BMI Calculator (Body Mass Index)
BMI Calculator
BMI is the most widely used screening tool for weight classification. While it has limitations, it provides a quick assessment of weight status relative to height. The formula works the same in metric and imperial — enter your measurements in whichever system you prefer.
Conversion Formula
For metric: BMI = kg / m². For imperial: BMI = (lbs × 703) / in². The 703 factor converts the imperial calculation to match the metric result. Both produce the same BMI value.
Step-by-Step Examples
170 lbs, 70 in (5'10") = BMI 24.4 (Normal)
(170 × 703) / (70²) = 24.4 — that's 77 kg at 178 cm
77 kg, 178 cm = BMI 24.3 (Normal)
77 / (1.78²) = 24.3 — that's about 170 lbs at 5'10"
200 lbs, 66 in (5'6") = BMI 32.3 (Obese)
(200 × 703) / (66²) = 32.3 — that's 91 kg at 168 cm
History
BMI was devised by Adolphe Quetelet between 1830-1850. The term "Body Mass Index" was coined by Ancel Keys in 1972.
Common Use Cases
- Health screening
- Weight management tracking
- Insurance risk assessment
- Clinical research
Frequently Asked Questions
What is BMI?
Body Mass Index (BMI) is a numerical value calculated from height and weight. It is used as a screening tool to categorize weight status. For example, a person who weighs 170 lbs (77 kg) and is 5'10" (178 cm) tall has a BMI of about 24.4.
What is a healthy BMI range?
A BMI of 18.5 to 24.9 is considered normal weight. Below 18.5 is underweight, 25-29.9 is overweight, and 30+ is obese. For a person who is 5'9" (175 cm) tall, a healthy weight is roughly 125-169 lbs (57-77 kg).
Is BMI accurate for athletes?
BMI does not distinguish between muscle and fat. Athletes and muscular individuals may have a high BMI despite being healthy. For example, a 6'0" (183 cm) athlete weighing 210 lbs (95 kg) of mostly muscle would have a BMI of 28.5 (overweight), even though they may be very fit.
How is BMI calculated in both unit systems?
In metric: BMI = weight(kg) / height(m)². In imperial: BMI = (weight(lbs) × 703) / height(in)². Both give the same result. For example, 77 kg / (1.78 m)² = 24.3, and (170 lbs × 703) / (70 in)² = 24.4 — the same person in different units.
Should I use BMI alone to assess my health?
No, BMI is a screening tool only. Consult a healthcare provider who can consider waist circumference (ideally under 40 in / 102 cm for men, 35 in / 88 cm for women), body fat percentage, and other factors.
Does BMI differ by age or gender?
The BMI formula is the same for adults of all ages and genders. For children, BMI is interpreted using age- and gender-specific percentiles.
What BMI is considered obese?
A BMI of 30 or higher is classified as obese. For a 5'6" (168 cm) person, that means weighing 186 lbs (84 kg) or more. Severe obesity is a BMI of 40 or higher.