BMI Calculator kg: How to Calculate Your Body Mass Index

This BMI calculator works out your Body Mass Index from your weight in kilograms and your height in meters (or centimeters). You enter your weight and height, and the calculator returns your BMI value plus the standard weight category (underweight, normal, overweight, or obese). The result is a quick screening number that helps you see where your weight falls relative to your height.

Formula

The BMI formula is simple: divide your weight in kilograms by the square of your height in meters.

BMI = weight (kg) ÷ height² (m²)

Where:

• weight is your body mass in kilograms (kg)
• height is your height in meters (m). If you measure in centimeters, divide by 100 first.

The result is expressed in kg/m². For example, a person who weighs 70 kg and is 1.70 m tall has a BMI of 70 ÷ (1.70 × 1.70) = 24.2 kg/m².

Worked Example

Using the default values from the calculator above (weight = 70 kg, height = 170 cm):

1. Convert height to meters: 170 cm ÷ 100 = 1.70 m
2. Square the height: 1.70 × 1.70 = 2.89 m²
3. Divide weight by height squared: 70 ÷ 2.89 = 24.22 kg/m²

Rounded to one decimal place, the BMI is 24.2 kg/m², which falls in the “normal weight� category (18.5–24.9).

How to Interpret Your BMI Result

BMI is a screening tool, not a direct measure of body fat. The standard adult categories, set by the World Health Organization, are:

• Below 18.5 – Underweight
• 18.5 to 24.9 – Normal weight
• 25.0 to 29.9 – Overweight
• 30.0 and above – Obese

These cutoffs apply to adults aged 18 and over. For children and teens, BMI is plotted on age- and sex-specific growth charts because body fat changes with growth. A BMI in the “normal� range is associated with lower risk of many chronic conditions, but it is not a guarantee of health, nor does a high BMI always mean a person is unhealthy.

Why Use Kilograms and Meters?

The original BMI formula was developed in the 1830s by Adolphe Quetelet, a Belgian mathematician, using the metric system. The formula is inherently metric: weight in kilograms and height in meters. If you measure weight in pounds and height in inches, you must convert to metric or use a different formula (BMI = 703 × weight(lb) / height²(in²)).

Using kilograms directly avoids conversion errors. Most of the world uses kilograms for body weight, and medical records typically list height in centimeters. The calculator above accepts height in centimeters, which is the most common way people know their height in metric countries. If you only know your height in feet/inches, convert to centimeters first: 1 foot = 30.48 cm, 1 inch = 2.54 cm.

Common Mistakes and Misconceptions

One frequent error is mixing units – for example, entering height in centimeters but treating it as meters. That gives a wildly low BMI. Another is using the wrong formula for pounds/inches. Always double-check your inputs.

A bigger misconception is that BMI measures body fat directly. It does not. It only compares weight to height. Two people with the same BMI can have very different body compositions. A muscular athlete may have a high BMI (because muscle weighs more than fat) but a low body fat percentage. Conversely, an older person with low muscle mass might have a “normal� BMI but excess fat. For this reason, BMI is best used as a population-level screening tool, not a personal diagnostic.

Also, BMI does not account for fat distribution. Fat stored around the abdomen (apple shape) carries higher health risk than fat stored around the hips and thighs (pear shape). Waist circumference is often used alongside BMI to assess that risk.

Limitations of BMI

BMI has several well-documented limitations:

• Muscle mass: Athletes and bodybuilders may be classified as overweight or obese despite low body fat.
• Age: Adults lose muscle and gain fat with age, so BMI may underestimate fat in older adults.
• Ethnicity: Some populations have different health risks at the same BMI. For instance, people of Asian descent may have higher metabolic risk at lower BMIs, and some health organizations use lower cutoffs for them.
• Pregnancy: BMI is not applicable during pregnancy, as weight gain is expected and necessary.
• Children: BMI categories for children are age- and sex-specific; adult cutoffs do not apply.

Despite these limits, BMI remains a quick, inexpensive, and non-invasive screening tool. It is widely used in public health research and clinical settings to flag potential weight-related issues that warrant further assessment.

Alternatives to BMI

If BMI doesn’t give you the full picture, other methods measure body composition more directly:

• Waist-to-hip ratio: Divide waist circumference by hip circumference. A ratio above 0.90 (men) or 0.85 (women) indicates abdominal obesity.
• Body fat percentage: Measured via skinfold calipers, bioelectrical impedance scales, DEXA scans, or hydrostatic weighing. Healthy ranges vary by sex and age.
• Waist-to-height ratio: Keep your waist less than half your height. This simple ratio correlates well with cardiovascular risk.
• DEXA scan: A low-dose X-ray that measures bone density, lean mass, and fat mass precisely – considered a gold standard, but expensive.

For most people, BMI plus waist circumference gives a reasonable picture. If you have concerns about your weight or health, discuss them with a healthcare provider, who can recommend the most appropriate assessment.

Frequently Asked Questions

How do I calculate my BMI in kg and cm?

Take your weight in kilograms and divide it by your height in meters squared. If your height is in centimeters, divide by 100 first. For example, 170 cm becomes 1.70 m. Then square that number (1.70 × 1.70 = 2.89) and divide your weight by it. A 70 kg person at 170 cm has a BMI of 24.2.

What is a healthy BMI for an adult?

For most adults, a BMI between 18.5 and 24.9 is considered healthy. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese. However, these cutoffs may not apply to athletes, older adults, or certain ethnic groups, so use them as a general guide.

Is BMI accurate for athletes?

Not really. BMI does not distinguish muscle from fat. A muscular athlete often has a BMI above 25 (overweight) or even 30 (obese) despite having very low body fat. For athletes, body fat percentage or waist-to-height ratio may be more useful.

Does BMI work for children?

BMI is calculated the same way for children, but the interpretation is different. For kids and teens, BMI is plotted on growth charts that account for age and sex, because body fat changes as they grow. A child’s BMI is compared to percentiles, not fixed adult cutoffs.

What is the difference between BMI and body fat percentage?

BMI is a mathematical ratio of weight to height. Body fat percentage is the actual proportion of your weight that is fat. Two people with the same BMI can have very different body fat percentages. BMI is easier to measure, but body fat percentage gives a more accurate picture of body composition.

Can I use the same BMI formula with pounds and inches?

Yes, but you need a conversion factor. The formula is BMI = 703 × weight(lb) / height²(in²). The 703 converts the units to the same kg/m² scale. If you prefer, convert pounds to kg (divide by 2.205) and inches to meters (multiply by 0.0254) and use the standard metric formula.

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in weight | Weight | kg | 70 | 0
in height | Height | cm | 170 | 0
out bmi | Body Mass Index | kg/m² | weight / ((height/100)^2) | 1