What Is BMI? How to Calculate It, What the Ranges Mean, and Its Limits

BMI, or body mass index, is a number calculated from your weight and height that gives a rough indication of whether your weight is in a typical range for your height. For adults, a BMI from 18.5 to 24.9 is generally classed as a healthy weight, 25 to 29.9 as overweight, and 30 or above as obese.

BMI is quick, free, and needs no equipment, which is why doctors and health organizations use it as a first screening tool. It is also limited in ways worth understanding before you put too much weight on your result.

What is BMI and how is it calculated?

BMI divides your weight by the square of your height.

Metric: BMI = weight (kg) / height (m) squared

Imperial: BMI = (weight in lb / height in inches squared) x 703

Worked example

Someone who weighs 82 kg and is 1.75 m tall:

1. Square the height: 1.75 x 1.75 = 3.0625
2. Divide weight by that number: 82 / 3.0625 = 26.8

The same person in imperial units weighs about 181 lb and is about 69 inches tall:

1. 69 x 69 = 4,761
2. 181 / 4,761 = 0.038
3. 0.038 x 703 = about 26.7

Small differences come from rounding. Either way, this BMI falls in the overweight range.

Adult BMI categories

These are the standard categories widely used for adults:

Some health bodies use different cutoffs for certain ethnic groups, because health risks can begin at lower BMI levels for some populations. Children and teens are assessed with age- and sex-specific percentiles rather than these adult categories. A doctor can tell you which reference applies to you.

What BMI does well

BMI earned its place for practical reasons:

• It is easy. You need only a scale and a tape measure.
• It is consistent. Everyone calculates it the same way, so it is useful for comparing over time.
• It correlates with health risk at the population level. Higher BMI ranges are associated with higher risk of conditions such as type 2 diabetes and heart disease across large groups of people.

For an individual, the most useful thing about BMI is often the trend. If you are working on weight loss, watching your BMI move from one category toward another is a clear, motivating signal of progress.

The limits of BMI

BMI measures weight relative to height. It does not know what that weight is made of.

It does not distinguish muscle from fat

A muscular athlete can have a BMI in the overweight range while carrying little body fat. Muscle is denser than fat, and BMI cannot tell them apart.

It ignores where fat is stored

Fat stored around the abdomen is linked with higher health risk than fat stored elsewhere. Two people with the same BMI can have very different risk profiles.

It does not account for age or sex differences

Older adults often have less muscle and more fat at the same BMI as younger adults. Women typically carry more body fat than men at the same BMI.

It is not a diagnosis

A BMI number on its own does not mean you are healthy or unhealthy. It is a prompt for a broader conversation.

Measures to use alongside BMI

For a fuller picture, pair BMI with:

• Waist circumference. Measure around your waist just above the hip bones, after breathing out. Larger waist measurements are associated with higher health risk.
• Waist-to-height ratio. A commonly used guideline is to keep your waist measurement to less than half your height.
• How you feel and perform. Energy, sleep, strength, and stamina matter.
• Medical checks. Blood pressure, blood sugar, and cholesterol tell you far more about health than any body-size measure. Your doctor can order and interpret these.

How to use BMI if you are trying to lose weight

BMI works best as one dashboard light among several. A practical routine:

1. Weigh yourself at a consistent time, such as first thing in the morning after using the bathroom.
2. Track the weekly average rather than reacting to daily swings.
3. Recalculate BMI as your weight changes and watch the trend.
4. Measure your waist monthly. It can shrink even when the scale stalls, especially if you are also strength training.
5. Set a calorie target based on your maintenance needs. Our guide on how to calculate a calorie deficit walks through it.

NutriVision includes weight and BMI tracking alongside its food photo logging, so you can see your BMI update as you record your weight and view the trend over time in its reports. It is a convenient way to keep your eating and your progress in one place, and you can see all NutriVision features on the app page.

Tip: If your BMI is below 18.5, or you are losing weight without trying, talk to a doctor. Unintentional weight loss can have medical causes worth checking.

Frequently asked questions

What is a healthy BMI for adults?

For most adults, a BMI between 18.5 and 24.9 is considered a healthy weight range. Some populations may have different recommended cutoffs, so ask your doctor what applies to you.

Is BMI accurate for muscular people?

Not very. Because BMI cannot tell muscle from fat, very muscular people can be classed as overweight even with low body fat. Waist measurements and body composition give a better picture.

How often should I check my BMI?

If you are actively changing your weight, checking every week or two is plenty. Focus on the trend over several weeks rather than any single reading.

Does BMI apply to children?

Children and teens use BMI-for-age percentiles that account for growth and sex. The adult categories do not apply, so a pediatrician should interpret a child's BMI.

Want to see your BMI change as you make progress? Download NutriVision free on Google Play to track your weight, BMI, and meals in one app.