Body Mass Index (BMI), Explained

By Udit Agarwal · Updated October 17, 2026

Body mass index (BMI) is your weight in kilograms divided by the square of your height in metres. For adults, a BMI of 18.5 to 24.9 is classed as a healthy weight, 25 to 29.9 as overweight and 30 or more as obesity, the cut-offs used by both the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC). BMI is quick to work out and useful for spotting weight-related health risk across a population, but it does not measure body fat, so one person's number always needs context.

The BMI formula

The metric formula is the definition:

BMI = weight (kg) ÷ height (m)²

Height must be in metres, so 175 cm becomes 1.75 m, and it is squared before you divide. The result is in kilograms per square metre, although it is normally written as a bare number.

In pounds and inches, the same formula needs a conversion factor:

BMI = 703 × weight (lb) ÷ height (in)²

The 703 comes from the units themselves. A pound is exactly 0.45359237 kg and an inch exactly 0.0254 m, so the factor is 0.45359237 ÷ 0.0254² = 703.07. Rounding it to 703 changes a BMI by about 0.01 percent, a difference that almost never shows up in the first decimal place.

Worked examples in metric and imperial

Metric. Someone who weighs 70 kg and is 175 cm tall:

1. Height in metres: 1.75
2. Height squared: 1.75 × 1.75 = 3.0625
3. BMI: 70 ÷ 3.0625 = 22.86, which rounds to 22.9, in the healthy range.

Imperial. Someone who weighs 154 lb and is 5 ft 9 in tall:

1. Height in inches: 5 × 12 + 9 = 69
2. Height squared: 69 × 69 = 4,761
3. Weight times 703: 154 × 703 = 108,262
4. BMI: 108,262 ÷ 4,761 = 22.74, which rounds to 22.7, also healthy.

Two more for comparison. At 200 lb and 5 ft 10 in (70 inches), BMI is 703 × 200 ÷ 4,900 = 28.7, in the overweight range. At 100 kg and 170 cm, it is 100 ÷ 2.89 = 34.6, which is obesity class I.

What is a healthy BMI?

For adults, a healthy BMI is 18.5 to 24.9. This is the full adult classification, with the label the ArrayKit calculator shows for each band:

The WHO defines overweight as a BMI of 25 or more and obesity as 30 or more. The CDC uses the same cut-offs for adults aged 20 and over and calls 18.5 to below 25 "healthy weight". A BMI of exactly 25.0 therefore counts as overweight, and 24.9 is the top of the healthy range.

Healthy weight for your height

Turn the formula around and you get the weight range that matches a healthy BMI at any height: multiply 18.5 and 24.9 by your height in metres, squared. At 1.75 m that is 18.5 × 3.0625 = 56.7 kg up to 24.9 × 3.0625 = 76.3 kg.

The Healthy Weight Calculator gives this range for any height. The Ideal Weight Calculator shows a different, older approach: the clinical ideal-body-weight formulas of Devine, Robinson, Miller and Hamwi, each of which gives a single weight for your height and sex, plus their average.

Why BMI is only a screening number

BMI uses nothing but weight and height, so it cannot see what the weight is made of or where it sits. The CDC describes it as a screening measure, one indicator to consider alongside others. Its blind spots are well known:

• Muscle. BMI does not distinguish fat from muscle and bone. A muscular athlete can score as overweight while carrying little body fat.
• Age and sex. At the same BMI, women tend to carry more body fat than men, and older adults more than younger ones, partly because muscle is lost with age. A healthy BMI can sit alongside a high body-fat percentage.
• Where fat is stored. Fat around the waist is linked to more health risk than fat on the hips or thighs, and BMI cannot tell them apart. The US National Heart, Lung, and Blood Institute notes that a waist of more than 35 inches (88 cm) for women or more than 40 inches (102 cm) for men increases risk.
• Ethnicity. A 2004 WHO expert consultation, published in The Lancet, found that a substantial share of Asian people have a high risk of type 2 diabetes and heart disease at BMIs below 25. It kept the international categories but proposed extra public-health action points at 23 and 27.5, and some countries and health services use lower cut-offs for people of Asian origin.
• Pregnancy. The adult categories do not apply during pregnancy. Weight-gain guidance, such as the 2009 Institute of Medicine ranges used by the Pregnancy Weight Gain Calculator, starts from the BMI before pregnancy.

