Computing Body Mass Index- The Formula | Clear BMI Math

Body mass index, or BMI, turns your height and weight into one number that fits into common health screening ranges. The math is short, the units matter, and once you know the BMI formula you can repeat it any time with just a calculator and a notepad.

Health agencies across the world use BMI because it gives a quick snapshot of body size in relation to height. It does not tell the full story of anyone’s health, yet it helps flag values that sit far below or above ranges linked with lower risk in large population studies. When you know exactly how the BMI formula works, you can read that number with more context instead of treating it as a mystery label.

What Body Mass Index Measures

BMI is a ratio of weight to the square of height. That means it compares how heavy a person is against how tall that person is, instead of looking at weight alone. Two people who both weigh 80 kilograms can have very different BMI values if one person is much taller than the other.

Public health bodies describe BMI as a screening tool rather than a diagnosis. The CDC overview of BMI explains that this number helps flag underweight, overweight, and obesity ranges that link with higher chances of certain conditions, such as type 2 diabetes or heart disease, across large groups of adults. It should always sit beside other information such as blood tests, blood pressure, waist measures, and medical history.

The same formula applies to adults of all sexes, which makes BMI useful when a clinic or research group needs a standard measure across many people. At the same time, it does not separate muscle from fat and does not reflect where fat sits on the body, so it needs careful reading in people with very high muscle mass or unusual body proportions.

Body Mass Index Formula Explained Step By Step

The core BMI formula stays the same across the world:

BMI = weight ÷ height²

What changes is the unit system. In metric form, weight is in kilograms and height in metres. In imperial form, weight is in pounds and height in inches, and a conversion factor keeps the number in the same scale as the metric result.

Metric BMI Formula

In metric units, the formula looks like this:

BMI = weight (kg) ÷ [height (m)]²

If someone weighs 70 kilograms and stands 1.75 metres tall, you first square the height, then divide the weight by that squared value. The result comes out in kilograms per square metre (kg/m²), which is the standard unit used in health guidelines from groups such as the World Health Organization.

Imperial BMI Formula

In imperial units, the raw ratio uses pounds and inches. To keep the BMI scale the same as the metric version, a constant factor of 703 is added:

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

The 703 factor converts the pound and inch units into the same kg/m² style scale that international tables use. Without that constant, the numbers would not match standard BMI cut-offs and would not line up with published risk ranges.

How To Calculate BMI In Metric Units

Once you know the metric formula, you can run your own BMI calculation in three short moves. Here is the pattern many online calculators follow.

Step 1: Convert Height To Metres And Square It

If your height is already in metres, you can square it straight away. If your height is in centimetres, divide by 100 to get metres first.

• Height: 168 cm
• Convert to metres: 168 ÷ 100 = 1.68 m
• Square the height: 1.68 × 1.68 = 2.8224 m²

Step 2: Divide Weight By Squared Height

Now divide the weight in kilograms by that squared height:

• Weight: 72 kg
• Squared height: 2.8224 m²
• BMI = 72 ÷ 2.8224 ≈ 25.5 kg/m²

Step 3: Match The BMI Value To A Category

With a BMI of about 25.5 kg/m², this example adult sits just above the usual “healthy weight” range and into the “overweight” band in many adult charts used by groups such as the National Heart, Lung, and Blood Institute BMI guidance. A health professional would pair that number with other tests and with the person’s history before giving any advice.

How To Calculate BMI With Pounds And Inches

If you are more used to pounds and feet with inches, you can still compute BMI by following the same structure with the imperial formula.

Step 1: Convert Height To Total Inches

Take height in feet and inches and turn it into inches only:

• Height: 5 feet 6 inches
• Convert to inches: (5 × 12) + 6 = 60 + 6 = 66 inches
• Square the height: 66 × 66 = 4356 in²

Step 2: Apply The Imperial Formula

Now plug weight and squared height into the imperial BMI formula:

• Weight: 165 lb
• Squared height: 4356 in²
• 703 × 165 = 115,995
• BMI = 115,995 ÷ 4356 ≈ 26.6 kg/m²

This BMI value falls in the same range as the metric example above. That is the whole reason for the 703 factor: it keeps the scale aligned so the same cut-offs apply no matter which unit system you start from.

BMI Categories And What Your Number Means

Once you have a BMI value, the next step is to see how that number fits against common categories. Health agencies such as the CDC, WHO, and the UK’s National Health Service use very similar bands for adults, with small wording differences between sources. The list below reflects ranges often used in adult guidance.

• Underweight: BMI below 18.5
• Healthy weight: BMI 18.5 to 24.9
• Overweight: BMI 25.0 to 29.9
• Obesity class 1: BMI 30.0 to 34.9
• Obesity class 2: BMI 35.0 to 39.9
• Obesity class 3: BMI 40.0 and above

The CDC adult BMI categories and the NHS overview of BMI ranges both stress that these cut-offs apply to adults and that BMI is only one piece of health information. Some regions also use extra decision points for certain ethnic groups, since health risk can rise at lower BMI thresholds in some populations.

