BMI Calculator
How BMI Is Calculated
To calculate body mass index, the calculator uses the standard CDC adult formula. You can switch between US customary and metric units; the calculation method is documented below and applied consistently.
US Customary Units
Formula: BMI = 703 × weight (lb) ÷ [height (in)]²
The factor 703 converts the result to the same scale as the metric formula. This calculator uses the 703 factor everywhere for US units (not the exact conversion factor 703.06958…), so the result matches CDC published tables.
Metric Units
Formula: BMI = weight (kg) ÷ [height (m)]²
Height in centimeters is divided by 100 to obtain meters.
Category Assignment
The BMI is rounded to one decimal place (half up), and the category is assigned from that displayed value:
- Underweight: BMI < 18.5
- Healthy Weight: BMI 18.5 – 24.9
- Overweight: BMI 25.0 – 29.9
- Class 1 Obesity: BMI 30.0 – 34.9
- Class 2 Obesity: BMI 35.0 – 39.9
- Class 3 Obesity (Severe Obesity): BMI ≥ 40.0
This rounding rule reproduces the CDC's example table for a person 5 ft 9 in (69 in) tall (see Fixtures).
Worked Examples
These examples use the CDC 5 ft 9 in (69 in) reference height and the one-decimal rounding rule.
| Weight (lb) | Unrounded BMI | Displayed BMI (1 dp) | Category |
|---|---|---|---|
| 124 | 18.309599 | 18.3 | Underweight |
| 125 | 18.457257 | 18.5 | Healthy Weight |
| 168 | 24.806553 | 24.8 | Healthy Weight |
| 169 | 24.954211 | 25.0 | Overweight |
| 202 | 29.826927 | 29.8 | Overweight |
| 203 | 29.974585 | 30.0 | Class 1 Obesity |
| 236 | 34.847301 | 34.8 | Class 1 Obesity |
| 237 | 34.994959 | 35.0 | Class 2 Obesity |
| 270 | 39.867675 | 39.9 | Class 2 Obesity |
| 271 | 40.015333 | 40.0 | Class 3 Obesity |
| Height | Weight | Unrounded BMI | Displayed BMI (1 dp) | Category |
|---|---|---|---|---|
| 175 cm | 70 kg | 22.857143 | 22.9 | Healthy Weight |
Fixtures (Exact-Arithmetic Test Values)
These values were computed with exact decimal arithmetic (Python Decimal, 60 digits) and are used to verify the calculator's arithmetic.
FX-BMI-1: CDC 69-inch sweep (US method, 703 factor)
Category assigned on BMI rounded to 1 decimal. Every whole pound from 80 to 400 lb at 69 in matches the CDC published boundaries: Underweight ≤124; Healthy Weight 125–168; Overweight 169–202; Class 1 203–236; Class 2 237–270; Class 3 ≥271.
FX-BMI-2: Rounding matters
125 lb at 69 in gives unrounded 18.457257 (below 18.5) but rounds to 18.5 → Healthy Weight, matching CDC. 169 lb gives 24.954211 → 25.0 → Overweight. 203 lb gives 29.974585 → 30.0 → Class 1.
FX-BMI-3: Metric (175 cm, 70 kg)
Unrounded 22.857143 → displayed 22.9 → Healthy Weight.
FX-BMI-4: US factor vs exact conversion
69 in, 125 lb: 703 factor gives 18.457257; exact conversions (0.45359237 kg/lb, 0.0254 m/in) give 18.459084. This calculator uses the 703 factor consistently.
FX-BMI-5: Category boundaries on displayed BMI (1 dp)
18.4 Underweight; 18.5 Healthy; 24.9 Healthy; 25.0 Overweight; 29.9 Overweight; 30.0 Class 1; 34.9 Class 1; 35.0 Class 2; 39.9 Class 2; 40.0 Class 3.
FX-BMI-6: Height conversion
5 ft 9 in = 69 in = 175.26 cm exactly (69 × 2.54).
Comparison Tables
BMI Categories at a Glance
| Category | BMI Range (1 dp) | Health Implication |
|---|---|---|
| Underweight | < 18.5 | May indicate insufficient nutrition or underlying health condition |
| Healthy Weight | 18.5 – 24.9 | Associated with lowest risk of weight-related health problems |
| Overweight | 25.0 – 29.9 | Increased risk for hypertension, type 2 diabetes, cardiovascular disease |
| Class 1 Obesity | 30.0 – 34.9 | Moderate risk; clinical evaluation recommended |
| Class 2 Obesity | 35.0 – 39.9 | High risk; clinical evaluation strongly recommended |
| Class 3 Obesity (Severe) | ≥ 40.0 | Very high risk; clinical evaluation strongly recommended |
US vs Metric Input Comparison
| Entry Method | Height | Weight | Displayed BMI | Category |
|---|---|---|---|---|
| US | 5 ft 9 in | 160 lb | 23.6 | Healthy Weight |
| Metric | 175 cm | 72.6 kg | 23.7 | Healthy Weight |
Visual Guides
✓ Included in this calculator
- CDC adult BMI formula (703 factor for US units)
- Categories assigned from displayed BMI (1 decimal, half-up)
- Healthy weight range at your height (in lb or kg)
- Step-by-step arithmetic with substituted numbers
- Unit toggle with lossless conversion
- Adult scope check (age ≥ 20)
✗ Not included
- Child/teen BMI percentiles (ages 2–19)
- Sex-specific adult categories (CDC uses same for all adults)
- Body fat percentage, muscle mass, or fat distribution
- Clinical diagnosis or treatment recommendations
- Pregnancy adjustments
- Ethnicity-specific thresholds
Common Mistakes and Key Conditions
- Using unrounded BMI for category: The category must come from the BMI rounded to one decimal. An unrounded 24.95 is Overweight (rounds to 25.0), not Healthy Weight.
