BMI Calculator

Calculate your Body Mass Index (BMI) instantly

Your Body Mass Index (BMI) is a simple measure that uses your height and weight to estimate whether you are in a healthy weight range. This free BMI calculator works for both metric (kilograms and centimetres) and imperial (pounds, feet and inches) measurements, shows your BMI category according to WHO guidelines, and tells you the healthy weight range for your height.

How BMI is calculated

The formula is straightforward: BMI = weight (kg) ÷ height (m)². For example, if you weigh 70 kg and are 175 cm tall (1.75 m), your BMI is 70 ÷ (1.75 × 1.75) = 22.9, which falls in the normal weight range. If you use imperial units, the calculator converts pounds and feet/inches to metric internally before applying the same formula.

BMI categories explained

The World Health Organization classifies adult BMI as follows: under 18.5 is underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or above is obesity (subdivided into Classes I, II and III). This calculator displays your category alongside a colour-coded scale so you can see where you stand at a glance.

Limitations of BMI

BMI is a useful screening tool for most adults but it does not directly measure body fat. It may overestimate body fat in muscular athletes and underestimate it in older adults who have lost muscle mass. It is also not intended for pregnant women or children. For personal health decisions, always consult a qualified healthcare professional.

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How the formula works step by step

BMI compares your weight to your height squared. Squaring height in the denominator stops tall people from being penalised purely for being tall — a heavier build is more expected when someone is 190 cm than when they are 150 cm.

BMI = weight (kg) ÷ [height (m)]²

In metric, convert centimetres to metres first: 175 cm = 1.75 m. Then square that height: 1.75 × 1.75 = 3.0625 m². Divide weight by that number.

In imperial, this calculator converts pounds to kilograms (1 lb = 0.453592 kg) and feet/inches to metres (1 inch = 2.54 cm), then applies the same formula. You do not need to convert manually — but knowing the steps helps if you are checking a result on paper.

From BMI back to a healthy weight range

To find the weight that corresponds to a target BMI, rearrange the formula:

weight (kg) = BMI × [height (m)]²

For a healthy BMI between 18.5 and 24.9, calculate both ends. That gives you the weight band shown in the results panel for your height.

Worked examples

Common mistakes to watch for

• Entering height in the wrong unit. Typing 175 into a feet field, or 5.9 meaning "five foot nine" as a decimal, produces nonsense results. Use cm for metric or separate feet and inches for imperial.
• Comparing BMI across different standards. WHO adult cut-offs (18.5 / 25 / 30) are what this tool uses. Some Asian health authorities suggest lower overweight thresholds (around 23–24) because health risks can rise at lower BMI in those populations.
• Using adult BMI for children. Paediatric BMI is plotted on age- and sex-specific growth charts. A BMI of 19 can be healthy for a 14-year-old and underweight for a 40-year-old.
• Treating BMI as a body-fat percentage. BMI is a weight-to-height index. Two people with identical BMI can have very different body-fat levels depending on muscle, age and sex.

When to use this calculator

Use this tool when you want a quick screening number from height and weight — checking whether you fall inside the normal band, comparing your current weight to the healthy range for your height, or converting imperial measurements without doing the arithmetic yourself.

It is not a substitute for medical assessment. If you need to convert stones to kilograms or inches to centimetres without calculating BMI, use the Unit Converter. If you are tracking body composition over time, pair BMI with other measures (waist size, strength, how you feel) rather than relying on the index alone.

Because everything runs on your device, there's nothing to install and nothing is uploaded. Your measurements stay completely private, and the calculator works even when you're offline. This is one of many free browser tools on Uzetool — explore more tools as we add them.

Disclaimer: This BMI calculator is for informational purposes only and is not medical advice. Always consult a healthcare provider for health-related decisions.