BMI Range (kg/m2)	Weight Category	General Notes
< 16.0	Severe Thinness	May reflect undernutrition or another medical issue; needs prompt medical review.
16.0–18.4	Moderate To Mild Thinness	Can link with low energy reserves and mineral or vitamin lack in some people.
18.5–24.9	Healthy Weight	Often linked with lower risk of many weight-related conditions at a population level.
25.0–29.9	Overweight	Higher body weight for height; risk depends on fat distribution, fitness, and other factors.
30.0–34.9	Obesity Class 1	Linked with raised risk of conditions such as type 2 diabetes and high blood pressure.
35.0–39.9	Obesity Class 2	Risk level rises further; medical teams often suggest more active monitoring.
≥ 40.0	Obesity Class 3	Very high BMI value; usually leads to close follow-up and tailored care plans.

This table draws on ranges used in international guidance from the WHO and national bodies. Exact wording can vary slightly, yet the numeric cut-offs remain close across sources, which helps people compare the same BMI number across tools and leaflets.

Sample BMI Calculations For Common Height And Weight Pairs

Once you understand the BMI formula, worked examples help the math feel routine. The table below shows sample values for adults using the standard kg/m² scale. All figures are rounded to one decimal place to keep the table neat while still showing useful detail.

Height And Weight	BMI (Rounded)	Category
1.60 m, 45 kg	17.6	Underweight
1.60 m, 58 kg	22.7	Healthy weight
1.70 m, 72 kg	24.9	Upper end of healthy weight
1.70 m, 80 kg	27.7	Overweight
1.80 m, 65 kg	20.1	Healthy weight
1.80 m, 95 kg	29.3	Near top of overweight
1.80 m, 115 kg	35.5	Obesity class 2

These examples keep height and weight combinations realistic for adults while showing how sensitive BMI can be to either variable. A change of only a few kilograms can move a person across a cut-off when height stays the same.

Limits Of BMI As A Health Measure

BMI formula results are easy to compute and compare, which explains why they appear in many health tools and national campaigns. At the same time, the index has well known limits that matter when you read your own result.

Muscle Mass And Body Composition

BMI does not separate lean tissue from fat. A strength athlete with high muscle mass can land in the overweight or even obesity range while carrying low levels of body fat. A person with low muscle mass may sit in the healthy band while holding more fat in the abdomen than the number suggests.

Because BMI treats every kilogram the same, it works best as a broad screening figure. Measures such as waist circumference, waist-to-height ratio, and body fat estimates fill in the gaps. Many clinics now use these alongside BMI, as shown in advice linked from tools such as the NHS adult BMI calculator.

Age, Ethnic Background, And Health History

Age and ethnic background can change how BMI links with health risk. For some Asian populations, for example, type 2 diabetes and heart disease appear at lower BMI values than in white European groups, so guidelines sometimes use extra cut-offs around 23 and 27.5 kg/m² for closer watching.

Older adults often have more body fat and less muscle than younger adults at the same BMI, which can change the way a number maps to health risk. Health history also matters: someone with long-standing high blood pressure, high cholesterol, or a strong family history of early heart disease may need more attention at a BMI that would raise fewer concerns in someone else.

How To Use Your BMI Number Wisely

Once you compute BMI with the formula, the next step is to decide what you want to do with that information. A single number on its own rarely calls for instant action. It works best as a prompt for a wider chat about health habits, symptoms, and goals.

Pair BMI With Other Simple Checks

Waist circumference gives extra detail that BMI cannot show. A higher waist measure at the same BMI points toward more fat around the abdomen, which relates closely to risk for conditions such as type 2 diabetes and fatty liver disease. Blood pressure, resting heart rate, and basic blood tests round out the picture.

Online tools from agencies such as the CDC BMI resource hub and national health services often combine BMI calculators with plain-language advice. Those tools can help you form questions to bring to a doctor, dietitian, or other qualified health professional who knows your full history.

Talk With A Health Professional

If your BMI sits outside the healthy range or has changed sharply over a short time, it makes sense to share that change with a health professional. Sudden weight loss can point toward underlying illness even when BMI still sits in a typical range. Rapid weight gain can link with fluid buildup or hormone issues as well as lifestyle patterns.

A doctor or nurse can place your BMI, waist measure, blood results, and daily habits side by side. From there, you can agree on next steps that fit your situation, such as changes in food patterns, movement, sleep, or targeted treatment. The BMI formula supplies one neat number in that wider conversation, not the entire answer.

Bringing The BMI Formula Into Everyday Life

The BMI formula is short, the units are clear, and once you run through a few examples the steps become second nature. You square your height in metres or inches, divide your weight by that squared value, and then place the result into ranges drawn from large long-term studies.

Used in that way, BMI helps you track broad shifts over time and spot values that fall far from average for your height. When you pair this number with waist measures, activity patterns, and medical checks, it becomes a practical part of looking after long-term health. The formula stays the same every time; the meaning comes from the wider context you and your health team build around it.