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.
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How to Use the BMI Calculator
Choose metric or imperial units, enter your weight and height, then click Calculate. Your BMI, category, healthy weight range and how much to lose or gain will show instantly.
How Is BMI Calculated?
BMI is calculated by dividing your weight in kilograms by your height in metres squared.
Metric formula: BMI = Weight (kg) ÷ Height² (m²)
Imperial formula: BMI = 703 × Weight (lbs) ÷ Height² (inches²)
For example someone who weighs 70kg and is 1.75m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9 — which is in the healthy range.
BMI Categories
- Underweight — below 18.5
- Healthy Weight — 18.5 to 24.9
- Overweight — 25 to 29.9
- Obese — 30 to 34.9
- Severely Obese — 35 and above
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 used in GP surgeries, hospitals and public health monitoring to categorise populations and flag individuals who may benefit from further health assessment. While it is not a diagnostic tool on its own, a BMI outside the healthy range can indicate an increased risk of conditions including type 2 diabetes, heart disease, high blood pressure and joint problems.
BMI by Age
The standard BMI scale applies to adults aged 18 and over. For adults, 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 older adults over 65 where a small amount of extra weight can provide a protective buffer. For children and teenagers, BMI is measured differently using age and gender specific percentile charts rather than fixed thresholds.
How to Reach a Healthy BMI
If your BMI is in the overweight or obese range, even modest weight loss of 5 to 10 percent of your body weight can significantly improve your health. The most sustainable approach combines a moderate calorie deficit with regular physical activity. Aim for at least 150 minutes of moderate exercise per week such as brisk walking, cycling or swimming. Reducing ultra-processed foods, increasing protein and vegetable intake and improving sleep quality all support healthy weight management. Always consult your GP before making significant changes to your diet or exercise routine.
If your BMI is in the underweight range, focus on increasing calorie intake through nutritious foods rather than processed options. A GP or registered dietitian can provide personalised guidance.
Limitations of BMI
BMI is a useful screening tool but doesn’t directly measure body fat. Athletes with high muscle mass may have a high BMI despite being very lean. BMI also doesn’t account for age, gender differences in body composition, or where fat is distributed in the body. Use BMI as a general guide alongside other health measures.
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.