BMI Calculator
Calculate your Body Mass Index, healthy weight range, and more — instantly
What Is BMI?
Body Mass Index (BMI) is a screening measure that estimates whether your weight is healthy relative to your height. It's widely used by health organizations like the WHO and CDC because it's quick, free, and correlates with health risk across large populations — though it doesn't measure body fat directly.
How to Use
- Choose Metric or Imperial units
- Enter your weight and height
- Optionally add your waist measurement in Advanced Options
- Click "Calculate BMI" to see your results
BMI Formula
BMI = weight (kg) ÷ height (m)²
For imperial units: BMI = 703 × weight (lb) ÷ height (in)². This calculator converts imperial inputs to metric automatically and uses the same underlying formula either way.
Explanation of Every Result
Your BMI
Your calculated Body Mass Index value.
Category
Which WHO weight-status band your BMI falls into — Underweight, Normal, Overweight, or Obese.
BMI Prime
Your BMI divided by 25 (the upper bound of "normal"). A value under 1.0 means you're under that threshold; over 1.0 means you're above it.
Healthy Weight Range
The weight range (at your entered height) that corresponds to a BMI of 18.5–24.9 — useful context, not a personal target.
Waist-to-Height Ratio
Only shown if you enter a waist measurement; compares waist circumference to height as an additional indicator alongside BMI.
BMI Categories (WHO Reference)
| BMI Range | Category |
|---|---|
| Below 16.0 | Severe Thinness |
| 16.0 – 16.9 | Moderate Thinness |
| 17.0 – 18.4 | Mild Thinness |
| 18.5 – 24.9 | Normal Weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Obese (Class III) |
Limitations of BMI
- Doesn't distinguish between muscle mass and fat mass.
- Can be misleading for athletes, bodybuilders, and very muscular individuals.
- Doesn't account for age, sex, or ethnicity-specific differences in body composition.
- Not designed for children, teens, or pregnant women.
- Doesn't indicate where fat is distributed on the body, which also affects health risk.
Waist-to-Height Ratio Explained
Waist-to-height ratio (WHtR) divides your waist circumference by your height. Some researchers consider it a useful companion to BMI because it reflects abdominal fat specifically, which is more strongly linked to certain health risks than overall body weight alone. A commonly cited guideline is keeping your waist circumference under half your height.
Why BMI Is Still Widely Used
Despite its limitations, BMI remains popular because it requires only two simple, easily-measured numbers and correlates reasonably well with health risk at a population level. Health professionals typically use it as a starting point alongside other measurements — not as a standalone diagnosis.
Common Mistakes to Avoid
- Treating BMI as a precise measure of body fat rather than a general screening tool.
- Comparing your BMI directly to an athlete's without accounting for muscle mass.
- Using adult BMI categories to assess a child's or teen's weight status.
- Making major health decisions based on BMI alone without professional guidance.
A Note on Health and Weight
BMI is one data point, not a verdict. Overall health depends on many factors — nutrition, physical activity, sleep, muscle mass, genetics, and mental wellbeing among them. If your BMI result concerns you, or if you have questions about your weight or health, a doctor or registered dietitian can offer guidance tailored to your individual situation.
Worked Examples
Underweight Example
Weight: 50 kg
Height: 170 cm
BMI: 17.3
Category: Underweight
A BMI below 18.5 may warrant a conversation with a healthcare provider about nutrition and overall health, especially if unintentional.
Sarah's BMI (Normal Weight)
Weight: 70 kg
Height: 175 cm
BMI: 22.9
Category: Normal weight
Falls within the standard 18.5–24.9 "normal weight" range used by the WHO and CDC.
Overweight Example
Weight: 85 kg
Height: 175 cm
BMI: 27.8
Category: Overweight
A BMI of 27.8 falls in the overweight range (25–29.9), which is often used as a general prompt to review diet and activity habits with a professional.
Obese (Class I) Example
Weight: 100 kg
Height: 170 cm
BMI: 34.6
Category: Obese (Class I)
A BMI at or above 30 is classified as obese and is generally associated with a higher risk of several health conditions — a good reason to check in with a healthcare provider.
Athletic Build (BMI Limitation Example)
Weight: 90 kg
Height: 180 cm
BMI: 27.8
Category: Overweight (by BMI only)
This BMI lands in the 'overweight' category, but a muscular, low-body-fat athlete at this weight and height may actually be quite lean — illustrating why BMI alone can be misleading for very muscular people.
