BMI Calculator

Use this free BMI calculator to check your Body Mass Index instantly. Enter your height and weight to find out whether you are underweight, a healthy weight, overweight, or obese — with a full explanation of what your result means and what to do next.

BMI Calculator

Check your Body Mass Index using metric or imperial units.

Underweight: Below 18.5
Normal: 18.5–24.9
Overweight: 25–29.9
Obese: 30+

How to Use the BMI Calculator

  1. Select your preferred unit — metric (kg/cm) or imperial (lbs/ft)
  2. Enter your height
  3. Enter your weight
  4. Click Calculate — your BMI score and category appear instantly

What Is BMI?

BMI (Body Mass Index) is a widely used screening measure that estimates whether your weight is appropriate for your height. It is calculated by dividing your weight in kilograms by the square of your height in metres. While BMI does not directly measure body fat, it is a reliable indicator of weight-related health risks for most adults and is used by doctors and health professionals worldwide.

BMI Formula

BMI = Weight (kg) ÷ Height² (m)

Example: Weight = 75 kg, Height = 1.80 m
BMI = 75 ÷ (1.80 × 1.80) = 75 ÷ 3.24 = 23.1 (Healthy weight)

For imperial measurements: BMI = (Weight in lbs × 703) ÷ Height in inches²

BMI Categories (Adults)

BMI RangeCategoryHealth Risk
Below 18.5UnderweightNutritional deficiency, bone loss risk
18.5 – 24.9Healthy weightLowest health risk
25.0 – 29.9OverweightModerate health risk
30.0 – 34.9Obese (Class I)High health risk
35.0 – 39.9Obese (Class II)Very high health risk
40.0 and aboveSeverely obese (Class III)Extremely high health risk

What Does My BMI Result Mean?

Once you have your BMI score, here is what each category means for your health:

  • Underweight (below 18.5): You may not be getting enough nutrients. Speak to your GP if you’re struggling to maintain weight.
  • Healthy weight (18.5–24.9): Your weight is in the recommended range. Maintain it through a balanced diet and regular activity.
  • Overweight (25–29.9): You are carrying excess weight that may increase the risk of type 2 diabetes, high blood pressure, and heart disease.
  • Obese (30+): Your weight poses a significant health risk. Speaking to a healthcare professional about a weight management plan is strongly recommended.

BMI for Men vs Women

The standard BMI ranges apply to both men and women. However, women naturally carry more body fat than men at the same BMI due to hormonal and physiological differences. For example, a woman with a BMI of 23 may have a higher body fat percentage than a man with the same BMI — yet both fall within the healthy range. For a more detailed breakdown, use our body fat calculator.

BMI for Children and Teenagers

For children and teenagers (ages 2–18), BMI is interpreted differently. Instead of fixed ranges, BMI is compared to other children of the same age and sex using percentile charts. A child’s BMI is considered healthy if it falls between the 5th and 85th percentile for their age group. Always consult a paediatrician for children’s weight assessments.

Limitations of BMI

BMI is a useful starting point but has important limitations that you should be aware of:

  • Muscle mass: Athletes and muscular people often have a high BMI despite very low body fat — muscle weighs more than fat
  • Age: Older adults tend to have more body fat at the same BMI as younger adults
  • Ethnicity: Some ethnic groups face higher health risks at lower BMI thresholds — for example, health risks for people of South Asian descent begin at a lower BMI
  • Pregnancy: BMI is not a reliable measure during pregnancy and should not be used
  • Height extremes: Very tall or very short individuals may get misleading results

Always use BMI alongside other health indicators such as waist circumference, body fat percentage, blood pressure, and cholesterol levels for a complete health picture.

How to Improve Your BMI

If your BMI falls outside the healthy range, there are practical steps you can take:

  • For weight loss: Create a modest calorie deficit (500 kcal/day) through diet and exercise — use our calorie calculator to find your targets
  • For weight gain: Increase calorie intake with nutrient-dense foods and add strength training to build lean muscle
  • Track progress: Recheck your BMI every 4–6 weeks as you make changes
  • Seek guidance: A GP, dietitian, or certified personal trainer can create a personalised plan for your situation

BMI Calculator FAQ

What is a healthy BMI for women?

A healthy BMI for women is between 18.5 and 24.9, the same range as for men. However, women typically carry more body fat at the same BMI due to natural physiological differences, so pairing BMI with a body fat percentage check gives a more complete picture.

What BMI is considered obese?

A BMI of 30 or above is classified as obese, split into Class I (30–34.9), Class II (35–39.9), and Class III or severe obesity (40 and above). Each level carries progressively higher health risks.

Is BMI accurate for athletes?

Not always. Athletes with high muscle mass often have an elevated BMI that suggests overweight or obese, despite having very low body fat. In these cases, a body fat calculator gives a far more accurate picture of health and fitness.

How do I lower my BMI?

BMI decreases when you reduce your weight relative to your height. This is best achieved through a combination of a balanced, calorie-controlled diet and regular physical activity. Use our calorie calculator to find your daily calorie targets and our BMR calculator to understand your metabolism.

How often should I check my BMI?

If you are actively trying to change your weight, checking your BMI every 4–6 weeks is enough to track meaningful progress. Daily weigh-ins can be misleading due to natural fluctuations in water weight.

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