PureTools
BMI Calculator
Calculate your Body Mass Index instantly with WHO standards, animated gauge, healthy weight range, exact distance from normal, metric and imperial units. Fully private, runs entirely in your browser.
Enter your weight and height to see your BMI.
How to use the BMI Calculator
- Pick your unit system: Imperial is the default for the English site, or switch to Metric for kilograms and centimeters. Switching converts your values automatically.
- Enter your weight and height. The tool calculates your BMI live as you type.
- Read the animated gauge: the needle shows where you fall on the WHO spectrum, and the colored card below highlights your current category.
- Check your healthy weight range and the distance to normal to know exactly what gain or loss would put you in the healthy bracket.
- Use Copy summary to grab a clean text recap, or Reset to start over.
What is BMI?
Body Mass Index is a quick screening tool used by the World Health Organization and most healthcare systems worldwide. It estimates whether a person's weight is in a healthy range for their height. While BMI is useful at the population level, it has known limitations on the individual level: it does not distinguish between fat and muscle, and does not account for fat distribution, age, sex, or ethnicity. Always interpret BMI alongside other health markers.
Special considerations
- Adults only: This calculator uses adult BMI thresholds and does not classify anyone under 18.
- Older adults (65+): Several large studies suggest that the optimal BMI range may shift slightly upward with age. A BMI of 25–27 in someone over 70 is not necessarily concerning. Discuss with a healthcare provider if you are unsure.
- Athletes and very muscular builds: Muscle is denser than fat, so BMI can overestimate body fat in athletes. Body fat percentage or waist-to-hip ratio gives a better picture in those cases.
- Pregnancy and post-partum: Standard BMI does not apply during pregnancy. Use a tool dedicated to gestational weight monitoring instead.
BMI categories (WHO)
- Underweight (< 18.5): May indicate insufficient nutritional intake. Discuss with a healthcare provider, especially if rapid weight loss occurred.
- Normal weight (18.5 – 24.9): Associated with the lowest health risk for most adults aged 20 and over.
- Overweight (25 – 29.9): Mildly elevated risk of type 2 diabetes, hypertension, and cardiovascular disease. Even modest weight loss (5–10%) measurably reduces these risks.
- Obese Class I (30 – 34.9): Moderate risk. A structured plan combining nutrition, activity and medical follow-up is generally recommended.
- Obese Class II (35 – 39.9): High risk. Medical intervention and structured support are strongly recommended.
- Obese Class III (≥ 40): Very high risk. A comprehensive medical evaluation should be a priority.
Frequently asked questions
What is BMI and how is it calculated?
Body Mass Index (BMI) is a screening tool used by the World Health Organization to categorize weight relative to height. The metric formula divides weight in kilograms by height in meters squared (kg/m²). The imperial formula is 703 × weight(lb) / height(in)². It is a quick proxy for population-level health screening, not a direct measure of body composition.
What are the WHO BMI categories?
The World Health Organization defines six adult categories: Underweight (below 18.5), Normal weight (18.5 to 24.9), Overweight (25 to 29.9), Obese Class I (30 to 34.9), Obese Class II (35 to 39.9), and Obese Class III (40 and above). These thresholds apply to adults aged 20 and older and may be adjusted by clinicians for specific populations.
Why does the calculator ask for sex and age?
Sex and adult age refine the interpretation, not the number. The BMI formula itself does not change. However, body composition differs between men and women, and adults aged 65 or older may need extra context. When you provide these inputs, the tool adds tailored notes such as waist-risk context or older-adult guidance. All inputs remain optional and stay on your device.
Is BMI accurate for athletes or older adults?
BMI does not distinguish between fat and lean mass, so muscular athletes can score in the overweight range despite low body fat. For older adults, slightly higher BMI ranges have been associated with reduced mortality in some studies. BMI is best interpreted alongside waist circumference, body fat percentage, and a clinical assessment from a healthcare provider. To estimate daily calorie and macro needs based on your stats, see the Calorie Calculator.
How is the healthy weight range calculated?
The healthy weight range is computed by applying the lower (18.5) and upper (24.9) bounds of the normal BMI category to your specific height. For example, a 175 cm person has a healthy weight range of approximately 56.7 to 76.3 kg. The exact figure depends only on your height, not on your current weight.
Is my data sent anywhere?
No. The entire calculation runs in your browser using vanilla JavaScript. Nothing you type is transmitted, logged or stored on any server. Your last entered values are saved only to your local browser storage so the tool remembers them on your next visit; you can reset them at any time using the Reset button. Your data is never used to train AI models or improve machine learning systems.
How often should I recalculate my BMI?
BMI is a snapshot, not a continuous signal. Recalculating every four to eight weeks is sufficient for most adults who are monitoring gradual changes in body weight. More frequent calculations rarely add useful information because the scale can shift several kilograms within a single day from food, hydration and clothing. For clinical tracking, healthcare providers typically compare BMI at regular physical examinations spaced several months apart. If you are also tracking calorie intake alongside weight changes, the Calorie Calculator gives a complementary view of your daily energy balance.