South Asian Threshold: Healthcare guidelines note that individuals of South Asian descent have a higher risk of diabetes and cardiovascular complications at lower BMI thresholds. Overweight indicators for South Asian backgrounds begin at a BMI of 23.0.
Mathematical Audit LogVerified against WHO standard and CDC adult calculations. Calculation precision conforms to standard clinical floating points.Last Verified: 2026-07-10
Mathematical Formulas
Standard BMI:
BMI = Weight (kg) / Height² (m²)
Nick Trefethen "New BMI":
New BMI = 1.3 × Weight (kg) / Height².⁵ (m².⁵)
Core Assumptions
Assumes an adult body frame (age 20 or older).
Assumes typical muscle-to-fat proportions.
Limitations & Exclusions
Does not distinguish between lean muscle and fat tissue. Ideal weights may vary widely for athletes.
Cannot be used for children or teens (which require percentile curves).
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About the BMI Calculator
Body Mass Index is a screening measure that relates a person's weight to their height, calculated as weight in kilograms divided by the square of height in metres. Devised in the nineteenth century by the Belgian statistician Adolphe Quetelet as a way of describing populations rather than diagnosing individuals, it was adopted by health organisations in the twentieth century because it is cheap, fast and requires nothing more than a scale and a tape measure. That practicality is the whole reason it endures. What BMI cannot do is measure body fat, distinguish muscle from adipose tissue, or say anything about where fat is distributed on the body — and since abdominal fat carries substantially different health implications from fat elsewhere, that last omission matters. BMI is best understood as a first-pass population screening tool that flags who might benefit from further assessment, not as a diagnosis or a verdict on any individual's health.
Mathematical Formula & Logic
BMI relates mass to the square of height.
1. Metric formula:
BMI = weight (kg) ÷ height (m)²
2. Imperial formula:
BMI = 703 × weight (lb) ÷ height (in)²
The 703 factor converts pounds-per-square-inch into kilograms-per-
square-metre, so both formulas produce the same number.
3. Standard WHO categories for adults:
Underweight BMI < 18.5
Normal weight 18.5 ≤ BMI < 25.0
Overweight 25.0 ≤ BMI < 30.0
Obesity class I 30.0 ≤ BMI < 35.0
Obesity class II 35.0 ≤ BMI < 40.0
Obesity class III BMI ≥ 40.0
4. Lower thresholds recommended for Asian populations:
Overweight from 23.0 and obesity from 27.5, reflecting evidence of
elevated metabolic risk at lower BMI values in these groups.
5. Healthy weight range for a given height:
Weight range = 18.5 × height² to 24.9 × height²
6. Units:
BMI is expressed in kg/m², though the unit is conventionally omitted.
Step-by-Step Example
Calculate BMI for a person of 1.75 m and 78 kg, then work out their healthy weight range:
Metric calculation
1. Height squared = 1.75 × 1.75 = 3.0625 m²
2. BMI = 78 ÷ 3.0625 = 25.47
3. This falls in the 25.0 to 29.9 band, categorised as overweight under the standard WHO thresholds.
The same person in imperial units
4. 1.75 m = 68.9 inches, and 78 kg = 171.96 lb
5. BMI = 703 × 171.96 ÷ (68.9)² = 120,888 ÷ 4,747.2 = 25.47
6. Identical result, confirming the 703 conversion factor.
Healthy weight range for this height
7. Lower bound = 18.5 × 3.0625 = 56.7 kg
8. Upper bound = 24.9 × 3.0625 = 76.3 kg
9. So a healthy-range weight at 1.75 m is roughly 56.7 kg to 76.3 kg, meaning this person sits about 1.7 kg above the upper bound.
An important caveat
10. If this individual is a trained athlete carrying substantial muscle mass, the same 25.47 could reflect a lean, highly conditioned body rather than excess fat. BMI cannot tell the difference, which is why waist circumference and body composition measures belong alongside it rather than after it.
Reference Data & Values
category
bmi range
weight at_1_75m
Underweight
Below 18.5
Below 56.7 kg
Normal weight
18.5 – 24.9
56.7 – 76.3 kg
Overweight
25.0 – 29.9
76.6 – 91.6 kg
Obesity class I
30.0 – 34.9
91.9 – 106.9 kg
Obesity class II
35.0 – 39.9
107.2 – 122.2 kg
Obesity class III
40.0 and above
122.5 kg and above
Frequently Asked Questions
No. BMI uses only height and weight, so it cannot distinguish fat from muscle, bone or water, and it says nothing about where fat sits on the body. A heavily muscled athlete and a sedentary person of identical height and weight receive identical scores despite very different body composition. Because muscle is denser than fat, well-trained individuals frequently register as overweight while carrying low body fat. BMI is a screening indicator for populations rather than a measurement of any individual's fatness, and it works best when read alongside waist circumference or a direct body composition assessment.
Research has consistently found that people of South Asian, East Asian and South-East Asian descent tend to develop elevated body fat percentage and increased risk of type 2 diabetes and cardiovascular disease at lower BMI values than European-descent populations, largely because of differences in body composition and fat distribution at a given BMI. A WHO expert consultation therefore recommended lower action points, commonly applied as overweight from 23.0 and obesity from 27.5. Some national health bodies have adopted these thresholds for their populations, so the category a given BMI falls into can depend on which guideline is being applied.
No. For children and adolescents, BMI is interpreted against age- and sex-specific percentile charts rather than fixed adult cut-offs, because healthy body composition changes substantially through growth, so a raw BMI number is meaningless without the corresponding percentile. In older adults the picture shifts again: height loss from spinal compression inflates BMI, while age-related muscle loss can leave the number unchanged even as body composition deteriorates. Some evidence suggests a slightly higher BMI may be protective in advanced age, so adult thresholds should be applied cautiously in the elderly.
Waist circumference is the most valuable companion measure, because it captures abdominal fat, which is more strongly associated with metabolic and cardiovascular risk than fat carried elsewhere. Waist-to-height ratio, with a commonly cited guideline of keeping waist below half of height, is simple and applies reasonably across body sizes. Body fat percentage from skinfold callipers, bioelectrical impedance or DEXA gives direct composition data that BMI cannot. Blood pressure, blood glucose and lipid profile describe actual metabolic health rather than inferring it from body dimensions.
Because human body mass does not scale with the cube of height the way geometric similarity would predict. If people simply grew proportionally in all three dimensions, mass would scale with height cubed, but empirical measurement of real populations showed that weight tracks more closely with height squared, since taller people are not proportionally wider and thicker. Quetelet chose the square because it produced the flattest relationship between the index and height across a population. This is also a known weakness: BMI still tends to slightly overestimate for very tall people and underestimate for very short ones.
No. BMI is a screening tool that indicates whether further assessment might be worthwhile, not a diagnosis and not a measure of health. Two people with the same BMI can differ enormously in fitness, metabolic health and disease risk, and the number reflects nothing about diet quality, physical activity, blood pressure, blood glucose or family history. Treat a result outside the normal range as a prompt for a conversation with a qualified healthcare professional who can consider your full clinical picture, rather than as a conclusion in itself.