Last updated: March 2026
BMI Calculation in Metric Units
The metric BMI formula is the original and simplest version: BMI = weight (kg) / height (m)². Unlike the Imperial version, it requires no conversion factor. You take your weight in kilograms, divide by your height in meters squared, and the result is your Body Mass Index.
For example, if you weigh 68 kg and stand 170 cm tall, the calculation is: 68 / (1.70 × 1.70) = 68 / 2.89 = 23.5. That places you in the normal weight category (18.5 to 24.9). If you weigh 90 kg at the same height: 90 / 2.89 = 31.1, which falls into the Obese Class I range.
The reason metric is simpler is historical. Belgian mathematician Adolphe Quetelet designed the formula in the 1830s using the metric system. The formula naturally produces a value in kg/m². When you use Imperial units (pounds and inches), you need to multiply by 703 to convert the result back to the same kg/m² scale. Both methods produce identical BMI values — the metric path simply gets there with less arithmetic.
Metric vs. Imperial: Formula Differences Explained
The two BMI formulas look different but are mathematically equivalent. Metric: BMI = kg / m². Imperial: BMI = (lbs × 703) / inches². The factor of 703 compensates for the difference between pounds/inches and kilograms/meters. Specifically, 703 approximates the ratio needed to convert lbs/in² to kg/m².
Here is the unit math. One kilogram equals 2.20462 pounds. One meter equals 39.3701 inches. So 1 kg/m² = 2.20462 lbs / (39.3701 in)² = 2.20462 / 1549.997 lbs/in². Inverting this gives approximately 703.07, which rounds to 703 in clinical practice. The rounding introduces an error of less than 0.01%, which is insignificant for a screening tool.
In practice, the formula you use makes no difference to the result. A person who is 5'10” (177.8 cm) and 170 lbs (77.1 kg) gets a BMI of approximately 24.4 whether calculated in metric or Imperial. This calculator defaults to metric inputs for convenience if you think in kilograms and centimeters, but you can switch to Imperial using the toggle at the top.
Global Usage of Metric BMI
The vast majority of the world uses the metric system for everyday measurements, and medical research worldwide reports BMI in kg/m². Only the United States, Liberia, and Myanmar officially use Imperial units. Even in the US, scientific and medical literature typically reports BMI using the metric formula, with Imperial conversions provided for patient-facing materials.
International health organizations including the WHO, NHS, and health ministries across Europe, Asia, Africa, and South America all use metric BMI. The standard BMI categories (underweight below 18.5, normal 18.5 to 24.9, overweight 25 to 29.9, obese 30+) were established using metric measurements and are applied globally. Some Asian countries have adopted modified cutoffs — Japan uses 25 as the obesity threshold, while China and India use 24 for overweight — reflecting that health risks emerge at lower BMI levels in certain populations.
For international travelers, expatriates, or anyone accustomed to metric measurements, this calculator provides a friction-free experience. Enter your weight in kilograms and height in centimeters, and the result appears instantly. No need to convert units first — the tool does the math directly in the system you already think in.
Health Disclaimer: BMI is a general screening tool and is not a diagnostic measure. It does not account for muscle mass, bone density, age, sex, or body composition differences. The same BMI categories apply regardless of unit system used. Consult a healthcare professional for personalized health guidance.
Frequently Asked Questions
What is the metric BMI formula?
In metric units, BMI = weight (kg) / height (m)². For example, if you weigh 70 kg and are 1.75 m tall: BMI = 70 / (1.75 × 1.75) = 70 / 3.0625 = 22.9. The metric formula is the original — no conversion factor needed.
How do I convert my height to centimeters?
Multiply feet by 30.48 and add inches × 2.54. For example, 5'8" = (5 × 30.48) + (8 × 2.54) = 152.4 + 20.32 = 172.72 cm. Or use our unit converter tool for instant conversions.
Is the metric BMI formula more accurate than Imperial?
Both formulas produce identical BMI values — they're mathematically equivalent. The metric formula (kg/m²) is simpler because it doesn't need a conversion factor. The Imperial formula multiplies by 703 to compensate for using pounds and inches instead of kg and meters.
What BMI is considered healthy in metric countries?
The WHO categories are the same worldwide: underweight below 18.5, normal weight 18.5–24.9, overweight 25–29.9, obese 30+. Some Asian countries use adjusted thresholds (overweight starting at 23) because health risks increase at lower BMI levels in those populations.
Why do most of the world use metric for BMI?
The BMI formula was originally designed using metric units (kg/m²) and most countries use the metric system. Only the US, Liberia, and Myanmar officially use Imperial measurements. The metric version is simpler, requires no conversion factor, and is used in virtually all international medical research.
Is this metric BMI calculator free?
Yes, completely free. No signup, no data collection, no ads. All calculations happen in your browser. Your weight and height data is never sent to any server.