conversely, a sedentary person at 70 kg and 175 cm has a BMI of 22.9 (normal), but might have 30% body fat. Use the [Body Fat Calculator](/body-fat-calculator/) alongside BMI for a more meaningful assessment of body composition.
By global WHO standards, obesity begins at a BMI of 30. It is divided into three classes — Class I is 30–34.9, Class II is 35–39.9, and Class III (sometimes called severe or morbid obesity) is 40 or above. For South Asian populations including Indians, the threshold is lower: a BMI of 27.5 and above is classified as obese due to greater metabolic risk at lower BMI values. Obesity increases the risk of type 2 diabetes, hypertension, heart disease, sleep apnoea, and certain cancers, making early identification with the [BMI Calculator](/bmi-calculator/) a useful first step before consulting a doctor.

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