BMI Calculator (Free — Metric & Imperial)
Use this free BMI calculator to instantly find your Body Mass Index. Works in both metric (kg and cm) and imperial (lbs, feet and inches). See your BMI category, healthy weight range for your height and personalised guidance. Once you have your result, use our free AI Health Summary below to get personalised recommendations based on your BMI, age and goals.
🤖 AI Health Summary
Enter your BMI result below — include your age, gender and any health goals — and get a personalised AI health summary with recommendations instantly.
Ask a follow-up question or request changes
How to Use the BMI Calculator
Choose metric or imperial units at the top of the calculator, enter your weight and height, then click Calculate BMI. Your result shows instantly — including your BMI score, category, healthy weight range for your height, and how much to lose or gain to reach the healthy range.
Once you have your result, scroll down to the AI Health Summary to get a personalised analysis based on your BMI, age, gender and goals.
How Is BMI Calculated?
BMI is calculated by dividing your weight in kilograms by your height in metres squared. There's also an imperial formula for those using pounds and inches.
BMI Categories
The World Health Organisation defines the following BMI categories for adults aged 18 and over:
| BMI Range | Category | Health Implication |
|---|---|---|
| Below 18.5 | Underweight | May indicate nutritional deficiency |
| 18.5 – 24.9 | Healthy Weight | Associated with lowest health risk |
| 25.0 – 29.9 | Overweight | Increased risk of some conditions |
| 30.0 – 34.9 | Obese Class I | High risk — health intervention advised |
| 35.0 and above | Obese Class II/III | Very high risk — medical advice recommended |
What Is BMI Used For?
BMI is used by healthcare professionals as a quick screening tool to identify potential weight-related health risks. It is widely used in GP surgeries, hospitals and public health monitoring to categorise populations and flag individuals who may benefit from further assessment.
BMI by Age
The standard BMI scale applies to adults aged 18 and over. The healthy range of 18.5 to 24.9 remains consistent regardless of age, though some research suggests slightly higher BMI values may be acceptable for adults over 65, where a small amount of extra weight can provide a protective buffer against illness.
For children and teenagers, BMI is measured differently — using age and gender specific percentile charts rather than fixed thresholds. This calculator is designed for adults only.
How to Reach a Healthy BMI
If your BMI is in the overweight or obese range, even modest weight loss of 5–10% of your body weight can significantly improve your health markers.
- Aim for at least 150 minutes of moderate exercise per week — brisk walking, cycling or swimming all count
- Reduce ultra-processed foods and increase protein and vegetable intake
- A moderate calorie deficit of 300–500 calories per day leads to sustainable weight loss
- Improving sleep quality supports healthy weight management
- Always consult your GP before making significant changes to diet or exercise
Limitations of BMI
BMI is a useful screening tool but it has well-documented limitations. It does not directly measure body fat — it only uses height and weight, which means it can misclassify some individuals:
- Athletes and people with high muscle mass may have a high BMI despite being very lean
- BMI doesn't account for where fat is stored — abdominal fat carries higher health risk than fat stored elsewhere
- It doesn't account for age-related changes in body composition
- BMI thresholds may not apply equally across different ethnicities — some health organisations use lower thresholds for South Asian populations
Use BMI as a general guide alongside other health measures such as waist circumference, body fat percentage and blood test results for a fuller picture of your health.
FAQs
What is a healthy BMI?
A BMI between 18.5 and 24.9 is considered healthy for most adults. However healthy BMI ranges can vary slightly by age and ethnicity.
What is the difference between BMI and body fat percentage?
BMI is a simple calculation based on height and weight. Body fat percentage measures the actual proportion of fat in your body and is more accurate but requires specialist equipment.
Does BMI work for children?
This calculator is designed for adults. Children and teenagers should use age and gender specific BMI charts as healthy ranges differ during growth stages.
Can I use this in pounds and feet?
Yes — simply toggle to Imperial at the top of the calculator and enter your weight in pounds and height in feet and inches.
Is BMI accurate?
BMI is a widely used screening tool but has limitations. For a fuller picture of your health use it alongside other measures like waist circumference and body fat percentage.
What BMI is considered obese?
A BMI of 30 or above is classified as obese. This is further divided into obese class 1 (30-34.9), obese class 2 (35-39.9) and obese class 3 (40 and above, sometimes called severely or morbidly obese). At these levels the health risks associated with excess weight increase significantly.
Can I get a personalised health summary based on my BMI?
Yes — scroll down to the AI Health Summary below the calculator. Enter your BMI result along with your age, gender and goals to get a personalised analysis with practical recommendations instantly.
Is this calculator free?
Yes completely free with no sign-up needed.