Tape Test History: How the Circumference Method Estimates Body Fat
Estimate body fat with a tape measure. The method’s origins, how to measure and what its limits are.
BMI History: Quetelet’s Index and Why It Has Limits
Body mass index (BMI) is the most widely used single number for judging whether a person’s weight is in proportion to their height. It is quick, free and easy to calculate, which is why doctors, insurers, public-health agencies and researchers all use it. It was never designed to diagnose an individual, and understanding where it came from helps explain both its usefulness and its limits.
How to calculate BMI
BMI = weight (kg) ÷ height (m)². In imperial units, BMI = 703 × weight (lb) ÷ height (in)². A man who weighs 80 kg and is 1.78 m tall has a BMI of 80 ÷ 3.1684 = 25.2. The same person at 176 lb and 70 inches gives 703 × 176 ÷ 4,900 = 25.2, the same answer.
The World Health Organization’s adult categories are underweight (below 18.5), healthy range (18.5–24.9), overweight (25–29.9) and obesity (30 and above). For a person 1.78 m tall, the healthy range corresponds to about 58.6–78.9 kg.
Quetelet and the 1830s
The Belgian mathematician, astronomer and statistician Adolphe Quetelet studied human measurements as part of what he called “social physics”. In the 1830s and 1840s, drawing on data from large groups, he noticed that for adults of normal build weight increased roughly with the square of height. His “Quetelet index” was meant to describe the average person and to compare populations, not to assess individuals.
How it became “BMI”
In 1972 the American physiologist Ancel Keys and colleagues compared several height-and-weight indices in a large study and concluded that Quetelet’s index was the best of the simple ones as a proxy for body fatness across groups. Keys proposed calling it the body mass index, and the name stuck. Through the following decades health agencies adopted cut-offs, and BMI became embedded in guidelines for everything from drug dosing to insurance underwriting.
Where BMI falls short
- It cannot tell muscle from fat. A muscular athlete and a sedentary person can share a BMI.
- It ignores where fat sits. Fat stored around the abdomen carries more cardiometabolic risk than fat stored on the hips and thighs.
- Risk differs between populations. A 2004 WHO expert consultation concluded that Asian populations can have higher health risk at lower BMI values and suggested additional action points at 23 and 27.5.
- Age and height matter. BMI can overestimate fatness in very tall people and underestimate it in older adults who have lost muscle.
- It is not used the same way for children. Children and teenagers are assessed with BMI-for-age percentiles.
What is changing
In January 2025 a commission published in The Lancet Diabetes & Endocrinology proposed that clinicians should diagnose “clinical obesity” by confirming excess body fat with measures such as waist circumference, waist-to-height ratio or direct body-fat measurement, together with evidence that excess fat is affecting organ function or daily life. BMI remains a sensible first screen, but the direction of travel is clear: it is the start of an assessment, not the end of one.
Better companions to BMI
- Waist-to-height ratio. Keeping your waist under half your height is a simple rule of thumb.
- Body-fat estimates. Tape-measure methods, skinfold calipers or DEXA scans offer more detail.
- Blood pressure, blood sugar and cholesterol. These show metabolic health directly.
If your BMI is outside the healthy range, it is worth a conversation with a healthcare professional, but a single number never tells the whole story.
Frequently asked questions
Is BMI different for men and women?
The formula is the same for both. Body composition differs, so a clinician may use other measures alongside it.
Is BMI accurate for athletes?
Often not. Muscle weighs more than fat, so waist measurement or a body-fat estimate is usually more informative.
What is a healthy BMI?
For most adults 18.5–24.9 is considered the healthy range, although some groups have lower or higher thresholds.
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This article is for general information only and is not financial, medical or professional advice.