BMI Calculator

How this calculator works

BMI, or body mass index, is your weight divided by your height squared. In metric units that is kilograms divided by meters squared. In US units the calculator multiplies your weight in pounds by 703 and divides by your height in inches squared. A 5'6" woman at 160 lb has a BMI of 25.8. A 5'10" man at 190 lb has a BMI of 27.3.

It does not ask your age or sex because the adult categories from the CDC and the WHO are the same for women and men from age 20 on. A BMI calculator for women and one for men give the same number; what differs is how to read it, covered below.

Enter your height and weight as they are today, ideally weighed in the morning after the bathroom and before breakfast. Shoes off, and be honest about height: many of us lose half an inch or more after 60, and an old driver's license number will push the result down.

How to read your result

The CDC and the WHO use the same adult BMI chart. The healthy BMI range, often called "normal weight," runs from 18.5 to 24.9. Above 30, the categories are split into three obesity classes because health risk keeps climbing.

Lower cutoffs for people of Asian descent

A WHO expert consultation in 2004 found that many Asian populations carry more body fat and more metabolic risk at a lower BMI. It suggested additional public-health action points at 23 (increased risk) and 27.5 (high risk). If you are of South Asian, Chinese, Japanese or Southeast Asian background, a BMI of 24 is not automatically reassuring.

Where the number misleads

Is BMI accurate? For populations, yes: across millions of people, higher BMI tracks with higher rates of type 2 diabetes, heart disease and some cancers. For one person standing on a scale, it is a screening number, not a diagnosis.

Muscle mass

BMI cannot tell fat from muscle. A 5'9" man who lifts three times a week at 195 lb reads 28.8, "overweight," with a waist that says otherwise. The reverse is more common after 50: a normal BMI with little muscle and a lot of fat around the middle, sometimes called normal-weight obesity. This is the core of the BMI vs body fat debate. The two are related but not the same measurement.

Age, and BMI over 50 and over 65

For adults over 65, a slightly higher BMI does not carry the penalty it does at 40. In a 2014 meta-analysis of studies in people 65 and older, mortality risk began to rise below a BMI of about 23, while BMIs in the overweight range were not linked with higher mortality. Part of that is illness causing weight loss, but a small reserve of weight also protects against falls, fractures and long hospital stays. For a BMI calculator for adults over 50 or 65, the practical shift is this: worry more about unintended weight loss and lost muscle than about a 26.

Ethnicity

The Asian cutoffs above are the best-documented example, but at the same BMI, body-fat percentage, fat distribution and diabetes risk differ between ethnic groups. BMI was built on European data and travels imperfectly.

Pregnancy, fluid and medication

BMI is not meaningful during pregnancy. Swelling from heart, kidney or liver conditions, and weight gain from steroids, insulin or some antidepressants, all change the number without saying anything about body fat. Weight swings on a new prescription belong with your clinician, not a calculator.

The better companion: waist circumference

A 2020 consensus statement in Nature Reviews Endocrinology argued that waist circumference should be measured alongside BMI as a routine "vital sign," because it captures the belly fat that drives most of the metabolic risk. The NHLBI's simple thresholds: above 40 inches for men and above 35 inches for women signals higher risk, at any BMI under 35. Measure at the top of the hip bones, tape snug but not squeezing, after breathing out.

What to do with it

1. Add your waist. Write down BMI and waist together. A BMI of 26 with a 33-inch waist and a BMI of 26 with a 41-inch waist are two different situations.
2. Track the trend, not the day. Weigh once a week under the same conditions. A pound of water can move BMI by 0.2 overnight; three months of the same direction means something.
3. If you are over 25 and the waist agrees, the two habits with the best evidence per minute are a 10-minute walk after each meal and moving protein to the front of the day, which we covered in protein at breakfast and belly fat.
4. If you are over 65 and under 23, or losing weight without trying, that is the number to take to a clinician. Our retirement checklist for your body lists the screenings and the strength work that matter most at that stage.
5. Do not diet on BMI alone. Blood pressure, fasting glucose or A1C, and a lipid panel tell you whether the weight is doing harm. BMI tells you whether to ask.