BMI Calculator – Check Your Body Mass Index Free (kg & lb)
Measure — Body
BMI Calculator
Readout
Working out your Body Mass Index by hand means remembering a formula, converting units, and doing the division correctly — easy to get wrong, especially when switching between pounds/inches and kilograms/centimeters. This free calculator handles both unit systems and gives you an instant, accurate BMI reading with its standard category.
What Is BMI and How Is It Calculated?
Body Mass Index is a simple ratio of weight to height, used as a quick population-level screening tool to categorize weight status. The formulas are:
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = [weight (lb) ÷ height (in)²] × 703
The 703 multiplier in the imperial formula exists purely to convert pounds and inches into the equivalent of the metric kilograms-per-square-meter unit, so both formulas produce the same BMI number for the same person regardless of which unit system is used.
Standard BMI Categories for Adults
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These ranges are the standard adult classification used by major public health bodies, including the CDC's adult BMI guidance, and are the same category thresholds this calculator applies to your result.
Why "How Much Should I Weigh for My Height" Isn't a Simple Question
BMI is widely used because it's fast and requires only two measurements, but it has well-documented limitations that are worth understanding before treating the number as a complete health picture:
- It doesn't distinguish muscle from fat. A muscular athlete can register as "overweight" or even "obese" by BMI despite having low body fat, because muscle is denser than fat tissue.
- It doesn't account for fat distribution. Two people with identical BMI can have very different health risk profiles depending on whether weight is carried around the waist versus the hips.
- It's a population-level tool, not an individual diagnosis. BMI was originally developed for statistical studies across large groups, not as a precise individual health assessment.
- Age, sex, and ethnicity affect interpretation. Some health organizations apply adjusted BMI thresholds for certain populations, since body composition norms vary.
For these reasons, BMI is best treated as a quick screening starting point — useful for tracking trends over time — rather than a definitive verdict on health, and any concerns about weight or body composition are worth discussing with a doctor who can consider the fuller picture.
Tracking BMI Over Time vs. a Single Reading
A single BMI reading is a snapshot; a series of readings over weeks or months tells a more useful story. If you're tracking progress toward a health goal, note your BMI (or simply your weight, since height doesn't change) at consistent intervals — weekly or monthly — rather than daily, since normal water-weight fluctuation can vary a reading by a pound or more day to day without reflecting any real change in body composition.
Frequently Asked Questions
Is BMI accurate for athletes and very muscular people?
Not particularly. Because muscle weighs more than fat by volume, muscular individuals often score in the "overweight" range despite having low body fat. Athletes are generally better assessed using body fat percentage or other composition measures alongside BMI.
Does BMI work the same way for children?
No — children and teens are assessed using age- and sex-specific BMI percentiles rather than the fixed adult categories shown above, since body composition changes significantly during growth. This calculator is intended for adult use.
What's a healthy BMI range for most adults?
18.5 to 24.9 is generally classified as the normal weight range for adults, according to standard public health guidelines, though individual health depends on many factors beyond BMI alone.
Can I calculate BMI in pounds and inches instead of kilograms and centimeters?
Yes — switch the "Units" dropdown above to "Imperial (lb / in)" and the calculator automatically applies the correct formula for those units.
Is my height and weight data stored anywhere?
No. The calculation happens entirely in your browser using JavaScript. Nothing is uploaded, logged, or saved to any server.
Why did my doctor use a different measurement than BMI?
Many healthcare providers now combine BMI with other measurements — such as waist circumference, waist-to-hip ratio, or body fat percentage — to get a more complete picture of health risk, since BMI alone doesn't account for fat distribution or muscle mass.
Try the calculator above, or explore our other free tools: Age Calculator, Word Counter, Percentage Calculator, and Text Case Converter.
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