What Is a BMI Calculator?
A BMI calculator estimates your Body Mass Index — a simple ratio of weight to height used worldwide as a first-pass screening tool for weight categories. Public health agencies including the CDC and NIH rely on BMI because it requires only two measurements, correlates reasonably with body fat at the population level, and helps identify adults who may benefit from further clinical assessment. Our free BMI calculator accepts imperial or metric units, shows your numeric result, and maps it to standard adult weight categories.
BMI does not measure body fat directly, and it cannot distinguish muscle from fat. Still, it remains one of the most widely cited metrics in medical guidelines, insurance underwriting, and wellness programs because large epidemiological studies link BMI ranges to risks for type 2 diabetes, cardiovascular disease, and certain cancers. Use this calculator to get a quick snapshot, then explore related health tools for a fuller picture of your energy needs and target weight. For how we verify formulas and cite medical sources, see our editorial policy and methodology.
If your BMI suggests you are underweight, overweight, or obese, pair the result with our calorie calculator to estimate total daily energy expenditure, or check whether your current weight falls within a healthy range using our ideal weight calculator. Resting metabolism — the calories your body burns at complete rest — is modeled separately in our BMR calculator.
How to Use This BMI Calculator
- Choose your preferred unit system: imperial (feet, inches, pounds) or metric (centimeters, kilograms).
- Enter your height. In imperial mode, provide feet and inches separately; in metric mode, enter centimeters.
- Enter your current body weight in pounds or kilograms.
- Review your BMI value and the weight category assigned by CDC adult standards.
- Compare your result against the category table below to understand where you fall relative to underweight, healthy weight, overweight, and obesity ranges.
- Explore related calculators — calorie needs, basal metabolic rate, or ideal weight — if you are planning next steps with a healthcare provider.
How to Calculate BMI Manually
Manual BMI calculation requires converting your measurements into consistent units, then applying one of two standard formulas. In metric form, divide weight in kilograms by height in meters squared. In imperial form — common in the United States — multiply weight in pounds by 703, then divide by height in inches squared. Both methods produce the same BMI when inputs are converted correctly.
Let us work through a concrete example. Suppose you are 5 feet 10 inches tall and weigh 180 pounds. First convert height to total inches: 5 × 12 + 10 = 70 inches. Square the height: 70 × 70 = 4,900. Multiply weight by the imperial conversion factor: 180 × 703 = 126,540. Divide: 126,540 ÷ 4,900 ≈ 25.8. Your BMI is approximately 25.8, which falls in the overweight category (25.0 to 29.9) under CDC adult guidelines.
To verify with metric units, convert 180 lb to about 81.6 kg and 70 inches to 1.778 m. BMI = 81.6 ÷ (1.778)² = 81.6 ÷ 3.161 ≈ 25.8 — the same result. Small rounding differences are normal when converting between unit systems. The CDC BMI resources and the NIH NHLBI BMI calculator publish the same formulas, which is why our calculator aligns with government screening tools.
Remember that BMI is a population-level screening metric. A person with substantial muscle mass may score in the overweight range despite low body fat, while someone with normal BMI may still carry excess visceral fat. Waist circumference, body fat percentage, and clinical labs provide context that BMI alone cannot. Treat your number as a starting point for conversation with a qualified provider, not a final verdict on health.
BMI Formula
Imperial equivalent (US customary units):
- kg = kilograms
- m = meters
- lb = pounds
- in = inches
- 703 = conversion factor linking imperial units to the metric formula
CDC Adult BMI Categories
The table below reflects standard adult cutoffs used by the CDC for people aged 20 and older. Pediatric and adolescent BMI uses percentile charts rather than these fixed thresholds.
| Category | BMI Range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity | 30.0 and above |
Obesity is sometimes subdivided into Class I (30.0–34.9), Class II (35.0–39.9), and Class III (40.0 and above) in clinical settings, but the four-row table above covers the screening categories most people encounter in everyday use. If your BMI is 25.8 — as in our 5'10", 180 lb example — you would be classified as overweight and may wish to discuss lifestyle factors, waist measurement, and metabolic markers with a doctor before setting weight goals.
Frequently Asked Questions
What is a healthy BMI range?▼
For most adults, the CDC defines a healthy BMI as 18.5 to 24.9. Below 18.5 is underweight, 25.0 to 29.9 is overweight, and 30.0 or above is obesity. These ranges apply to adults aged 20 and older and do not account for muscle mass, age, or ethnicity.
Is BMI accurate for athletes and muscular people?▼
BMI can overestimate body fat in people with high muscle mass, such as athletes and weightlifters, because muscle weighs more than fat per unit volume. It may also underestimate health risk in older adults who have lost muscle. BMI is a screening tool, not a diagnostic measure of body composition.
How do I convert height and weight for BMI?▼
You can enter height in feet and inches or centimeters, and weight in pounds or kilograms. The calculator converts internally. In imperial units, BMI equals weight in pounds times 703, divided by height in inches squared. In metric units, BMI equals weight in kilograms divided by height in meters squared.
Does BMI differ for children and teens?▼
Yes. For people under 20, BMI is interpreted using age- and sex-specific percentile charts rather than fixed adult cutoffs. A pediatric BMI calculator compares a child result to CDC growth reference data. This tool is designed for adults aged 20 and older.
What should I do if my BMI is outside the healthy range?▼
A BMI outside 18.5 to 24.9 is a signal to talk with a healthcare provider, not a diagnosis on its own. Your doctor may consider waist circumference, blood pressure, cholesterol, family history, and lifestyle. Use BMI alongside tools like our calorie calculator to estimate daily energy needs if you are planning weight changes.
Explore more tools in our health calculators category, including BMR, calorie, and ideal weight tools for a complete picture of your anthropometric profile.