The FoodGPT BMI calculator: how it works and what to do with the result
The BMI calculator divides weight in kilograms by height in meters squared and names a category from the WHO scale. The arithmetic is exact; the health assessment is approximate: in a sample of 13,601 Americans, a BMI of 30 or more identified excess body fat in only 36% of men and 49% of women who had it by a body-composition measurement.
What the calculator does
Weight in kilograms is divided by height in meters squared. For 70 kg and 170 cm: 1.70 × 1.70 = 2.89, and 70 ÷ 2.89 ≈ 24.2. WHO writes the same formula as weight (kg) / height² (m²).
The page has three fields: height, weight and age. The answer has three parts: the number, the category, and a weight range for your height. The categories come from the WHO Europe table:
| BMI | Category in the calculator |
|---|---|
| below 18.5 | underweight |
| 18.5–24.9 | healthy weight |
| 25.0–29.9 | overweight |
| 30.0 and above | obesity |
A few rules of the calculator. The number is rounded to one decimal, and the category is chosen from the rounded value, that is, from what you see on screen. No result is shown if height is outside 120–230 cm, weight outside 30–250 kg, or age outside 2–100 years. Under 18 the number is calculated but no category is named: for children and teenagers BMI is compared with age norms. For ages 5–19 WHO uses the deviation from the median (+1 and +2 standard deviations), not adult cut-offs. The calculation runs in your browser, and your height and weight are not sent anywhere.
The weight range under the number comes from the limits of the healthy category: 18.5 and 24.9 multiplied by height squared. At 170 cm that is 53.5 to 72 kg.
Where the formula comes from
The ratio of weight to height squared was described by the Belgian mathematician Adolphe Quetelet: Eknoyan's review dates it to 1832, and in 1972 Ancel Keys named it the body mass index. Keys and colleagues compared several measures of relative weight in 7,424 “healthy” men from 12 cohorts in five countries. Weight divided by height squared proved slightly better than the simple ratio of weight to height in its relation to height and to measures of body fatness; the authors also cite the simplicity of the calculation.
The sample was men. WHO Europe describes BMI as a risk indicator of disease and bases the category limits on the link between excess fat and disease and death. That is why the Lancet commission mentioned below advises using it for groups and screening, not as a personal measure of health.
Why the 25 line is a convention
At 170 cm, 72 kg gives a BMI of 24.9: “healthy weight”. 72.5 kg gives 25.1: “overweight”. Half a kilogram changes the name of the category while the person stays the same. Body weight also swings over the course of a day.
Mortality data show a gradual climb, not a cliff. In the Global BMI Mortality Collaboration analysis (239 studies, never-smokers without chronic disease, first 5 years of follow-up excluded), all-cause mortality was lowest at a BMI of 20–25. The hazard ratio was 1.07 at 25–27.5, 1.20 at 27.5–30, 1.45 at 30–35 and 1.13 at 18.5–20.
The Flegal meta-analysis painted a different picture: against a BMI of 18.5–25, the 25–30 category had a hazard ratio of 0.94 (95% CI 0.91–0.96), grade 1 obesity 0.95 (0.88–1.01) and grades 2–3 1.29. The results depend on who was included: the first study deliberately excluded smokers, people with chronic disease and the first five years of follow-up, to limit distortion from smoking and illness already under way. Neither study says anything about an individual; both describe groups.
Where the calculator is wrong
It cannot see body composition. A Mayo Clinic study of NHANES III data (13,601 people aged 20–79, body fat estimated by bioimpedance) compared BMI with body fat percentage. A BMI of 30 or more detected obesity by fat in 36% of men and 49% of women who had it. Specificity was 95% in men and 99% in women: people without excess fat were almost never labeled obese. Accuracy fell with age, and in the 25–29.9 range BMI could not separate fat from lean mass in either sex.
One scale for everyone. A 2004 WHO consultation concluded that in Asian populations the risk of type 2 diabetes and heart disease is noticeable below a BMI of 25: the observed risk starts between 22 and 25 depending on the population. The common scale was kept, with extra public-health action points at 23.0, 27.5, 32.5 and 37.5. The UK NHS uses lower thresholds for people with an Asian, Chinese, Middle Eastern, Black African or African-Caribbean family background: 23 to 27.4 is overweight, 27.5 and above is obese. The calculator has no field for background and applies the general scale.
