It calculates your body mass index (BMI) from your weight and height, in kilograms and centimetres or in pounds, feet and inches, and tells you which category that BMI falls into in two tables: the World Health Organization's international classification for adults, and the lower Asia-Pacific cut-offs that many doctors in Vietnam and the rest of East and Southeast Asia use. It also shows the weight range that gives a normal BMI at your height, your basal metabolic rate (BMR, the energy your body uses at complete rest) and an estimate of your daily energy needs for five activity levels. Everything is calculated in your browser.
How it is calculated
BMI is weight in kilograms divided by the square of height in metres. 70 kg at 1.70 m is 70 ÷ 2.89 = 24.2. Imperial values are converted with the exact definitions (1 lb = 0.45359237 kg, 1 in = 2.54 cm), so 154 lb at 5 ft 7 in gives 24.1. The BMI is shown with one decimal and the category is read from that shown value, so a BMI displayed as 25.0 is never labelled normal.
WHO categories for adults: below 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight (pre-obese), 30 to 34.9 obesity class I, 35 to 39.9 class II, 40 and above class III.
Asia-Pacific cut-offs: the 2000 report "The Asia-Pacific perspective: redefining obesity and its treatment" (WHO Western Pacific Region with IASO and IOTF) proposed, provisionally, that adults in the region count as overweight (at risk) from 23 and obese from 25, because the risk of type 2 diabetes and heart disease rises at lower BMIs in many Asian populations. A WHO expert consultation in 2004 (The Lancet) kept the international categories but added 23, 27.5, 32.5 and 37.5 as public health action points, noting that the BMI at which risk rises differs between Asian populations. The tool shows the 2000 table as a second reference, not as a diagnosis.
BMR uses the Mifflin-St Jeor equation: 10 × weight in kg + 6.25 × height in cm − 5 × age, plus 5 for men or minus 161 for women. Daily energy needs are BMR times an activity factor: 1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 active and 1.9 very active. Results are rounded to whole kilocalories once, at the end.
A worked example
A 30-year-old man who weighs 70 kg and is 170 cm tall has a BMI of 24.2. In the WHO table that is normal weight; in the Asia-Pacific table it is overweight, at risk, because it is above 23. A normal BMI at his height is 53.5 to 71.9 kg (WHO) or 53.5 to 66.1 kg (Asia-Pacific). His BMR is 700 + 1,062.5 − 150 + 5 = 1,617.5, shown as 1,618 kcal a day, and with moderate exercise his energy needs are about 1,617.5 × 1.55 = 2,507 kcal a day.
What BMI cannot tell you
BMI is a screening number for groups of adults, not a measurement of body fat or health. A muscular person can have a high BMI and little fat; an older person can have a normal BMI and too little muscle. It is not used for children and teenagers (they are compared with growth charts for their age and sex), during pregnancy, or for athletes in heavy training. Waist circumference, blood pressure, blood sugar and how you feel tell a doctor much more. Calorie formulas are estimates too: real needs can differ by a few hundred kilocalories a day. Use the numbers as a starting point for a conversation with a health professional, not as medical advice.
Splitting a meal with friends? The tip and bill split calculator works out who owes what, and the age calculator gives an exact age in years, months and days. For a deeper look at the two BMI tables, read what BMI can and cannot tell you.