Imperial Units Example
Weight: 160 lb
Height: 5 ft 7 in
BMI: 25.1
Category: Overweight (borderline)
Shows the same formula applied after converting pounds to kilograms and feet/inches to centimeters — the calculator does this conversion for you automatically.
Frequently Asked Questions
Is BMI accurate for everyone?
BMI is a useful screening tool for most adults, but it doesn't distinguish between muscle and fat, so it can be misleading for athletes, bodybuilders, pregnant women, and older adults with reduced muscle mass. It's a starting point for a conversation with a healthcare provider, not a diagnosis on its own.
What is a healthy BMI range?
For most adults, a BMI between 18.5 and 24.9 falls into the "normal weight" range used by the WHO and CDC. Healthy ranges can vary somewhat by population and individual body composition, so treat this as a general guideline rather than an exact target.
What is BMI Prime?
BMI Prime is your BMI divided by the upper limit of the normal range (25), showing how close you are to that threshold as a ratio. A BMI Prime of 1.0 means you're exactly at the boundary between "normal" and "overweight"; above 1.0 means you're over it, below means under.
Can I use this calculator for children or teens?
No — this calculator is designed for adults. Children and teens grow at different rates, so their BMI is interpreted using age- and sex-specific percentile charts rather than the fixed adult categories used here. Ask a pediatrician for a child-appropriate assessment.
Why might BMI be misleading for athletes?
Muscle is denser than fat, so a muscular, low-body-fat athlete can register a BMI in the "overweight" range despite having a healthy or even low body fat percentage. In cases like this, body fat percentage or waist measurements are more informative than BMI alone.
What is waist-to-height ratio, and why does it matter?
Waist-to-height ratio (WHtR) compares your waist circumference to your height, and is considered by some researchers to be a better indicator of abdominal fat and related health risk than BMI alone, since it accounts for where fat is carried rather than just overall weight.
Does BMI mean the same thing for men and women?
The same BMI formula and category thresholds are used for both sexes, though men and women naturally carry different amounts of muscle and fat at the same BMI. This is one reason BMI is best used alongside other measurements rather than in isolation.
Does BMI account for age?
The standard adult BMI categories don't adjust for age, even though body composition naturally shifts — often toward more fat and less muscle — as people get older. Some clinicians apply age-adjusted judgment when interpreting BMI in older adults.
How is BMI different across ethnic groups?
Research has shown that health risks associated with a given BMI can differ by ethnicity — for example, some Asian populations show increased health risks at lower BMI thresholds than the standard categories suggest. Some health organizations use adjusted cutoffs for specific populations.
Is BMI used in clinical settings?
Yes — BMI remains a widely used first-pass screening tool in clinical practice because it is quick, free, and correlates with health risk at a population level, even though clinicians typically combine it with other measurements for individual assessments.
How often should I check my BMI?
There's no strict rule, but checking periodically (such as during routine health checkups) is generally more useful than frequent tracking, since natural day-to-day weight fluctuations don't reflect meaningful changes in health.
What should I do if my BMI is outside the normal range?
A BMI outside the 18.5–24.9 range is worth discussing with a healthcare provider, who can consider your full health picture — including muscle mass, waist measurements, family history, and lab results — rather than relying on BMI alone.
Does this calculator support pounds and feet/inches?
Yes — use the Metric / Imperial toggle above the form to switch between kilograms and centimeters, or pounds and feet/inches, and the calculator will convert automatically.
What is the difference between BMI and body fat percentage?
BMI estimates weight status from height and weight alone, while body fat percentage measures the actual proportion of your body composed of fat tissue, typically via skinfold calipers, bioelectrical impedance, or DEXA scans. Body fat percentage is more precise but requires specialized measurement.
Can BMI change quickly?
BMI reflects your current weight and height at the moment you calculate it, so it changes as your weight changes — but meaningful, sustained changes in body composition typically happen gradually rather than overnight.
Conclusion
BMI offers a quick, free way to understand where your weight falls relative to your height, but it's most useful as one piece of a bigger health picture rather than a final answer. Use this calculator to check your numbers, then talk to a healthcare professional about what they mean for you specifically.