BMI Calculator: Check Your Body Mass Index

A body mass index (BMI) calculator online works out your BMI from your height and weight, giving you a number that places you in a category such as underweight, healthy, overweight, or obese. You enter your weight and height (in metric or imperial units), and the calculator returns your BMI value and the category it falls into. This page explains the formula, how to use the calculator correctly, and what the result does and doesn’t tell you.

Formula

The BMI formula is the same everywhere in the world:

BMI = weight (kg) ÷ height² (m²)

Where:

• weight is your body mass in kilograms (kg).
• height is your height in metres (m).

If you measure in pounds and inches, first convert: 1 lb = 0.4536 kg and 1 inch = 0.0254 m. Then apply the same formula. The result is expressed in kg/m², though the unit is often omitted in everyday use.

Worked Example

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

1. Convert height to metres: 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.
4. Round to one decimal place: BMI = 24.2 kg/m².

This matches the calculator’s output exactly.

How to Use the Calculator

Enter your weight and height in the units you know best. The calculator above accepts metric (kg and cm), but if you prefer imperial, convert first: 1 kg = 2.2046 lb, 1 cm = 0.3937 inches. Then use the formula with those numbers. For most adults, the World Health Organization (WHO) classifies BMI as:

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

These cutoffs apply to adult men and women of all ages, but not to children, who use age- and sex-specific percentile charts.

What Your Result Means

BMI is a screening tool, not a diagnostic test. A high BMI is associated with increased risk of heart disease, type 2 diabetes, and certain cancers, but it does not measure body fat directly. Two people with the same BMI can have very different body compositions. For example, a muscular athlete may have a BMI of 28 (overweight) yet have very low body fat, while an older adult with low muscle mass might have a BMI of 22 but a high fat percentage. BMI also doesn’t account for where fat is stored; abdominal fat (apple shape) carries more health risk than fat stored in the hips and thighs (pear shape).

For a more complete picture, consider measuring your waist circumference (over 88 cm for women and 102 cm for men indicates increased risk in many populations) or ask a healthcare professional about body fat percentage measurement.

Common Mistakes and Limitations

• Unit mix-ups: Mixing pounds with metres or kilograms with inches gives wildly wrong numbers. Always use consistent units.
• Rounding too early: Rounding height to the nearest centimetre is fine, but rounding weight to the nearest 5 kg can shift your category. Use your actual current weight.
• Using BMI for children: Adult BMI categories do not apply to those under 18. Children’s BMI is plotted on growth charts that adjust for age and sex.
• Ignoring muscle mass: BMI can overestimate body fat in very muscular people and underestimate it in older adults who have lost muscle.
• Pregnancy: BMI is not meaningful during pregnancy; weight gain guidelines are different and based on pre-pregnancy BMI.
• Ethnic differences: Some populations (e.g., South Asian, Chinese, Japanese) have higher health risks at lower BMI values. The WHO suggests lower cutoffs (e.g., 23 for overweight) for some Asian groups.

Why BMI Is Still Useful

Despite its limitations, BMI remains a standard first-step screening tool because it is cheap, non-invasive, and correlates well with body fat for most of the population. It is used in large-scale studies to track obesity trends and in clinical settings to flag patients who may need further assessment. It is also a useful self-tracking metric over time: if your BMI is creeping up, it is a signal to review your diet and activity level.

Alternatives to BMI

If you want a more direct measure of body fat, consider these options:

• Waist-to-height ratio: Divide waist circumference by height. A ratio above 0.5 is associated with increased risk.
• Body fat percentage: Measured via skinfold calipers, bioelectrical impedance scales, or DEXA scans. Healthy ranges vary by sex and age.
• Waist-to-hip ratio: Divide waist by hip circumference. Higher ratios indicate more abdominal fat.
• DEXA scan: A low-dose X-ray that precisely measures bone, fat, and muscle. It is the gold standard but not always accessible.

Each has its own pros and cons in terms of cost, accuracy, and convenience.

Frequently Asked Questions

What is a healthy BMI?

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. These cutoffs come from the WHO and are based on large population studies linking BMI to mortality risk.

Does BMI account for muscle mass?

No. BMI only uses weight and height, so it cannot distinguish between muscle and fat. A bodybuilder with low body fat may have a BMI in the overweight range. If you are very muscular, your BMI may overestimate your body fat level.

Is BMI accurate for athletes?

Not really. Athletes often have higher muscle mass, which increases weight and thus BMI, even though their body fat percentage is low. For athletes, body fat percentage or waist-to-height ratio is a better indicator of health.

Is BMI different for men and women?

The same BMI categories apply to both sexes, but women naturally have a higher body fat percentage than men at the same BMI. The categories are population-level averages and do not adjust for sex, though some research suggests that slightly different cutoffs might be more appropriate for different sexes.

Can I use BMI if I’m pregnant?

No. BMI is not used to assess weight status during pregnancy because the weight gain is not due to fat alone. Instead, healthcare providers use pre-pregnancy BMI to recommend a healthy weight gain range during pregnancy.

What should I do if my BMI is high?

If your BMI is 25 or above, consider it a prompt to talk to a healthcare provider. They can assess your overall risk and suggest lifestyle changes such as increasing physical activity, improving diet, and monitoring waist circumference. Even a 5–10% weight loss can significantly improve health markers.

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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