What does this BMI calculator actually work out?

It divides your weight by the square of your height, and nothing else. With Metric selected, the script reads height in centimeters and weight in kilograms and computes weight ÷ height ÷ height × 10,000. With Standard selected, it reads height in decimal feet, multiplies by 12 to get inches, then computes weight in pounds ÷ height ÷ height × 703. The answer is rounded to a whole number before it is shown.

That is the whole calculation. The form has four controls — unit, height, weight, Calculate — and it never asks for your age, sex, waist or activity. Check the unit buttons before you type: the page loads with Standard selected, so centimeters and kilograms entered as they stand come back as a BMI of 0.

How do you read a BMI number?

The World Health Organization defines BMI as weight (kg)/height² (m²) and sets overweight at a BMI of 25 or above and obesity at 30 or above — the same lines as the classification table this page shows with the result. The NHS lists the same figures for the general population, but adds that people from a South Asian, Chinese, other Asian, Middle Eastern, Black African or African–Caribbean background carry the same risk at a lower number: 23 to 27.4, and 27.5 or higher.

Those population differences are not a rounding argument. A 2004 WHO expert consultation published in The Lancet found that across Asian populations the BMI at which risk was observed ranged from 22 to 25, and the group proposed 23.0, 27.5, 32.5 and 37.5 as public-health action points rather than redefining the cut-offs country by country.

What a BMI number does not tell you

It cannot separate muscle from fat. The NHS says this outright: a very muscular person can land in the overweight or obese band without carrying much body fat. Two people with the same BMI can have completely different body composition, and the calculator has no way to see the difference.

• It is not built for under-18s. WHO classifies children aged 5–19 by BMI-for-age against a growth reference, in standard deviations — not by the fixed 25 and 30 lines this tool applies to everyone.
• It ignores pregnancy, age and where fat sits. WHO calls BMI a surrogate marker of fatness and notes that waist measurement adds information the index cannot.
• It rounds first. Because the script compares a rounded BMI against strict greater-than and less-than tests, a value that rounds to exactly 25, 30, 35 or 40 falls between two labels and shows the number with no wording after it.
• It says nothing about what you eat. That part is measurable too — see the doses actually used in the trials behind anti-inflammatory foods for joint pain, or the 18 calories and 2,573 µg of lycopene in our tomato nutrition breakdown.

This calculator is an estimate, not a diagnosis. It is not medical advice — talk to a doctor or a registered dietitian before making changes based on a number you get here.

Sources

• World Health Organization, fact sheet “Obesity and overweight” — who.int
• NHS, “Overweight and obesity” — BMI bands, including lower thresholds by ethnic background — nhs.uk
• WHO Expert Consultation, “Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies,” The Lancet, 2004;363(9403):157–63 — PMID 14726171