For a closer look at body composition, the Body Fat Calculator estimates body-fat percentage from tape measurements of your neck and waist (and hips, for women) using the US Navy method.

BMI for children and teens

Children's body fat changes as they grow, and boys and girls develop differently, so a child's BMI is not read against the adult cut-offs. The CDC compares the BMI of anyone aged 2 through 19 with growth charts for their age and sex and expresses it as a percentile; obesity, for example, is a BMI at or above the 95th percentile. The WHO uses a similar BMI-for-age reference for ages 5 to 19.

The ArrayKit BMI Calculator does not ask for age or sex and applies the adult categories only, so it is not suitable for children or teenagers.

Using the ArrayKit BMI Calculator

The BMI Calculator works in Metric (kilograms and centimetres) or US units (pounds, feet and inches). It shows your BMI to one decimal place, the label from the table above, and the healthy weight range for your height at a BMI of 18.5 to 24.9, with a button to copy the summary. In US mode it converts pounds and inches to metric with the exact factors before applying the formula, so it agrees with the 703 version to within a rounding difference.

A few things to know. In Metric mode, height goes in as centimetres (175, not 1.75). The two unit modes keep separate fields, so re-enter your figures after switching. All three obesity classes share one "Obese" label, and the tool does not account for age, sex, muscle or ethnicity. The calculation runs in your browser, and the numbers you type are not sent anywhere.

BMI is an estimate from a standard formula and not medical advice, so talk to a doctor or registered dietitian before making decisions about your weight or health.

Frequently asked questions

What is a healthy BMI for adults? A BMI of 18.5 to 24.9 is classed as a healthy (normal) weight by both the WHO and the CDC. Below 18.5 is underweight, 25 to 29.9 is overweight and 30 or more is obesity. These cut-offs are for adults; children are assessed with percentiles instead.

What is the BMI formula in pounds and inches? Multiply your weight in pounds by 703, then divide by your height in inches squared. For 154 lb at 69 inches that is 108,262 ÷ 4,761 = 22.7. The 703 converts pounds and inches into the kilograms and metres BMI is defined in.

Is a BMI of 25 overweight? Yes. In the WHO and CDC classification the overweight range starts at exactly 25.0, and 24.9 is the top of the healthy range. The same applies at 30.0, where obesity class I begins.

Is BMI accurate for athletes and muscular people? Often not. BMI cannot tell muscle from fat, so people with a lot of muscle can score as overweight, or even obese, while carrying little body fat. A body-fat estimate or a waist measurement says more about them.

Why are BMI cut-offs lower for some Asian populations? A WHO expert consultation in 2004 found that a substantial share of Asian people face a high risk of type 2 diabetes and heart disease at BMIs below the standard overweight cut-off of 25. The WHO kept the international categories but suggested extra action points at 23 and 27.5, and some countries use lower thresholds for overweight and obesity.

Can I use a BMI calculator for my child? Not one built for adults. For ages 2 to 19, BMI has to be compared with growth charts for age and sex and expressed as a percentile, which the ArrayKit BMI Calculator does not do. The CDC offers a separate child and teen BMI calculator for that.

Is BMI different for men and women? The formula and the adult cut-offs are the same for everyone. What differs is body composition: at the same BMI, women typically carry more body fat than men.

Keep going

• Healthy Weight Calculator — the weight range for a BMI of 18.5 to 24.9 at any height.
• Body Fat Calculator — estimate body-fat percentage from tape measurements.
• Ideal Weight Calculator — compare the Devine, Robinson, Miller and Hamwi formulas.
• Calorie Calculator — estimate maintenance calories from your size, age and activity.
• Daily Calorie Needs, Explained — BMR, TDEE and what a calorie deficit means in numbers.
• Health calculators — every health and fitness calculator on ArrayKit.

BMI Calculator

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