Assessment
Body mass index calculator
Calculate body mass index (BMI) from weight and height — enter metric or imperial units and see the adult CDC/WHO category plus the full worked setup behind the answer, not just the number. Free, no signup, runs in your browser.
Interactive calculator
Body mass index (BMI) calculator
Enter weight and height — in metric or imperial units — to calculate body mass index, and see the worked setup and adult category below. Results update as you type.
Result
Enter weight and height to see the BMI, category, and worked setup.
For education and practice only. This tool is a study aid, not a substitute for clinical judgment or a medical assessment. BMI is a screening measure, not a diagnosis — interpret it with full clinical context and, for any real health decision, with a clinician.
How it works
The BMI formula
Body mass index relates weight to height so a single number can be compared across body sizes. The metric and imperial forms give the same value in kg/m² — the imperial form just folds in the unit conversion with the constant 703.
Metric BMI (kg/m²)
BMI = weight(kg) ÷ height(m)²
Divide weight in kilograms by height in meters squared. If height is in centimeters, divide by 100 first to get meters. The result is usually reported to one decimal.
Imperial BMI (kg/m²)
BMI = 703 × weight(lb) ÷ height(in)²
Multiply weight in pounds by 703, then divide by height in inches squared. The 703 constant converts pounds and inches into the metric kg/m² scale so both formulas agree.
Adult categories (CDC/WHO)
< 18.5 Underweight · 18.5–24.9 Normal · 25.0–29.9 Overweight · ≥ 30.0 Obesity (I 30–34.9, II 35–39.9, III ≥ 40)
These adult cut-offs apply to people 20 and older. Children and teens use BMI-for-age percentiles by age and sex instead.
Worked example
A BMI calculation, step by step
The patient
An adult who weighs 80 kg and is 1.80 m tall. What is the body mass index?
The setup
- 1. Square the height: 1.80 m × 1.80 m = 3.24 m².
- 2. Divide weight by height²: 80 ÷ 3.24 = 24.7.
- 3. Read the category: 24.7 falls in 18.5–24.9.
Answer: BMI 24.7 → Normal weight.
Reading the result
A BMI of 24.7 sits at the top of the normal range (18.5–24.9), just below the 25.0 overweight threshold. Because it is so close to the cut-off, small changes in weight or a measurement rounding could nudge the category — a reminder that BMI is a screen, not a hard line.
The imperial form checks out too: 176 lb and 71 in gives 703 × 176 ÷ (71 × 71) = 703 × 176 ÷ 5041 ≈ 24.5 — close to 24.7, with the small gap coming from rounding the converted units.
Enter 80 kg and 180 cm in the calculator above to see this exact worked setup returned.
Common questions
Body mass index, explained
- How do you calculate BMI?
- In metric units, BMI equals weight in kilograms divided by height in meters squared: BMI = weight(kg) ÷ height(m)². If height is in centimeters, divide by 100 first to get meters. In imperial units, BMI = 703 × weight(lb) ÷ height(in)². Both give the same value in kg/m², usually reported to one decimal.
- What are the adult BMI categories?
- For adults, the CDC and WHO use: below 18.5 = underweight; 18.5 to 24.9 = normal weight; 25.0 to 29.9 = overweight; and 30.0 or higher = obesity, split into Class I (30.0–34.9), Class II (35.0–39.9), and Class III (40.0 or higher). These cut-offs apply to adults 20 and older, not to children.
- Why is BMI a screening tool and not a diagnosis?
- BMI only uses height and weight, so it does not distinguish muscle from fat or show where fat is stored. A very muscular athlete can score in the overweight range without excess fat, and BMI can misclassify older adults who have lost muscle. It is a quick screen that flags people who may benefit from further assessment — not a diagnosis on its own.
- Does BMI work the same way for children?
- No. Children and teens use BMI-for-age percentiles specific to age and sex, because body composition changes as they grow. The same numeric cut-offs used for adults do not apply. Pediatric BMI is plotted on growth charts and read as a percentile, so use an age-and-sex percentile tool for anyone under 20.
- Can I enter pounds and inches instead of kilograms and centimeters?
- Yes. Enter weight in kilograms or pounds and height in centimeters or inches — this calculator converts internally (pounds ÷ 2.2 = kilograms, inches × 0.0254 = meters, centimeters ÷ 100 = meters) and shows the conversion in the worked steps. Convert feet-and-inches to total inches first (for example, 5 ft 11 in = 71 in).
- Is this BMI calculator free and mobile-friendly?
- Yes. It is completely free, needs no account, and runs entirely in your browser, so it works on a phone. It is a study and practice aid for learning how BMI is calculated and categorized — not medical advice. Any real assessment of weight and health should be made with a clinician who can consider the full picture.
Keep practicing
From assessment to med-math
BMI is one of the assessment numbers you will meet on the wards and in practice questions. When you are ready for dosing math, use the full dosage calculator, read the dosage study guide, or explore the other free calculators.
This calculator and all study material on this site are provided for practice and study only — they are not medical advice or a substitute for clinical judgment or a medical assessment. BMI is a screening measure, not a diagnosis. NCLEX® is a registered trademark of the National Council of State Boards of Nursing, Inc. (NCSBN), which does not endorse or sponsor this site.