- Mixing unit systems: Do not enter height in feet/inches and weight in kilograms. Use the unit toggle to switch the entire form.
- Entering total inches in the feet field: Use the feet field for whole feet (e.g., 5) and the inches field for remaining inches (e.g., 9).
- Forgetting the adult scope: This calculator is for ages 20+. Children and teens need percentile charts.
- Treating BMI as a diagnosis: BMI is a screening measure only. It does not measure body fat, muscle, bone density, or fat distribution.
Frequently Asked Questions
- Is BMI interpreted differently for women and men?
- No. The CDC uses the same adult BMI categories for women and men. BMI is a screening measure based on height and weight only; it does not account for sex, muscle mass, bone density, fat distribution, or ethnicity.
- How do I calculate BMI?
- how to calculate bmi: BMI = 703 × weight in pounds ÷ (height in inches)², or weight in kilograms ÷ (height in meters)². This calculator uses the 703 factor for US units and shows the arithmetic step by step.
- What is a healthy BMI range?
- For adults, a BMI of 18.5 to 24.9 is classified as Healthy Weight by the CDC. The calculator also shows the weight range in pounds (or kilograms) that corresponds to that category at your height.
- Can I use this calculator for children or teens?
- No. This calculator is for adults only (age 20 and older). Children and teens (ages 2–19) need age- and sex-specific BMI percentiles. See the CDC's Child and Teen BMI Calculator for that assessment.
- Does BMI measure body fat?
- No. BMI is a screening tool that uses height and weight. It does not measure body fat, muscle mass, bone density, or fat distribution. Athletes with high muscle mass may have a high BMI without excess fat.
- What is the difference between the 703 factor and exact conversion?
- The 703 factor is the CDC's published constant for US units. Exact unit conversions (0.45359237 kg/lb, 0.0254 m/in) imply a factor of about 703.06958. This calculator uses 703 everywhere for consistency with CDC tables.
- Why does 125 lb at 5 ft 9 in give Healthy Weight when the unrounded BMI is below 18.5?
- The CDC assigns categories from the BMI rounded to one decimal. 125 lb at 69 in gives unrounded 18.457…, which rounds to 18.5 → Healthy Weight. This matches the CDC's published table.
Glossary
- BMI
- Body Mass Index — a screening measure calculated from height and weight.
- CDC
- Centers for Disease Control and Prevention — the US national public health agency.
- Healthy Weight
- The CDC category for BMI 18.5–24.9 (displayed to one decimal).
- Obesity Class 1/2/3
- CDC subcategories for BMI 30.0–34.9, 35.0–39.9, and ≥40.0 respectively.
- Percentile
- A statistical measure used for children and teens (ages 2–19) instead of fixed BMI categories.
- Screening measure
- A tool that identifies potential risk; not a diagnosis.
Related Calculators
Official References and Last Checked
- CDC Adult BMI Calculator — formula, categories, FAQ (reference starting point)
- CDC Adult BMI Categories — category definitions and ranges (reference starting point)
- CDC BMI FAQ — common questions about BMI (reference starting point)
Rules version: 2026-10-18 2026-10-18 Sources listed above are reference starting points. See limitations.
Limitations, Exclusions, and Assumptions
- Not a diagnosis: BMI is a screening measure only. It does not measure body fat, muscle mass, bone density, age effects, ethnicity, or fat distribution.
- Adults only: This calculator is for people aged 20 and older. Children and teens (ages 2–19) need age- and sex-specific percentiles.
- No sex-specific categories: The CDC uses the same categories for all adults. This calculator does not offer separate female/male categories.
- No clinical advice: Results do not constitute medical advice, treatment recommendations, or weight-loss guidance.
- Excluded populations: Pregnancy, very high muscle mass, and clinical conditions requiring professional assessment are outside scope.
- Unit convention: US units use the 703 factor (not exact conversions) to match CDC published tables.
- Rounding convention: Category is assigned from displayed BMI rounded half-up to one decimal.
- Validation ranges:
- No data storage: Calculations run in your browser only. No health inputs are saved, transmitted, or shared.
Summary and Next Steps
Your BMI places you in a standard CDC adult weight category. This is a screening result, not a diagnosis. If your category is Underweight, Overweight, or any Obesity class, consider discussing weight and health with a clinician.
