BMI Calculator – Free Body Mass Index Checker
Calculate your Body Mass Index, BMI Prime, Ponderal Index, and get your healthy weight range. Supports metric (kg/cm) and imperial (lb/in) units. No sign‑up, completely private.
What is BMI (Body Mass Index)?
BMI is a measurement used to estimate body fat based on a person's weight and height. It provides a quick, easy‑to‑understand number that classifies individuals as underweight, normal weight, overweight, or obese. The World Health Organization (WHO) uses BMI to track weight‑related health risks at a population level. For adults, a healthy BMI ranges from 18.5 to 24.9. While BMI doesn't directly measure body fat, it is a useful screening tool and a starting point for further health assessments.
How to Calculate BMI (Formula & Examples)
The formula for BMI is straightforward:
- Metric: BMI = weight (kg) / [height (m)]²
- Imperial: BMI = 703 × weight (lbs) / [height (in)]²
For example, a person weighing 70 kg with a height of 1.70 m has a BMI of 70 / (1.70 × 1.70) = 24.2, which falls in the normal weight range. Our calculator handles all the math instantly, converting between units as needed.
BMI Categories & Ranges
- Underweight: BMI less than 18.5
- Normal weight: BMI 18.5 – 24.9
- Overweight: BMI 25 – 29.9
- Obese Class I: BMI 30 – 34.9
- Obese Class II: BMI 35 – 39.9
- Obese Class III (Severe obesity): BMI 40 and above
These ranges are the same for men and women, though individual body composition (muscle mass, bone density) can affect accuracy. The tool also provides BMI Prime (BMI divided by 25) and the Ponderal Index (weight / height³) for a more three‑dimensional perspective.
Why Use Nyxbo's BMI Calculator?
- 100% free & private – No sign‑up, no data collection. All calculations happen in your browser.
- Dual units – Switch between metric (kg/cm) and imperial (lbs/in) with one click.
- Extra metrics – Get BMI Prime, Ponderal Index, and your personal healthy weight range.
- Works offline – After the page loads, you can keep using the calculator without an internet connection.
- Evidence‑based – Follows WHO and CDC guidelines for weight classification.
Frequently Asked Questions
How is BMI calculated?
BMI = weight (kg) / height² (m²). For imperial units: BMI = 703 × weight (lbs) / height² (in²). Our tool does the conversion automatically.
What is a healthy BMI range?
According to the WHO, a healthy BMI for adults is between 18.5 and 24.9.
Is BMI different for men and women?
The calculation is the same, but healthy body fat percentages differ. BMI doesn't distinguish between muscle and fat; athletes may have a high BMI but low body fat.
Can I use BMI for children?
BMI for children is calculated the same way, but interpreted using age‑ and sex‑specific percentiles. Our adult calculator is designed for ages 20 and above. For children, consult a pediatrician or a child BMI calculator.
What is BMI Prime?
BMI Prime is the ratio of your BMI to the upper limit of the normal range (25). A value < 0.74 = underweight, 0.74–1.0 = normal, > 1.0 = overweight/obese. It's a single number that shows how close you are to the healthy limit.
What is the Ponderal Index?
The Ponderal Index (PI) is weight divided by height cubed (kg/m³). It is used sometimes for more accurate body fat assessment, especially in very tall or short individuals.
Is BMI accurate for everyone?
BMI is a screening tool, not a diagnostic tool. It doesn't account for muscle mass, bone density, or fat distribution. Athletes may have a high BMI but low body fat, and older adults may have a normal BMI but higher body fat.
How do I know my healthy weight range?
Our calculator shows your personal healthy weight range for your height based on a BMI of 18.5–24.9. For example, if you're 170 cm tall, your healthy weight range is roughly 53.5–72 kg.
Does this tool collect my data?
No. All calculations are performed locally in your browser. Weight, height, and results are never transmitted or stored.
Can I use this BMI calculator offline?
Yes. Once the page is loaded, the calculator works without an internet connection.
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