Age and muscle. WHO Europe gives this table for adults over 20; the calculator applies it from 18. The NHS notes that very muscular people, such as professional sportspeople, can have a high BMI without much fat. The calculator page separately warns that BMI does not apply during pregnancy.
It does not show where fat sits. WHO calls BMI a “surrogate marker of fatness” and adds that waist circumference can help the diagnosis. The 2025 Lancet Diabetes & Endocrinology Commission (58 experts) went further and recommends using BMI as a measure of risk for populations, research and screening, not as an individual measure of health. Excess fat is confirmed by direct measurement of body fat or at least one additional measure besides BMI, such as waist circumference. The exception: above a BMI of 40, excess fat can be assumed without further measurements.
What to add to the result
The NHS suggests waist-to-height ratio alongside BMI. Divide your waist circumference by your height in the same units; 0.5 or above may mean increased health risks. Measure halfway between the lowest rib and the hip bone, breathing naturally and not pulling the stomach in. The NHS uses this in adults with a BMI under 35. The site has a waist and hip calculator and a body fat calculator. How the waist behaves when weight holds steady is covered in the scale is stable, the waist is shrinking, and fat around the organs in the article on visceral fat.
Smart scales with bioimpedance estimate body composition but have their own error: how to read it.
How to use the answer
- The number is useful as a starting point and for a conversation with a doctor. A single value says less than the change over months.
- The weight range under the number shows how the scale is built. It is not a target: the range spans about 18 kg at 170 cm, and where you sit inside it depends on build and body composition.
- There is no point recalculating after every weighing, for the same reason: weight swings through the day.
- The wider link between body weight and health is covered in the articles on BMI and excess weight and health.
When to see a doctor
- Your weight is falling although your eating and activity have not changed. The NHS advises seeing a doctor: the cause can be stress or a serious illness, and the earlier it is found, the sooner it is treated.
- A BMI below 18.5. WHO calls this underweight; a doctor works out the cause.
- A BMI of 30 or above. Under the Lancet commission's criteria, the next step is to confirm excess fat with another measure and assess how it affects organ function. A doctor does this.
- Pregnancy, a child or a teenager: the adult scale does not apply.
- Thoughts about weight and food take up a lot of room and get in the way of life. That is a reason to see a doctor or a psychotherapist, not a calculator.
The calculator: BMI from height and weight. For a daily energy estimate see daily calorie needs, and for the formulas behind “ideal weight”, a separate breakdown.
Frequently asked questions
- How does the calculator work out BMI?
- It divides weight in kilograms by height in meters squared, rounds to one decimal and names the WHO category: below 18.5, 18.5–24.9, 25.0–29.9, 30 and above. For 70 kg and 170 cm the result is 24.2.
- Why does half a kilogram change the category?
- The scale's limits are single numbers. At 170 cm, 72 kg gives 24.9 and 72.5 kg gives 25.1. Mortality data show a gradual rise in risk, not a jump at the line.
- How accurate is BMI for judging body fat?
- In the NHANES III study of 13,601 people, a BMI of 30 or more detected obesity by body fat percentage in 36% of men and 49% of women who had it, and wrongly labeled 5% of men and 1% of women without it as obese. Accuracy fell with age.
- Why does the calculator give no category under 18?
- For children and teenagers BMI is judged against norms for age and sex. For ages 5–19 WHO uses deviation from the median, not adult cut-offs. A pediatrician makes the assessment.
- Does the calculator account for ethnic background or muscle mass?
- No. It applies one WHO scale to all adults. The NHS gives lower thresholds, 23 and 27.5, for several ethnic groups, and in very muscular people BMI can be high without excess fat.
Read next
- Body mass index: how to calculate it and where it goes wrong — Body mass index takes ten seconds to calculate and is therefore used where it shouldn't be: for an individual.
- Excess weight and health: what risks are confirmed — This topic is usually presented in a way meant to induce fear.
- Visceral fat: why it matters and how to measure it — Two people at the same weight, with the same BMI, can carry very different metabolic risk.
- The scale is steady but your waist is shrinking: what it means — If your weight is steady but your waist measures smaller, change may be happening without a visible shift on the scale.
- The ideal weight calculator: where the formulas come from and what they cannot see — There is no such thing as an ideal weight: the name is given to the output of formulas derived from insurance height and weight tables, and the Devine formula was published to calculate drug doses.
This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.
Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.