BMI Calculator
Calculate Body Mass Index and understand your healthy weight range.
About the BMI Calculator
This BMI (Body Mass Index) calculator estimates body fatness based on height and weight using the standard Quetelet formula adopted by the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC). Enter your weight and height, choose metric (kg/cm) or imperial (lb/in) units, and the tool returns your BMI value, the WHO weight category, and the healthy weight range for your height.
BMI is the most widely used screening tool for overweight and obesity because it is simple, free, and correlates reasonably well with body fat in populations. However, it is a screening measure, not a diagnostic one — it does not directly measure body fat and can misclassify certain individuals (athletes with high muscle mass, older adults with reduced muscle, pregnant women, children). Use it as a starting point, not as the final word on your health.
The WHO classification thresholds used here are: underweight (BMI < 18.5), normal weight (18.5-24.9), overweight (25.0-29.9), obese class I (30.0-34.9), obese class II (35.0-39.9), and obese class III (≥ 40.0). These thresholds apply to adults aged 20+ and are based on Caucasian populations; some Asian populations face elevated health risks at lower BMIs (23-27), prompting WHO to suggest Asian-specific cut-offs.
How It Works
The BMI formula is:
BMI = weight (kg) / height (m)^2In imperial units, the formula becomes: BMI = 703 * weight (lb) / height (in)^2. Our calculator converts imperial inputs to metric internally and uses the standard metric formula for consistency.
The WHO classification thresholds applied:
- < 18.5: Underweight
- 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: Obese Class III
The healthy weight range for a given height is computed as 18.5 * h^2 to 24.9 * h^2, where h is height in meters. For a 175 cm (1.75 m) adult, this is 56.6 - 76.2 kg.
Worked Examples
Suppose you weigh 70 kg and are 175 cm tall.
- Height in meters: 175 / 100 = 1.75 m
- BMI = 70 / (1.75 * 1.75) = 70 / 3.0625 = 22.9
- Category: Normal weight (18.5 - 24.9)
- Healthy weight range for 175 cm: 56.6 - 76.2 kg
Now consider a 90 kg person at the same height: BMI = 90 / 3.0625 = 29.4 (Overweight, just below the obesity threshold). At 95 kg, BMI = 31.0 (Obese Class I).
In imperial: a 154 lb person at 5'9" (69 in): BMI = 703 * 154 / (69 * 69) = 108262 / 4761 = 22.7 (Normal weight).
When to Use This Tool
Use this BMI calculator when:
- Screening yourself for overweight or obesity risk during a routine health check
- Tracking changes in body composition over time as part of a weight-management program
- Estimating whether you fall within the healthy weight range for your height
- Completing medical intake forms or insurance applications that require BMI
- Comparing your BMI to population averages (U.S. adult median is about 29)
- Educational use in health classes demonstrating the relationship between height, weight, and body composition
- Setting a target BMI for weight-loss or weight-gain goals
BMI is a screening tool, not a diagnostic. A high BMI in a muscular athlete does not indicate excess fat, and a normal BMI in an older adult may mask age-related muscle loss. For more accurate body composition, use our Body Fat Calculator or get a DEXA scan.
Limitations & Disclaimer
This BMI calculator uses the standard Quetelet formula and WHO classification thresholds for adults aged 20+. It does not distinguish muscle from fat, does not account for body composition distribution (visceral vs subcutaneous fat, which carry different health risks), and does not apply to children (use CDC BMI-for-age percentiles), pregnant women (use IOM pregnancy weight gain guidelines), athletes with high muscle mass, or adults over 65 (where the standard thresholds may underestimate risk). The Asian-specific cut-offs (overweight at 23+) are not applied here. BMI is a screening tool only and does not diagnose body fatness or health. Consult a healthcare provider for individualized assessment. See our disclaimer for full terms.
Frequently Asked Questions
What BMI is considered healthy?
The WHO defines a healthy BMI as 18.5-24.9 for adults aged 20+. Below 18.5 is underweight (associated with malnutrition and weakened immunity), and above 25 is overweight (associated with elevated risk of type 2 diabetes, hypertension, and cardiovascular disease). Above 30 is obese, with progressively higher health risks at each class.
Why does BMI not work well for athletes?
BMI cannot distinguish muscle from fat. A bodybuilder with 10% body fat may have a BMI of 30+ (technically 'obese') because muscle is denser than fat. Athletes with high lean mass should use body-fat percentage (via our <a href='/tools/body-fat.php'>Body Fat Calculator</a>) rather than BMI for a more accurate assessment.
Is BMI accurate for older adults?
Less so. Older adults lose muscle mass (sarcopenia) while maintaining or gaining fat. A normal BMI in an older adult may mask excess body fat and reduced muscle. The WHO and geriatric medicine use additional measures like calf circumference and grip strength to assess nutritional status in this population.
What about pregnant women?
BMI should not be used to assess pregnant women, as the weight includes the fetus, placenta, amniotic fluid, and increased blood volume. Use our <a href='/tools/pregnancy-weight-gain.php'>Pregnancy Weight Gain Calculator</a> instead, which applies Institute of Medicine guidelines based on pre-pregnancy BMI.
Do Asian populations need different BMI cut-offs?
Yes. Asian populations face elevated type 2 diabetes and cardiovascular risk at lower BMIs than Caucasians. The WHO suggests Asian-specific cut-offs: overweight at BMI 23+, obese at BMI 27+. Some countries (Japan, India) have adopted these for clinical use; this calculator uses the international (Caucasian) thresholds.
How does BMI relate to body fat percentage?
BMI correlates with body fat in populations (r ≈ 0.7-0.8) but is a poor predictor for individuals. A BMI of 25 may correspond to 15% body fat in a muscular male or 30% body fat in a sedentary one. For individual body-fat estimation, use our <a href='/tools/body-fat.php'>Body Fat Calculator</a> (U.S. Navy method) or get a DEXA scan, hydrostatic weighing, or Bod Pod measurement.
Last updated: July 21, 2026 · Author: HT99 Tools Editorial Team · Reviewed by: HT99 Tools Editorial Team