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BMI Calculator

Calculate your Body Mass Index using metric or imperial measurements.

Your Measurements

kg
cm

Your BMI

Body Mass Index
—
Under 18.518.5–24.925–29.930+
UnderweightBelow 18.5
Healthy weight18.5 – 24.9
Overweight25 – 29.9
Obese30 or above

BMI is a general indicator and doesn't account for muscle mass, bone density, age, or sex. Consult a healthcare professional for personalised advice.

Frequently Asked Questions

BMI = weight in kg ÷ height in metres squared. For example, 75kg at 175cm (1.75m): 75 ÷ (1.75 × 1.75) = 24.5 BMI.
A BMI between 18.5 and 24.9 is considered healthy for most adults. Under 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese.
BMI is a useful screening tool but has limitations. It doesn't distinguish between muscle and fat, and may overestimate risk in muscular people and underestimate it in those with low muscle mass. Always consult a healthcare professional.

Understanding BMI

Body Mass Index (BMI) is a simple ratio of weight to height, used as a screening tool to categorise weight status. The formula is: BMI = weight (kg) ÷ height (m)².

Worked Example

A person weighing 75 kg at 175 cm (1.75 m) tall: BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5 — within the healthy range.

In imperial: 12 stone (168 lb) at 5'9" (69 inches): BMI = (168 × 703) ÷ (69 × 69) = 118,104 ÷ 4,761 = 24.8.

BMI Categories

Under 18.5: Underweight — may indicate nutritional deficiency or an underlying health condition.

18.5 – 24.9: Healthy weight — associated with the lowest health risks for most people.

25 – 29.9: Overweight — may increase risk of heart disease, type 2 diabetes, and high blood pressure.

30+: Obese — significantly increased health risks. Subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (40+).

Limitations of BMI

BMI doesn't distinguish between muscle and fat. A muscular athlete may have a "overweight" BMI despite having very low body fat. It also doesn't account for age, sex, ethnicity, or where fat is distributed (abdominal fat is more dangerous than fat elsewhere). BMI is a useful starting point, but it's not a complete picture of health. Always consult a healthcare professional for personalised advice.

Related Calculators

Use the unit converter to convert between kg and stone/pounds. The cooking converter can help with portion sizes if you're managing your diet.

KwikSum's BMI calculator works in both metric (kilograms and centimetres) and imperial (stone, pounds, feet, and inches). Enter your measurements to see your BMI and which category it falls into, along with a visual scale showing where you sit.